This paper is published as part of a PCCP Themed Issue on:

Physical Chemistry of Aerosols

Guest Editors: Ruth Signorell and Allan Bertram (University of British Columbia)

Editorial

Physical Chemistry of Aerosols Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b916865f

Perspective

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols Barbara J. Finlayson-Pitts, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906540g

Papers

Water uptake of clay and desert dust aerosol particles at sub- and supersaturated water vapor conditions Hanna Herich, Torsten Tritscher, Aldona Wiacek, Martin Gysel, Ernest Weingartner, Ulrike Lohmann, Urs Baltensperger and Daniel J. Cziczo, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b901585j Secondary organic aerosol formation from multiphase oxidation of limonene by ozone: mechanistic constraints via two-dimensional heteronuclear NMR spectroscopy Christina S. Maksymiuk, Chakicherla Gayahtri, Roberto R. Gil and Neil M. Donahue, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b820005j DRIFTS studies on the photodegradation of tannic acid as a model for HULIS in atmospheric aerosols Scott Cowen and Hind A. Al-Abadleh, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905236d Infrared spectroscopy of ozone and hydrogen chloride aerosols Chris Medcraft, Evan G. Robertson, Chris D. Thompson, Sigurd Bauerecker and Don McNaughton, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905424n IR spectroscopy of physical and chemical transformations in cold hydrogen chloride and ammonia aerosols Evan G. Robertson, Chris Medcraft, Ljiljana Puskar, Rudolf Tuckermann, Chris D. Thompson, Sigurd Bauerecker and Don McNaughton, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905425c Formation of naproxen–polylactic acid nanoparticles from supercritical solutions and their characterization in the aerosol phase Moritz Gadermann, Simran Kular, Ali H. Al-Marzouqi and Ruth Signorell, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b901744e Measurements and simulations of the near-surface composition of evaporating ethanol–water droplets Christopher J. Homer, Xingmao Jiang, Timothy L. Ward, C. Jeffrey Brinker and Jonathan P. Reid, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904070f

Effects of dicarboxylic acid coating on the optical properties of soot Huaxin Xue, Alexei F. Khalizov, Lin Wang, Jun Zheng and Renyi Zhang, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904129j Spectroscopic evidence for cyclical aggregation and coalescence of molecular aerosol particles J. P. Devlin, C. A. Yinnon and V. Buch, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905018n Photoenhanced ozone loss on solid pyrene films Sarah A. Styler, Marcello Brigante, Barbara D Anna, Christian George and D. J. Donaldson, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904180j Quantifying the reactive uptake of OH by organic aerosols in a continuous flow stirred tank reactor Dung L. Che, Jared D. Smith, Stephen R. Leone, Musahid Ahmed and Kevin R. Wilson, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904418c Laboratory study of the interaction of HO2 radicals with the NaCl, NaBr, MgCl2·6H2O and sea salt surfaces Ekaterina Loukhovitskaya, Yuri Bedjanian, Igor Morozov and Georges Le Bras, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906300e Kinetics of the heterogeneous reaction of nitric acid with mineral dust particles: an aerosol flowtube study A. Vlasenko, T. Huthwelker, H. W. Gäggeler and M. Ammann, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904290n Timescale for hygroscopic conversion of calcite mineral particles through heterogeneous reaction with nitric acid Ryan C. Sullivan, Meagan J. K. Moore, Markus D. Petters, Sonia M. Kreidenweis, Greg C. Roberts and Kimberly A. Prather, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904217b Mid-infrared complex refractive indices for oleic acid and optical properties of model oleic acid/water aerosols Shannon M. McGinty, Marta K. Kapala and Richard F. Niedziela, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905371a A study of oleic acid and 2,4-DHB acid aerosols using an IR-VUV-ITMS: insights into the strengths and weaknesses of the technique Sarah J. Hanna, Pedro Campuzano-Jost, Emily A. Simpson, Itamar Burak, Michael W. Blades, John W. Hepburn and Allan K. Bertram, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904748d Deliquescence behaviour and crystallisation of ternary ammonium sulfate/dicarboxylic acid/water aerosols L. Treuel, S. Pederzani and R. Zellner, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905007h

Laboratory chamber studies on the formation of organosulfates from reactive uptake of monoterpene oxides Yoshiteru Iinuma, Olaf Böge, Ariane Kahnt and Hartmut Herrmann, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904025k Measurement of fragmentation and functionalization pathways in the heterogeneous oxidation of oxidized organic aerosol Jesse H. Kroll, Jared D. Smith, Dung L. Che, Sean H. Kessler, Douglas R. Worsnop and Kevin R. Wilson, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905289e Using optical landscapes to control, direct and isolate aerosol particles Jon B. Wills, Jason R. Butler, John Palmer and Jonathan P. Reid, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b908270k Reactivity of oleic acid in organic particles: changes in oxidant uptake and reaction stoichiometry with particle oxidation Amy M. Sage, Emily A. Weitkamp, Allen L. Robinson and Neil M. Donahue, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904285g Surface tension of mixed inorganic and dicarboxylic acid aqueous solutions at 298.15 K and their importance for cloud activation predictions Alastair Murray Booth, David Owen Topping, Gordon McFiggans and Carl John Percival, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906849j Kinetics of the heterogeneous conversion of 1,4-hydroxycarbonyls to cyclic hemiacetals and dihydrofurans on organic aerosol particles Yong Bin Lim and Paul J. Ziemann, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904333k Time-resolved molecular characterization of limonene/ozone aerosol using high-resolution electrospray ionization mass spectrometry Adam P. Bateman, Sergey A. Nizkorodov, Julia Laskin and Alexander Laskin, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905288g Cloud condensation nuclei and ice nucleation activity of hydrophobic and hydrophilic soot particles Kirsten A. Koehler, Paul J. DeMott, Sonia M. Kreidenweis, Olga B. Popovicheva, Markus D. Petters, Christian M. Carrico, Elena D. Kireeva, Tatiana D. Khokhlova and Natalia K. Shonija, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905334b

Effective broadband refractive index retrieval by a white light optical particle counter J. Michel Flores, Miri Trainic, Stephan Borrmann and Yinon Rudich, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905292e Influence of gas-to-particle partitioning on the hygroscopic and droplet activation behaviour of -pinene secondary organic aerosol Zsófia Jurányi, Martin Gysel, Jonathan Duplissy, Ernest Weingartner, Torsten Tritscher, Josef Dommen, Silvia Henning, Markus Ziese, Alexej Kiselev, Frank Stratmann, Ingrid George and Urs Baltensperger, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904162a Reactive uptake studies of NO3 and N2O5 on alkenoic acid, alkanoate, and polyalcohol substrates to probe nighttime aerosol chemistry Simone Gross, Richard Iannone, Song Xiao and Allan K. Bertram, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904741g Organic nitrate formation in the radical-initiated oxidation of model aerosol particles in the presence of NOx Lindsay H. Renbaum and Geoffrey D. Smith, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b909239k Dynamics and mass accommodation of HCl molecules on sulfuric acid–water surfaces P. Behr, U. Scharfenort, K. Ataya and R. Zellner, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904629a Structural stability of electrosprayed proteins: temperature and hydration effects Erik G. Marklund, Daniel S. D. Larsson, David van der Spoel, Alexandra Patriksson and Carl Caleman, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b903846a Tandem ion mobility-mass spectrometry (IMS-MS) study of ion evaporation from ionic liquid-acetonitrile nanodrops Christopher J. Hogan Jr and Juan Fernández de la Mora, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904022f Homogeneous ice freezing temperatures and ice nucleation rates of aqueous ammonium sulfate and aqueous levoglucosan particles for relevant atmospheric conditions Daniel Alexander Knopf and Miguel David Lopez, Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b903750k

Cloud condensation nuclei and ice nucleation activity of hydrophobic

and hydrophilic soot particles

Kirsten A. Koehler,*a Paul J. DeMott,*a Sonia M. Kreidenweis,a

Olga B. Popovicheva,bMarkus D. Petters,

aChristian M. Carrico,

a

Elena D. Kireeva,bTatiana D. Khokhlova

cand Natalia K. Shonija

c

Received 16th March 2009, Accepted 23rd June 2009

First published as an Advance Article on the web 28th July 2009

DOI: 10.1039/b905334b

Cloud condensation nuclei (CCN) activity and ice nucleation behavior (for temperatures r�40 1C)

of soot aerosols relevant for atmospheric studies were investigated. Soots were chosen to

represent a range of physico-chemical properties, from hydrophobic through a range of

hydrophilicity, to hygroscopic. These characteristics were achieved through generation by three

different combustion sources; three soots from natural gas pyrolysis (original: TS; graphitized:

GTS; and oxidized: TOS), soot from a diffusion flame in an oil lamp burning aviation kerosene

(TC1), and soot from a turbulent diffusion flame in an aircraft engine combustor (AEC). All of

the samples exhibited some heterogeneity in our experiments, which showed evidence of two or

more particle sub-types even within a narrow size cut. The heterogeneity could have resulted from

both chemical and sizing differences, the latter attributable in part to particle non-sphericity.

Neither GTS nor TS, hydrophobic particles distinguished only by the lower porosity and polarity

of the GTS surface, showed CCN activity at or below water supersaturations required for

wettable, insoluble particles (the Kelvin limit). TC1 soot particles, despite classification as

hydrophilic, did not show CCN activity at or below the Kelvin limit. We attribute this result to

the microporosity of this soot. In contrast, oxidized, non-porous, and hydrophilic TOS particles

exhibited CCN activation at very near the Kelvin limit, with a small percentage of these particles

CCN-active even at lower supersaturations. Due to containing a range of surface coverage of

organic and inorganic hydrophilic and hygroscopic compounds, up to B35% of hygroscopic

AEC particles were active as CCN, with a small percentage of these particles CCN-active at lower

supersaturations. In ice nucleation experiments below �40 1C, AEC particles nucleated ice near

the expected condition for homogeneous freezing of water from aqueous solutions. In contrast,

GTS, TS, and TC1 required relative humidity well in excess of water saturation at �40 1C for

ice formation. GTS particles required water supersaturation conditions for ice activation even

at �51 1C. At �51 to �57 1C, ice formation in particles with electrical mobility diameter of

200 nm occurred in up to 1 in 1000 TS and TC1 particles, and 1 in 100 TOS particles, at relative

humidities below those required for homogeneous freezing in aqueous solutions. Our results

suggest that heterogeneous ice nucleation is favored in cirrus conditions on oxidized hydrophilic

soot of intermediate polarity. Simple considerations suggest that the impact of hydrophilic soot

particles on cirrus cloud formation would be most likely in regions of elevated atmospheric soot

number concentrations. The ice formation properties of AEC soot are reasonably consistent with

present understanding of the conditions required for aircraft contrail formation and the

proportion of soot expected to nucleate under such conditions.

1. Introduction

The role of soot particles in the Earth’s climate system remains

a key outstanding uncertainty,1,2 with potentially large effects

of soot on the radiation budget and on the formation and

properties of both warm3–5 and cold6–8 clouds. Myriad sources

emit soot particles, including industrial combustion processes,

agricultural burning, forest fires, domestic heating and

cooking, and transportation sources. In the latter category,

emissions from aircraft engines deserve special attention,

since these are injected directly into the middle and upper

troposphere. Although the amount of soot, sulfur, metal

particles and nitrogen oxides emitted from airplanes is

small compared to the total loading in the atmosphere, at the

altitude of aircraft flight, these emissions may represent a

dominant source.8

The interactions of soot with water vapor in the atmosphere

are of particular interest, since they influence soot lifetime with

aDepartment of Atmospheric Science, Colorado State University,Fort Collins CO, 80523, USA

b Institute of Nuclear Physics, Moscow State University, 119991,Moscow, Russia

c Chemical Department, Moscow State University, 119991, Moscow,Russia

7906 | Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 This journal is �c the Owner Societies 2009

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

respect to nucleation scavenging and precipitation removal

and may influence cloud formation itself, as well as affect soot

optical properties and thus visibility and radiative balance.

Soot particles may serve as warm–cloud cloud condensation

nuclei (CCN), depending on their properties and the

ambient water supersaturation,9 where supersaturation,

Sw(%), is computed from the relative humidity with respect

to water, RHw(%), using Sw = RHw � 100. At colder

temperatures, soot particles may be involved in cirrus cloud

ice particle formation10–12 or in aircraft contrail

formation.13–15 This involvement may occur via either hetero-

geneous or homogeneous ice nucleation during water uptake

on the soot surfaces. Heterogeneous nucleation may occur if

the particle surfaces possess properties that induce ice

embryo formation on the surface, while homogeneous

nucleation could occur in aqueous solutions on soot surfaces

during condensation on surfaces at higher RHw. Upper

tropospheric temperatures are low enough for either type

of ice nucleation to occur, although heterogeneous ice

nucleation might be expected to ensue at lower relative

humidity. Contrail observations supported by theoretical

estimates based on simple models of plume evolution

suggest that contrails only form at high relative humidities,

when liquid saturation with respect to water is reached in the

plume.16,17 Nevertheless, the specific ice nucleation activity

of aircraft-generated soot particles and their impact on

threshold contrail formation conditions is still an important

open question.

Indeed, sparse documentation exists on the CCN and ice

nucleating properties of soot particles and on the relationship

between the physico-chemical characteristics of these

particles and their ability to uptake water. Laboratory studies

can provide relevant information concerning soot CCN and

ice nucleation behaviors. However, the literature concerning

soot laboratory studies shows the use of a great variety of

combustion sources and soot surrogates for atmospheric

studies. Commercially available soots,18–20 spark discharge

soot21,22 and samples produced by different burners and

fuels23–26 have been used to study the CCN and/or IN

activity of soot particles. These different particles represent

a variety of physico-chemical properties leading to differences

in their water/ice nucleability. Combustion aerosols generated

by burning acetylene fuel in a welding torch under high

oxygen conditions, as well as by flaming wood combustion,

have demonstrated the highest CCN activity; the ratio of the

number of CCN to the total number of condensation nuclei

(CN) is near 0.49 and 0.72 for these, respectively, at 1%

supersaturation.26 In contrast, the open burning of

aviation fuel JP-4 produced soot particles of weak CCN

activity (CCN/CN E8� 10�3) for similar conditions.

Unfortunately, not all such studies have characterized the

properties responsible for soot hygroscopicity and CCN

activity, such as composition and water soluble fraction. Thus,

the application of results to atmospheric systems is sometimes

questionable.

The most complex problem of CCN and IN activation is

determining the surface requirements for water/ice nucleation

at the microscopic level. It is expected that soot aerosols

may be incorporated into cloud droplets by the presence of

water soluble compounds.20,27,28 However, a comprehensive

evaluation of organic coverage composition in urban

areas and in diesel exhaust29,30 identified the presence of

different classes of polar compounds (aromatic, alkanoic and

alkanedioic acids, phenols) as well as non-polar polyaromatic

hydrocarbons (PAH), alkanes and alkenes. Published data

provide evidence that polar organic compounds uptake

water31 and thus increase the cloud and fog formation ability

of insoluble soot cores. Non-polar organic compounds

generally do not take up water32 and may also form films

that lower the accommodation coefficient of water on the

particles, thereby inhibiting absorption or absorption of water

on the surface.33,34 In addition, water adsorption is observed

on insoluble soot particles35–37 due to interaction with

surface active sites; water may condense on partly wettable

particles even below saturation conditions due to capillary

condensation into mesopores.38 Literature on the hetero-

geneous ice forming ability of insoluble particles yields a

number of requirements relating to size, microstructure,

insolubility, chemical bond, and surface active sites.39 Sites

which are capable of adsorbing water molecules are also sites

at which ice nucleation is likely to be initiated, while the

number of critical embryos per unit area of the substrate is

proportional to the number of water molecules adsorbed per

unit area.

Experiments in which actual combustion particles are

collected and characterized offer a direct method for studying

atmospheric soot water interactions while mitigating the

expense and difficulty of direct atmospheric sampling. Test

experiments at ground aircraft engine facilities are an example

of such characterization studies. In the PartEmis gas turbine

engine measurement experiments,3,27 the impact of the fuel

sulfur content on the hygroscopic growth factors and on CCN

activities of exhaust soot particles were reported. The

morphology, microstructure, and chemical composition of

original aircraft engine combustor (AEC) soot generated by

burning aviation TC1 kerosene in a typical gas turbine engine

combustor were described in Popovicheva et al.40 Possible

pathways of the CCN and IN activation of AEC soot through

the condensation freezing mode have been suggested in

Popovicheva et al.40 and Shonija et al.41

Laboratory measurements of water interactions on

hydrophobic and hydrophilic soots were performed using

water uptake techniques at conditions that simulate those in

the atmosphere.36 This includes applying a CCN counter for

measuring the activated fraction of particles representative of

anthropogenic BC aerosols over a wide range of particle size

and water supersaturations. A continuous flow diffusion

chamber (CFDC)42 is used to determine the relative humidity

conditions needed for IN activation at temperatures typical for

the upper troposphere (down to �60 1C). The present study

seeks direct evidence for CCN activity at environmentally-

relevant water supersaturation, and for ice nucleation behaviors

at contrail and cirrus formation conditions, using a set of

carefully selected soots, of varying and well-characterized

physical and chemical characteristics. These studies contribute

toward clarifying the relationship between the extent of soot

hydrophilicity and CCN/IN activated fraction at atmospheric

conditions.

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 | 7907

2. Samples and instrumentation

2.1 Samples and characterization

In a study related to the present one,36 we proposed a set of

soot samples selected because they exhibit a variety of physico-

chemical properties resulting in markedly different hydration

properties. We characterized the key soot properties responsible

for water uptake, namely surface area, texture parameters,

elemental composition, water soluble fraction, including ion

species, and surface polarity by a number of techniques such as

gravimetry, XREDS coupled to TEM, analytic chemistry and

ion chromatography. Observations of the water uptake below

100% RHw by two techniques were also reported. These

different hydration properties were classified on the basis of

the polarity of the soot surface (‘‘polarity’’ classification).

In this study, we added one more hydrophilic soot to our

experiments to accomplish representing the whole range of

hydration properties, from hydrophobic through a range of

hydrophilicity, to hygroscopic. We also adopt the more recent

concept of quantifying hydration properties suggested by

Popovicheva et al.,37 which we will refer to as the ‘‘water film’’

classification. Popovicheva et al.37 suggested that soot

demonstrating water uptake that exceeds the formation of a

water film of many layers even at low RHw be termed

‘‘hygroscopic soot’’. Its water uptake is attributed to the

presence of relatively large amounts of water-soluble material

associated with the particles; absorption via dissolution of this

soluble material is the water uptake mechanism. Low water

adsorption on some active sites following cluster formation is

a typical mechanism of water interaction with ‘‘non-hygroscopic’’

soot particles. These are qualitatively termed ‘‘hydrophilic’’ or

‘‘hydrophobic’’ based on the quantitative extent of water

uptake. Specifically, the spreading of clusters to form a water

film extending over the surface at or below 80% RHw, where

capillary condensation in inter-particle cavities does not yet

contribute to total water uptake, was proposed as a quantification

measure that separates hydrophilic from hydrophobic soot.

Table 1 summarizes the relevant properties and classifica-

tion for soot particles studied in this work. All of these soot

particles, except the graphitized sample, are the product of

actual combustion and therefore exhibit the typical features

for soot microstructure of graphite microcrystallites revealed

previously by Raman spectroscopy.43

Fig. 1 presents the water uptake (via bulk sample adsorption

data) on all soots with respect to the criterion used in the water

film classification. The area of existence of a water film on the

soot surface is indicated, with low and high boundaries

computed for 20 and 250 nm diameter primary particles,

respectively, as these diameters define the typical range of

primary soot particle sizes.

Thermal soot (TS). This soot was purchased from

Electrougly Ltd. (Moscow, Russia). Although previous studies

have used other commercially available soots (e.g. Printex

soot) we chose this sample because the primary particle size

close to that which is typically selected by a differential

mobility analyzer for monodisperse analysis. In doing so, we

minimize the fraction of particles that contain more than one

primary particle. The thermal soot chosen has a mean primary

particle diameter of 260 nm, as found by TEM analysis. Other

commercially available soots typically have primary particle

diameters less than 50 nm. Soot types with small primary

particle size will form agglomerated particles, which increases

the uncertainty in particle size and complicates the estimation

of particle size change in terms of the growth factor or particle

shrinkage. By selecting samples that likely only contain a single

primary particle, we are able to make clearer conclusions about

the role of the hydrophilicity of the surface itself. The thermal

soot is the product of pyrolysis of natural gas, with water

soluble fraction (WSF) B0.5%. Although our previous study

detected functional groups on the surface of TS, the low water

adsorption (due to low surface polarity) at up to 80% RHw

suggested that only water clusters, and not a surface film,

could form on these particles.37 Increased water uptake above

80% RHw was attributed to adsorption in mesopores. TS soot

is hydrophobic according to the water film classification.

Graphitized thermal soot (GTS). This sample was produced

by heating TS to B3000 1C. This treatment removes

any impurities from the particle surface and graphitizes the

structure, creating hydrophobic particles with a polyhedron

structure. Prior to CCN and IN measurements, the sample of

GTS soot was heated at 150 1C for 2 h to remove possible

contaminations accumulated during sample storage before

experiments.

Thermal oxidized soot (TOS). This soot was produced for

this study from TS by immersion in a 2.5 : 1 mixture of

concentrated nitric and sulfuric acids at room temperature.

The process of oxidation took 30 days. The reacted sample

was washed in flowing water for a few days up to pH about 7

to remove all soluble and unreacted materials from the

surface, and then dried at 120 1C for 4 h. The number of

active sites produced by this method was determined by

titration with 0.01 N NaOH aqueous solution, yielding an

estimate of 10.6 meq/g of acidic sites, corresponding to

B1 nm�2 acid groups present on the surface. For comparison,

a similar titration for untreated TS showed less than 0.1 nm�2

acid groups. N2 adsorption isotherms yielded a surface area of

6.5 m2 g�1, comparable to that of the original TS sample,

10 m2 g�1. The adsorption isotherm for TOS soot was

measured in this study by the method described in Popovicheva

et al.36 and is shown in Fig. 1. Oxidation of TS soot leads to an

increase in the density of active water adsorption sites and the

extent of surface polarity (intermediate polarity), and thus

increases the hydrophilicity of the surface, but does not

measurably change the soot microporosity. As seen in

Fig. 1, the adsorption isotherm of TOS soot approaches the

boundary of the area for existence of a water film on the soot

surface. Therefore, we infer that water cluster confluence and

water film formation occurs on the surface of this soot at quite

low RHw. Based on these observed characteristics, TOS is

classified as hydrophilic soot.

TC1 soot. TC1 kerosene flame soot was generated by

burning aviation kerosene TC1 in an oil lamp and collected

at 15–20 cm above the flame. This soot is typically composed

of agglomerates of 20–50 nm particles and is approximately

95% C and 5% O with no detectable sulfur, and with less than

7908 | Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 This journal is �c the Owner Societies 2009

1% WSF in the bulk sample. TC1 soot adsorbs water even

more readily than oxidized TOS. Thus, by either the water film

classification37 or the polarity classification36 we must classify

the surface of this soot as hydrophilic. Nevertheless, we know

from our previous studies36 that TC1 soot is highly

micro-porous and subject to a swelling phenomenon, as

revealed by hysteresis observed in adsorption/desorption

cycling. Therefore, we expect that the large amount of water

adsorbed below water saturation may be unavailable for

influencing CCN activation.

Aircraft engine combustor (AEC) soot. AEC soot was

generated by burning TC1 kerosene in a D30-KU aircraft

engine combustor and collected on a probe about 12 cm

from the exit of the engine combustor operating at cruise

conditions.40 The primary particle size of the main fraction

was 30–100 nm and contained essentially amorphous carbon

with small amounts of oxygen, sulfur and iron. A fraction of

impurities was characterized by various structures and large

amounts of contaminants, such as oxygen, iron, potassium

and sulfur. This soot had an estimated bulk sample water

soluble fraction (WSF) of around 13.5%,40 leading to multi-

layer water absorption at low RHw and its classification as

hygroscopic soot. The identifiable ion fraction (near 5%) was

dominated by sulfate ion along with organic ions including

acetate, formate and oxalate. Fourier transform infrared

(FTIR) spectroscopy was applied for measuring spectra of

AEC soot in Popovicheva et al.40 A large variety of –CH and

–CH2 groups in aromatics and aliphatics as well as oxygen-

containing functional groups such as –CQO carbonyl,

carboxyl, and –OH hydroxyl were found. These functional

groups may create hydrogen bounds of different strength,

thereby impacting soot hygroscopicity. Additionally, the

peaks of HSO4� ions and organic sulfates were found in the

spectra of AEC soot indicating the presence of hydrophilic

compounds on the surface of this soot. Characterization of the

organic species present was conducted as part of this study and

results are reported in Table 2.Table

1Sootsamplesstudiedin

thiswork

Meanprimary

particle

diameter/nm

Porosity

Surface

polarity

General

classification

(waterfilm

classification)

HTDMA

observations

(Popovichevaet

al.2008a,

exceptTOS)

CCN

activity

%Activatedat

Kelvin

line

(T=

301C)

Iceform

ation,

RH

w=

RH

Koop

(T=�40/�

57/

–51.5

1C)

Iceform

ation,

RH

w=

100%

(T=�40/�

57/

�51.5

1C)

AEC

30–100a

Low

micro-and

mesoporosity

High

Hygroscopic

Somerestructuring;

GF4

1forRH

w4

75%

B30%

ofparticles

activated

atSw4

1.5%

forDm=

250nm

25%

4%

/–/0.3%

3%

/–/3%

(Dm=

250nm)

TC1

55

Highmicro-and

mesoporosity

Interm

ediate

Hydrophilic

Minim

alrestructuring;

GF4

1forRH

w4

80%

Negligible(o

5%activated)upto

Sw=

2%forDm=

200nm

o1%

o0.01%

/–/0.1%

0.02%/–/0.1%

(Dm=

200nm)

TS

260

Low

micro-and

mesoporosity

Low

Hydrophobic

Norestructuring

GF=

1to

B90%

RH

w

Negligible(o

5%activated)

below

Sw=

2%for

Dm=

200nm;12%

activated

atSw=2%

o1%

0.03%

/–/0.1%

0.03%/–/0.13%

Stepincrease

at

RH

wB90%

toGFB1.1

(Dm=

200nm)

GTS

180

BZeromicro-

andmesoporosity

Verylow

Hydrophobic

Norestructuring

GF=

1to

B90%

RH

w

Negligible(o

5%activated)upto

Sw=

1.7%

forDmo

350nm

o1%

o0.01%

/o0.01%

/�o0.01%/

o0.01%/�

(Sw=

1%

)TOS

260

Low

micro-and

mesoporosity

Interm

ediate

Hydrophilic

Norestructuring

GF=

1to

B90%

RH

w

45%

ofparticles

activated

atSw=

1%forDm4

250nm

35%

1%

/2%

/�2%

/25%

/�(S

w=

1%

)

aReported

forthemain

fractionofAEC

(fordetails,seeDem

irdjianet

al.44)RH

Koopdefined

hereastheRH

wforactivatedfraction=

1.

Fig. 1 Water uptake on GTS,TS, TOS,TC1, and AEC soots. The

area of existence of a water film on the surface is indicated by the

grayscale lines, with low and high boundaries for 20 and 250 nm

diameter primary particles, respectively. For AEC soot the right axis is

used.

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 | 7909

Gas chromatography-mass spectroscopy (GC-MS) was

used to analyze the organic surface composition of AEC and

TC1 soots. Two solvents of different polarity (polar and

non-polar) were chosen, namely methanol and hexane, to

effectively extract surface compounds. Since the proper eluent

for efficiently removing and separating the organic compounds

is always a concern, a chloroform extraction was also

conducted after the first extraction by methanol. A soot

sample of 20 mg was extracted in 100–150 ml of solvent by

ultrasonic agitation for 45 min; the extract was then filtered

and a 2 ml portion of each filtrate was analyzed by GC-MS.

Analyses were performed on an Agilent system consisting of a

Series 6890 Gas Chromatographer and a 5973 Mass Selective

Detector. The columns were 30 m � 0.25 mm id � 0.25 mmfilm thickness (HP 5MS, Hewlett Packard). The column

temperature was programmed from 70 to 280 1C at a rate of

20 1C min�1. The injection was performed at 280 1C, and the

temperature of mass spectrometer interface was 280 1C. The

identification of compounds was made using the mass spectra

library NIST98.

Results of the GC-MS analyses are reported in Table 2.

For the AEC sample, all of the organic compound classes

identified, except acids, are likely to be both hydrophobic and

inhibitors of water uptake. Less is known about the water

uptake properties of representative aromatic and n-alkanoic

acids. In addition to the species listed in Table 2, about

1.5 wt% of organic ions such as acetate (CH3COO�), formate

(HCOO�) and oxalate (C2O4�) were extracted from AEC soot.40

These species may originate from low weight hygroscopic

carboxylic acids such as acetic, formic and oxalic acids, which

are commonly observed in atmospheric particulate matter,

and they may increase AEC soot hygroscopicity as proposed

in Demirdjian et al.44 In contrast, only representatives of one

class of hydrophobic organics, namely of PAH (phenanthrene,

pyrene, fluoranthene, and anthracene), were found in the

TC1 soot organic coverage, consistent with the relatively

hydrophobic nature of TC1 soot.

2.2 Instrumentation

Soot was dispersed without exposure to water using a fluidized

bed generator.45 Part of the flow was passed to a humidified

tandem differential mobility analyzer (HTDMA) system46 as

described in our previous publication.36 The remaining flow

was sent to a Differential Mobility Analyzer (DMA; TSI

model 3071A) for size-selecting aerosol particles. We note that

for the DMA flow ratios applied in this work, a selected midpoint

mobility diameter Dm = 200 nm passes particles with mobility

diameters of approximately 180–220 nm in the output size-selected

sample, along with some small number concentrations of multiply-

charged particles. This quasi-monodisperse sample was processed

in parallel in a cloud condensation nucleus counter (CCNc) and a

continuous flow diffusion chamber (CFDC). In some CFDC

experiments, a polydisperse flow of particles was used instead of

the size-selected flow.

The cloud condensation nuclei counter (CCNc) used was

manufactured by Droplet Measurement Technologies (Model

CCN-2). It is a continuous flow instrument that uses a

stream-wise temperature gradient to generate supersaturation

and activate particles to droplets. Details on its operation may

be found in Roberts and Nenes.47 The supersaturation was

varied between 0.1 and 2% using the factory calibration of the

instrument. The activated fraction is reported as the ratio of

the concentration of droplets to the concentration of total

particles, measured using a condensation particle counter

(CPC, TSI model 3010). The critical supersaturation, Sc, is

defined to be the supersaturation at which 50% of particles of

a given diameter activate as cloud droplets.

The CFDC technique is described by Rogers48 and Rogers

et al.49 The specific CFDC used in this study has been

described most recently by Koehler et al.42 It consists of two

concentric copper cylinders with aB1.2 cm gap between them.

Each wall is cooled to �25 1C and then coated with ice. The

walls are cooled further and held at different temperatures

giving a linear temperature and water vapor profile between

the two plates. This induces a non-linear RHw gradient

between the plates. The aerosol flow is sent between two

particle-free sheath flows providing a laminar flow between

the ice walls. By increasing the temperature gradient between

the plates, the RHw to which particles are exposed may be

increased while the temperature in the central aerosol location

is held relatively constant. The residence time of the aerosols in

the chamber is B11 s for all experiments reported herein.

Table 2 Particle-phase organic compounds of aircraft engine combustor soot

Class of organics Representative compounds Formulae

Alkenes Squalene C30H50

Cycloalkenes Diphenylcyclopropene C15H12

Alkanes Pentadecane, CH3–(CH2)n–CH3

Octacosane, icosane n = 13–15, 17–20, 22–26Benzene derivatives: Benzene, (1-butylheptyl)- C5H5C(R)(R1)Alkylbenzenes Benzene, (1-propyloctyl)- for R, R1 = CnH2n+1, n = 1�10Polyaromatics Antracene, C14H10

Phenanthrene, C14H10

Pyrene C16H10

Carboxylic acids:Aromatic acid 1,2 benzenedicarboxylic acid C6H4(COOH)2n-alkanoic acid n-hexadecanoic acid C15H31COOHOxyalkanoic acid 2-Hydroxy-hexadecanoic acid-1 C14H29CH(OH)COOHPolyaromatic ketone 9H-fluoren-9-one C13H8OEsters Phthalic acid, ester C6H4(COOR)2

for R = CH2CH(C2H5)(CH2)3CH3

Phenol, dialkyl 2,4-bis(1,1-dimethylethyl)-phenol C6H3(OH)((CH3)2C2H5)2

7910 | Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 This journal is �c the Owner Societies 2009

Aerosol and cloud particles are detected and binned by size

with an optical particle counter (Climet Model 7350A) inter-

faced to a multichannel analyzer. Only particles greater than

1.3 mm diameter are counted as ice crystals, as this is a

diameter which is too large for even soluble particles with

diameters less than 500 nm to have grown by water uptake

below water saturation. The use of a DMA to size select

particles at a range of midpoint sizes up to 250 nm introduces

the possibility of producing multiply-charged particles of

larger sizes in the flow to the CFDC. The number of larger

particles depends on the input aerosol size distribution. The

presence of larger hygroscopic particles in the sample stream

could create interference with ice detection, especially if one

desires detection of low fractions of particles activating at low

temperatures.50 Contamination of the freezing signal from

multiply-charged particles is expected to be a concern only

for the AEC samples; we address this issue further in the

results section. In our experiments, the RHw was gradually

increased from well below water saturation to several percent

above saturation so the activation curves could be traced out.

The particular CFDC used in this study does not distinguish

droplet versus ice activation above water saturation, although

the temperature range of experiments (below �40 1C)

suggests that any activation noted is likely to be ice formation

(i.e., water activation would be an initial and transient

behavior).

The CFDC used is designed for very low temperature

(primarily r �40 1C) studies and distinguished from the

device of Rogers et al.49 by a longer chamber and the absence

of a lower section for evaporating activated liquid particles.

The system also employs a pre-cooling section in which

aerosol is cooled before introduction to the CFDC chamber.

This section consists of a 1/200 copper tube surrounding a 3/800

copper tube. The dry aerosol stream flows through the center

tube and cooled Freon flows in the annular region, cooling the

aerosol flow to B�30 1C, as measured at the entrance of

the chamber. This ensures excess water vapor is removed from

the flow and that unresolved transient supersaturations are not

established as the aerosol flow enters the chamber and humidity

rises monotonically to the final value at lower temperature.

During these experiments, a modest temperature gradient

existed along the outer cylindrical ice surface, which was

configured to be the colder of the two ice surfaces. This

temperature gradient was typically a 1.5 1C decrease from

the top (entry) to the bottom (exit) of the chamber. Temperature

and RHw at the position of the aerosol lamina and of relevance

to ice nucleation conditions were therefore determined on the

basis of the thermocouple measurements in the lower half of

the chamber. Verification of this method was performed

by comparing results from freezing onset experiments for

quasi-monodisperse ammonium sulfate to predictions from

the water activity based homogeneous freezing theory of Koop

et al.,51 with good agreement.50

In most experiments, soot aerosol was size-selected by the

DMA prior to being introduced to the CFDC. However, for

the AEC soot particles, the test sample was limited, so very

low number concentrations of size-selected particles after a

period of fluidized bed use made it necessary to bypass the

DMA and send the flow with the entire particle size distribution

to the CFDC in some experiments. As a consequence, any

large soot particles which are counted by the OPC above the

cutoff size will be falsely identified as ice crystals. This resulted

in a ‘‘false IN’’ signal representing about 5 in 1000 particles

at low RHw values. This false signal has been removed from

the data and only subsequent increases in IN fraction with

increasing RHw are shown and taken as indicative of true

formation of ice crystals on the polydisperse AEC soot

particles.

3. Results

3.1 CCN activity measurements

The CCN activation experiments were carried out using two

experimental methodologies, shown schematically in Fig. 2a,

where the y-axis is the supersaturation Sw in the CCN instrument

and the x-axis is the selected dry diameter. The solid black line,

which we will refer to as the ‘‘Kelvin line’’, indicates the critical

supersaturation required to activate a spherical, insoluble, and

wettable particle of the indicated dry diameter (as computed

from standard Kohler theory, and corresponding to k = 0 in

Petters and Kreidenweis31). For reference, we also show the

critical supersaturations required to activate dry, spherical

ammonium sulfate particles. We used two methodologies

to conduct our CCN activation experiments. In some

experiments, the supersaturation in the CCN counter was held

constant, and the size-selected dry mobility diameter sent to

the instrument was increased stepwise. This procedure is

shown schematically by the horizontal arrow in Fig. 2a. The

fraction of particles that were activated in the instrument at

each selected diameter was then recorded. If this procedure

were conducted at Sw = 1% on a size-selected sample of a

hypothetical spherical, insoluble and wettable particle type, we

would expect to see zero particles activated for dry diameters

smaller than B200 nm, and 100% of the input particles

activated for diameters Z 200 nm—that is, as the diameter

of the selected particle crossed the ‘‘Kelvin line’’. The alternate

experimental procedure was to fix the selected dry diameter,

and then increase the instrument supersaturation stepwise,

corresponding to the vertical line in Fig. 2a. For supersaturations

exceeding the Sw corresponding to the Kelvin line for the

selected dry diameter, we would expect to transition from no

activation to complete activation of the hypothetical particles

discussed above. In our experiments, we do not expect to

observe these sharp transitions, for several reasons. First, the

DMA selects particles according to dry mobility diameter, Dm,

which is the same as the physical diameter only for spherical

particles, likely a poor assumption for soot particles;52 in

general, we expect the mobility diameter to be larger than

the volume-equivalent physical diameter. Second, as mentioned

in section 2, the quasi-monodisperse stream contains particles

within about 20 nm of the selected midpoint mobility

diameters, as well as some fraction of multiply-charged, larger

particles. These sizing variations can lead to at least a few

tenths of a percent spread in the expected Sc. Finally, there are

likely particle-to-particle variations in properties among soot

particles even within a small size range and we refer to this as

chemical heterogeneity.

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 | 7911

Henson53 and Sorjamaa and Laaksonen54 describe

modifications to Kohler theory to predict expected activation

behavior for particles that take up water only via adsorption.

Henson53 assumes the applicability of the BET isotherm to

describe water uptake over the full range of RHw, whereas

Sorjamaa and Laaksonen54 employ the FHH isotherm; both

assume perfectly wettable surfaces, i.e., zero contact angle.

Henson’s calculations show that this assumed activation

mechanism can change the slope of the critical supersaturation-

dry diameter relationship compared with that expected for

soluble species, and significantly reduces the diameter of the

critical droplet radius, but that all activation behaviors

are bounded by the Kelvin line for wettable but insoluble

particles.53 However, the data of Popovicheva et al.37 show

that the BET isotherm does not well-represent the adsorption

behavior of the soots we consider here. Sorjamaa and Laaksonen54

show schematically that for hydrophobic surfaces,RHw4 100%

may be required for continuous condensation on the surface to

occur. Thus, some particles that take up water by adsorption

but are not perfectly wettable could require Sw above that

predicted by the Kelvin line.

An important point to be made in interpreting the CCNc

data is that the corresponding HTDMA measurements for

RHwo100% for these soot types, presented in Popovicheva

et al.,36 and summarized in Table 1, were obtained for the

same particle sizes that were selected for the CCNc and CFDC

studies described here. However, bulk samples including all

particle sizes were used in the measurement of adsorption

isotherms. The heterogeneity of particle characteristics as a

function of particle diameter resulted in some differences in the

water interactions observed for bulk and monodisperse

samples, as discussed in Popovicheva et al.36 Our HTDMA

observations showed a range of observed growth factors even

within the size-selected sample, as expressed by growth curves

broader than those observed for uniform-composition samples

and some small fractions of particles that did not grow or even

shrank (restructured) upon exposure to relative humidities at

which the majority of the particles exhibited growth.

Fig. 3 shows the CCN activity measurements for AEC soot

particles of Dm = 250 nm, and for TCI and TS soot particles

of Dm = 200 nm. The procedure for this series of experiments

involved fixing the selected mobility diameter and scanning

water supersaturations in the CCNc. Error bars show 1

standard deviation of the range of activated fractions at a

given supersaturation. For comparison, we also show

computed activation curves for 200 and 250 nm dry midpoint

mobility diameter insoluble and wettable (‘‘Kelvin’’) particles.

The calculations assume all particles are spherical, but take

into account the population of multiply-charged particles that

would be passed for the selected midpoint mobility diameter,

resulting in the characteristic ‘‘S-curve’’ that is typically

observed experimentally, rather than a sharp step function at

the Kelvin line. The point at which the activated fraction is

50% of the total dry particle number concentration is taken at

the critical supersaturation, Sc; here, for the 200 and 250 nm

particles, Sc = 1.06 and 0.84%, respectively. It is immediately

clear from Fig. 3 that Sc could not be determined in this

manner for any of the three soots. For TS and TC1 particles,

only very small (o5% on average) activated fractions were

observed up to about Sw = 2%, with perhaps a slightly larger

fraction of TS particles activating at the highest measured Sw.

The observations for TS are entirely consistent with

Fig. 2 (a) The relationship between dry diameter (x axis) and the critical supersaturation, Sc (y axis) required to activate that particle size to a

cloud droplet. The dashed line indicates the critical supersaturations computed for ammonium sulfate particles, and the solid line (Kelvin line)

indicates the relationship computed for spherical, insoluble and wettable particles. The arrows indicate experimental methodologies in which dry

diameter is fixed and supersaturation is varied (vertical arrow), and in which supersaturation is fixed and dry diameter is varied (horizontal arrow).

(b) Solid line: the nucleated fraction (y axis) of 200 nm (dry diameter) aqueous ammonium sulfate particles freezing homogeneously in the CFDC

at �51.5 1C as a function of RHw (x axis), computed on the basis of the parameterization of Koop et al.51 Hatched area: region of water

supersaturation.

7912 | Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 This journal is �c the Owner Societies 2009

expectations from the related properties of this particle type

(Table 1): TS soot is classified as hydrophobic, and exhibited

no growth in the HTDMA up to 90% RHw, as well as no

restructuring. Total water uptake measured for the bulk

sample (Fig. 1) did not exceed one monolayer at 80% RHw,

the RHw range before capillary condensation into inter-particle

cavities can occur. Most of the TS particles did not activate at

the Kelvin line, and we can attribute this observation to

uncertain particle sizing, as discussed above, and possibly to

the hydrophobic surface. TC1 soot, on the other hand, was

classified more hydrophilic, with an adsorption isotherm

(Fig. 1) indicating earlier and larger water uptake than all of

other non-hygroscopic soot particles. Further, the TC1 size-

selected sample exhibited measurable, ‘‘step-like’’ growth in

the HTDMA for RHw 4 80% (Table 1). Popovicheva et al.36

attributed these observations to irreversible adsorption of

water in micropores inside the particle. This adsorbed water

led to swelling of particles that was observed as a diameter

growth factor (GF) 4 1 in the HTDMA. Based on the

CCNc observations in Fig. 3, we may conclude that, if our

interpretation is correct, this adsorbed water appears to be

unavailable for further condensational growth and thus does

not contribute to CCN activity that is measurably enhanced

over that expected for a Kelvin particle.

A plausible explanation of the observed CCN activity of TS

and TC1 soot particles may be found by employing classical

nucleation theory55 which suggests that for a spherical

particle, the freely growing droplet from the critical germ is

characterized by a contact angle, y, of water on the surface.

From the CCN activity at 2% supersaturation, we can

estimate that the most active 5% of 200 nm TC1 and TS soot

particles have at least y B 61. Measurements of contact angles

for bulk samples of such soot surfaces have found significantly

larger y values: B821 for TS and B681 for TC1,38 indicating

the prominent heterogeneity of soot particles. The most

wettable particles (probably containing a significant number

density of active sites) are the only particles observed to

activate in our CCN measurements.

The data in Fig. 3 for AEC, the only soot studied here that was

classified as hygroscopic, show a shallow activation curve,

compared to that computed for Kelvin particles. We also note

that the presence of larger, multiply-charged particles in the

quasi-monodisperse sample stream is generally manifested as a

constant ‘‘background’’ of particles that activate before the main

S curve, and might contribute to small fractions of particles

activating at the lowest tested supersaturations. For the AEC

soot, it was expected that due to the relatively high mass fraction

of soluble compounds on the hydrophilic soot particle surface,

standard Kohler theory might apply and the activation of AEC

particles could be modeled assuming their structure to be an

insoluble core surrounded by a soluble shell. But this was not the

case for the majority of the AEC particles, which did not activate

below the Kelvin line, and thus must have contained minimal or

no hygroscopic coating. At Sw = 2%, the upper limit of the

CCNc, only B35% of the particles were observed to activate as

CCN, despite observed HTDMA GF 4 1 for most of the

particles at RHw 4 75% (Table 1). Popovicheva et al.36 note

that several characterization methods indicated a high degree of

heterogeneity in AEC soot, with at least two fractions of different

composition and structure identified. Popovicheva et al.37 suggest

that AEC water uptake be modeled by ‘‘dual sorption theory’’,

with both ad- and absorption contributing to observed water

contents. Since AEC particles were observed to have complex

agglomerate structures, it is likely that Dm 4 volumetric

equivalent diameter, and thus only a small fraction of those

particles associated with little soluble material were able to

activate as Kelvin particles, similar to the observations for TS

and TC1 soots.

Fig. 4 shows the CCN activity measurements for GTS and

TOS soot particles of 100 nm r Dm r 350 nm, for Sw = 1

and 1.67%, together with the computed activation curves for

Kelvin particles, as discussed for Fig. 3. The procedure for this

series of experiments involved fixing the selected supersatura-

tion in the CCNc and scanning input particle dry mobility

diameter; the diameter at which 50% of the particles are

activated is selected as the critical dry diameter for the chosen

supersaturation. This methodology for exploring CCN

activity introduces an additional complication into the data

interpretation when particle characteristics vary with particle

size, as is suspected for the soot samples used here.

Nevertheless, the data for GTS are consistent with expectations

for this hydrophobic soot (Table 1): o2% of particles are

observed to activate as CCN at Sw = 1% for diameters as

large as 350 nm. The Kelvin line predicts that all of the

particles with dry diameters 4150 nm should activate at

Sw = 1.67%, whereas the data show that only a very small

percentage of GTS particles in this size range appear to be able

to serve as CCN at this Sw. In fact, the largest particle sizes

studied seem to have some of the lowest active fractions,

perhaps because the primary particle size is B180 nm and

the larger Dm correspond to irregular particles with much

smaller equivalent volume diameters. As with the other

hydrophobic soot, TS, limited wettability of the surface may

lead to lack of agreement with the Kelvin line predictions.

Fig. 3 Measurements of CCN activated fraction of TC1 and TS soot

particles having dry mobility diameters of 200 nm and AEC soot

particles with dry mobility diameters of 250 nm, as a function of

supersaturation, Sw. The predicted activation curves for wettable

particles selected at the electrical mobility sizes indicated are shown

for comparison.

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 | 7913

The TOS sample exhibited CCN activation behavior more

complex than that for GTS particles. For the experimental

conditions of Sw = 1%, all Kelvin particles larger than about

250 nm should be activated. The data show asymptotic

behavior as diameter is increased, with about 45% of the

TOS particles with Dm 4 250 nm activated, indicating the

presence of at least 2 particle types at these larger dry mobility

diameters. A small fraction of particles withDm = 100 nm was

activated, with the fraction steadily increasing as Dm was

increased. This behavior is qualitatively consistent with what

might be expected for activation of an insoluble particle as the

Kelvin line is ‘‘crossed’’, given the broad range of volume

equivalent diameters represented in our quasimonodisperse

sample. The results for TOS particles tested at Sw = 1.67%

are qualitatively consistent with the observations at Sw = 1%,

except that the data asymptote to a higher activated fraction

(B70%), reflecting the larger number concentrations of

smaller particles available for activation at the higher Sw.

We conclude from these experiments that increased surface

polarity introduced by the oxidation of TS soot particles may

enhance their wettability, leading to better agreement between

observations and the Sc-dry diameter relationship given by the

Kelvin line.

3.2 Ice nucleation

For the temperatures considered in this study, �40 1C and

below, it is useful to conceptually break down the nucleated

fraction-RHw parameter space (see Fig. 2b) into three main

regions, with different expectations for the mechanisms of ice

formation in each. We discuss ice formation mechanisms in

general terms rather than by microscopic definitions, since

such details are not readily discerned from an ice nucleation

signal in the CFDC. The first division occurs at RHw = 100%.

At RHw 4100%, particles will approach the conditions for

liquid cloud activation at an RHw that depends on their

specific hygroscopic/hydrophilic/hydrophobic properties,

although this phase transition may be complicated at the

microscopic scale by the low temperature phase state (more

liquid- versus more ice-like) of water molecules. Freezing of

adsorbed water or in solutions formed by absorbed water

(from trace amounts of solutes) on the particle surface may

occur before the point of predicted cloud droplet activation,

and will be a function of particle size because of the Kelvin

effect. However, homogeneous freezing should assuredly ensue

at the RHw corresponding to the region of condensation

without bound (cloud droplet activation RHw at warmer

temperatures). For RHw o100%, two further sub-regimes

can be distinguished, separated by the RHw point at which

homogeneous freezing of an ice germ from an aqueous

solution droplet occurs. This RHw for a pure hygroscopic

particle represents a lower bound for homogeneous freezing in

the case of presence of soluble material on the surface of a soot

particle of the same dry size. The RHw for this point depends

on temperature, hygroscopicity/water soluble fraction, and on

the size of the particle, as well as on the time available for ice

nucleation (i.e. residence time in the CFDC).50 We follow

DeMott et al.50 in calculating the fraction frozen as a function

of T and RHw for the CFDC residence time based on applying

the Koop et al.51 parameterization of homogeneous freezing as

a function of water activity.51 We refer to the resulting

activation curve as the ‘‘Koop line’’ and the RHw along this

line as RHKoop (Fig. 2b). At RHw o RHKoop, particles may

exist in a number of physical states depending on their

characteristics: for example, they may persist as dry, unwetted

particles; they may adsorb some water; and they may dissolve

and form an aqueous solution on their surface, too

concentrated (water activities too low) to support significant

homogeneous freezing rates. In any case, if freezing occurs for

RHw o RHKoop, we can attribute it to a heterogeneous

freezing mechanism—that is, the presence of a surface with

special activation properties is required for the formation of

ice at these conditions. The microscopic mechanism of hetero-

geneous ice formation in this case must involve either freezing

at some of the same sites active for water adsorption or

heterogeneous freezing of aqueous solutions. Finally, in the

regime RHKoop o RHw o 100%, more than one mechanism

may be active. It is generally not possible to distinguish

between homogeneous freezing in solutions versus the two

heterogeneous processes, unless the ice nucleating fraction

far exceeds the fraction of particles that are known to be

hygroscopic.

A few notes are in order with respect to observing these

three regimes in our experimental system. First, near 100%

RHw, the CCNc and CFDC have quite different resolutions.

In the CCNc, we can distinguish the transition from unactivated

to activated CCN with RHw resolution on the order of 0.1%

for the sizes of particles used in this study. In the CFDC,

the resolution achievable in RHw at the low temperatures

employed in this study, �3% RHw,56 is much lower. However,

as will be demonstrated in this section, our data suggest that

we can, under some conditions, distinguish this level of RHw

difference for ice formation between hygroscopic and non-

hygroscopic particles in the CFDC above 100% RHw. On the

other hand, the CFDC technique is very sensitive to detecting

Fig. 4 Measurements of CCN activated fraction of GTS and TOS

soot particles as a function of the selected dry mobility diameter at

supersaturation, Sw, of 1% (black symbols ) and 1.67% (grey symbols).

The Kelvin activation curves predicted at these two supersaturations

for a quasi-monodisperse particle sample (see text) are shown for

comparison.

7914 | Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 This journal is �c the Owner Societies 2009

small number concentrations of certain particle types. For

example, the sensitivity to detecting ice formation in the

CFDC may, under the right particle loading conditions,

extend down to below 1 in 104 particles freezing, whereas the

other analytical and experimental techniques applied in this

work are unable to resolve the small proportion of any of the

particle samples that may contain water soluble species.

Each soot sample was studied in the CFDC with multiple

sizes when possible. The difficulty in producing sufficient

concentrations of soot particles sometimes made this

impossible. The small mass of the sample used to generate

AEC particles, for example, permitted size selection with

sufficient number concentrations only for one experiment.

The whole particle distribution was sent to the CFDC in a

second experiment. Due to this additional complication,

and the fact that some of the particles contain hygroscopic

material, interpretation of the AEC measurements is more

challenging than for the other soot types. For TS and TC1,

quasi-monodisperse 100 and 200 nm dry mobility diameter

aerosols were sent to the CFDC. The larger primary particle

size of GTS soot made it impossible to size select 100 nm

particles; therefore, only results for ice nucleation on 200 nm

particles were obtained. Similarly, freezing results were

obtained only for 200 nm TOS soot.

The fraction of soot particles initiating ice formation is

shown in Fig. 5 for all types and sizes of particles examined.

Fig. 5a–b shows the activation for GTS, 5c–5d for TS, 5e–5f

for TC1, 5g–5h for TOS, and 5i–5j for AEC. This ordering

corresponds approximately to increasing hydrophilicity of

samples, from top to bottom. The left panels are for data

collected at �40 1C and the right panels are for data collected

at �57 1C, with the exception of the GTS and TOS data which

were collected at �51.5 1C. The ‘‘Koop lines’’ in each figure

indicate the conditions for which homogeneous freezing of

200 nm ammonium sulfate particles is predicted for the CFDC

residence times; these calculations have been confirmed in our

laboratory from separate CFDC experiments on pure solute

particles.50,56 The frozen fractions at RHKoop (for a predicted

activated fraction equal to 1) and at RHw = 100% are also

tabulated in Table 1. The data in Fig. 5 and Table 1 show

different behaviors of ice nucleation depending on temperature

and soot particle type, and reflect a variety of particle

interactions with water at low temperatures.

The hydrophobic GTS soot particles were observed to

initiate the ice phase only at water supersaturated conditions,

RHw 4 102%, at both �40 and �51.5 1C. No ice nucleation

was observed below water saturation, in association with

negligible water adsorption. Ice nucleation did not occur

until the particles were exposed to water supersaturations

approaching those (presumably) high enough to condense

water onto the particles. Since we could not measure CCN

activation above 102% RHw, we could not confirm the direct

association of the ice nucleation point with the condensation

point. Thus, it is not possible to distinguish whether ice

nucleation proceeded due to the nucleating properties of the

soot substrate, or simply via the homogeneous formation of ice

in condensed water. Ice formation was observed only onB1%

of the particles at up to several percent water supersaturation.

Although onset of detectable ice formation was observed at a

slightly lower RHw at �51 1C than at �40 1C, similar

maximum activated fractions were observed at both

temperatures.

As for GTS particles, ice formed on hydrophobic TS and

hydrophilic TC1 soot particles at �40 1C only in the water

supersaturated regime. A strong increase in the nucleated

fraction, up to 10% of the particles frozen, occurred only

above B102% RHw. This stronger ice activation RHw is

qualitatively consistent with the observed modest increase in

CCN activation of these particles only at this upper limit of

CCN supersaturation measurements. This indicates that these

particles freeze at �40 1C primarily above the supersaturation

at which they would act as cloud droplet nuclei in warm

clouds, a process that requires Sw above those typically found

in cold atmospheric clouds. At �57 1C, ice nucleation occurs

on a small fraction (o1 in 1000) of TS and TC1 starting at

RHw o RHKoop. Since TS and TC1 contain only small

amounts of soluble compounds, we did not expect a large

increase in activated fraction at the Koop line, and this is not

observed. Heterogeneous freezing appears as the most likely

explanation of ice formation atRHw well below water saturation.

Small amounts of soluble matter on some particles could still

explain some ice formation observed for RHw 4 RHKoop.

Nevertheless, the monotonic increase in activated fraction

with increasing RHw suggests that heterogeneous ice

nucleation is at play through all regimes. Ultimately, similar

maximum fractions of the particles freeze at �57 1C in the

water supersaturated regime as found at �40 1C. Possible size

effects on ice nucleation are indicated in the ice nucleation

results for these particle types. For TS and TC1 soots, larger

proportions of the 200 nm than the 100 nm particles typically

activated for similar T and RHw conditions.

The ice nucleation behavior observed for 200 nm TOS

particles at �40 1C displayed onset conditions close to

RHKoop. Interestingly, however, the HTDMA data for TOS

particles indicated Bzero water growth (GF = 1 up the

highest RHw), and the CCN data also did not clearly indicate

that TOS particles were more CCN active than expected for an

insoluble, wettable (Kelvin) surface. However, as mentioned

above, the CFDC technique is much more sensitive than either

the HTDMA or the CCNc in detecting small number fractions

of particles with special properties. Thus, the presence of less

than a few % of particles containing soluble matter that could

promote homogeneous freezing below water saturation cannot

be ascertained in those systems. We must also acknowledge

that at �40 1C, the RHw regime between the Koop line and

conditions for CCN activation spans only 2% RHw, within

our experimental RHw uncertainties. By 101% RHw, the

frozen fraction exceeds 20%, similar in proportion to the

35% active as CCN at this RHw at room temperature. Greater

than 70% of the particles were activated at high RHw, similar

to the higher CCN active fractions at B102% RHw at warm

temperatures. The higher freezing fractions of TOS may in

part reflect a greater homogeneity of particle surface properties

amongst the TOS particles as compared to AEC soot, and that

water was able to condense on most of the TOS particles at

high supersaturations. At �51.5 1C, 10�4 of the TOS particles

activated by RHw = 85%. More efficient ice nucleation

occurring in this lower-RH regime, up to BRHKoop, must

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 | 7915

proceed via a heterogeneous mechanism, as was also observed

for TS and TC1 particles, albeit to a lesser extent. In contrast

to the behaviors of TS and TC1 soot particle types, however,

ice formation on TOS particles displayed a steep increase in

the region RHKoop o RHw o100%, leading to activated

fractions of TOS exceeding 10% by RHw = 100%.

About 4% of the particles of hygroscopic AEC soot froze

at �40 1C near the predicted point for homogeneous freezing

of pure dissolved solute. Furthermore, more than 10% of the

particles froze for RHw = 102%, of the same order as the 30%

of monodisperse AEC activated as CCN at room temperature

and this RHw. From these observations, we infer that the

freezing of AEC particles is associated with the fraction

containing hygroscopic material. Similarly, at �57 1C, both

250 nm size-selected particles and the polydisperse particle

distribution showed significant increases in ice nucleated

fraction near the homogeneous freezing nucleation threshold.

We note here the loss of some of the data points for the

polydisperse particle sample that may have indicated more

clearly the onset of nucleation, because of the requirement to

remove points below a threshold value of several tenths of a

percent of the total number due to the background contamination

attributable to large particles, as discussed above. Activated

fractions at RHw o90% may represent a small fraction which

can freeze heterogeneously, but the limited resolution allows

us to state with confidence only that heterogeneous freezing

nuclei were not detectable above the level of 1 in 10 000 AEC

particles at below 90% RHw at �57 1C. In comparison to the

non-hygroscopic soot particles, the AEC particles nucleated

ice several to ten times more efficiently at �40 and �57 1C for

RHw 4 RHKoop, with B4% of particles frozen at water

saturation and 10% frozen at 102% RHw in both cases. These

observations appear at least consistent with the dissolution of

water in the soluble coverage of a large fraction of these

particles, leading to homogeneous freezing. Nevertheless, we

reiterate the inability to distinguish homogeneous and hetero-

geneous freezing mechanisms in the regime RHw 4 RHKoop.

4. Discussion of results and their implications for

cirrus and contrail formation by soot particles

Our experimental results suggest that the ability of soot

particles of various physico-chemical properties to act as cloud

condensation and freezing nuclei in the atmosphere is related

to their mechanisms of interaction with water vapor.

Specifically, processes that promote water uptake can facilitate

both CCN activation and ice nucleation. We found soot

particles to activate as CCN in measurable proportions at

below the Kelvin limit only if they were hygroscopic or

oxidized and hydrophilic, whereas most non-hygroscopic

particles required Sw at or above the Kelvin line, depending

on their hydrophilicity. Although the CCN experiments were

carried out only at a warm temperature (B30 1C), the

qualitative behavior with respect to uptake of water was

related to ice nucleation behavior at colder temperatures.

Specifically, for the only hygroscopic soot we examined in this

Fig. 5 Ice nucleation of soot particles at colder temperatures (T = �51.5 1C for GTS and TOS; T= �57 1C for TS, TC1 and AES, right panels)

and warmer temperatures (T=�40 1C, left panels) for GTS (a–b), TS (c–d), TC1 (e–f), TOS (g–h), and AEC (i–j). Grey symbols represent data for

dry Dm = 100 nm particles and black symbols represent data for dry Dm = 200 nm particles, unless otherwise indicated in the figure. Different

symbol types indicate different runs of the experiment. In each figure, the RHw resulting in the indicated fraction nucleated via homogeneous

freezing of dry Dm = 200 nm ammonium sulfate particles, computed for CFDC residence times at the indicated temperature using the

formulations of Koop et al.51 are shown for comparison.

7916 | Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 This journal is �c the Owner Societies 2009

study, AEC, we observed freezing only at temperatures and

RHw close to the homogeneous freezing conditions for aqueous

solutions. This tendency toward homogeneous freezing

conditions at low temperatures for hygroscopic soot is consistent

with at least one previous study21 and has been found recently

for hygroscopic carbonaceous particles produced from biomass

burning.50 In contrast, the non-hygroscopic soots exhibited a

wider range of ice nucleation behaviors, some clearly linked

with heterogeneous ice nucleation mechanisms. We conclude

that hygroscopicity does not necessarily favor heterogeneous

ice nucleation at cirrus temperatures. Rather, there is an

apparent range of hydrophilicity that is most favorable for

heterogeneous ice nucleation to occur under conditions that

will also potentially impact cirrus cloud formation due to

offering an advantage (lower RHw or warmer temperature)

versus the homogeneous freezing process.

Heterogeneous nucleation is typically promoted by so-called

active sites located on the surface of particles. These sites can

either be mechanical deformations of the surface (pores or

cracks in the crystal structure) or chemically modified surfaces

containing enhanced surface concentration of chemical groups

that can form hydrogen bonds with water molecules.57

Although some active sites must be present to promote ice

nucleation, too many active sites result in a polar surface

which may prevent ice nucleation because it orients ice dipoles

parallel to one another, and therefore it raises the free energy

of formation of embryos grown upon it.58 Our results for GTS

(no active sites, no heterogeneous IN activity), TS (low density

of hydrophilic nucleation sites, no heterogeneous IN activity)

and TOS (intermediate polarity and with a significant density

of hydrophilic nucleation sites, IN active) are consistent with

this conceptual picture.

However, microporosity of the soot particles complicates

the prediction of heterogeneous nucleation based on hydro-

philicity. TC1 is classified as a hydrophilic soot but much of its

water uptake is not available for influencing ice activation

because significant amounts of water is located within micro-

pores. We note that our set of soots, while covering a range of

surface types, does not fully explore the possible effects of

microstructure on CCN activity or on ice nucleation.

For example, one graphite spark-generated soot has indicated

strong ice nucleation activity in the cirrus temperature

regime,21 despite classification as a hydrophobic soot.37

Another complication is that surface active sites may be

bonded with the carbon matrix or originate from compounds

in the surface coverage. If the soot surface is covered by water

soluble inorganic and organic material, freezing at active sites

may also occur from the aqueous solutions. Freezing point

depression may impact the efficacy of active sites and mechanical

sites may be vulnerable to dissolution. These factors perhaps

play a role in the alignment of hygroscopic AEC soot with

conditions for homogeneous freezing of the soluble coverage.

We now discuss more specific implications that can be

drawn from the results with regard to cirrus and contrail

formation on soot particles. We will address only the situation

of cirrus forming in situ, not those driven by deep convection.

As for any aerosol particle type, the implications of our results

for impacts on cirrus depend sensitively on not only soot ice

nucleation properties, but soot number concentrations, the

number concentrations and ice nucleation properties of other

aerosols, the interplay of heterogeneous and homogeneous

ice nucleation mechanisms, and the dynamics of cirrus

formation).59 In the background atmosphere, many particles

are mixed, with some soluble components,60 whereas in

contrast the fractions of particles possessing specific

heterogeneous ice nucleation activity is relatively low,61 except

perhaps in relatively undiluted source plumes. For example,

elevated heterogeneous ice nuclei concentrations have been

measured in atmospheric mineral dust plumes.62,63 One can

imagine that a similar enhancement could occur in aircraft

exhaust, pollution and biomass burning plumes. Nevertheless,

numerical modeling accounting for the measured nucleation

properties and the atmospheric situation is required to fully

explore specific scenarios. Here we consider only the effects of

the possible physico-chemical nature of the soot, based on our

experimental observations, and order of magnitude number

concentration estimates of soot in the atmosphere.

We first consider the two scenarios of soot hydrophilicity.

We can assume that hydrophobic soot particles without active

sites, as represented by GTS in this study, should not alter

cirrus cloud formation due to requiring high water super-

saturations for ice nucleation at temperatures below �40 1C.

For soot particles that have developed a surface coverage of

significant amounts of water soluble material (e.g., as was the

case for some fraction of AEC soot particles), ice nucleation

may be limited to occurring only at relative humidities near to

or exceeding values required for homogeneous freezing of

similar sizes of soluble particles in the upper troposphere.

Their freezing is then indistinguishable from that of other

particles containing water soluble species and they must

compete with these other particles. Numerical modeling

studies of homogeneous freezing have shown that cirrus ice

crystal concentrations and sizes are much more sensitive to

cirrus dynamics (cloud updraft) than they are to changes in the

aerosol population that feeds homogeneous freezing.64,65

Therefore, only special circumstances could allow hygroscopic

soot particles to alter cirrus properties by acting as centers for

homogeneous freezing. This could occur in situations where

soot particles dominate aerosol numbers overall or especially

at larger particle sizes. In this case, they would be the first ones

to freeze in the homogeneous freezing regime, but cloud

microphysical and radiative properties might not be altered.

A greater impact on cirrus formation and properties is

expected if soot particles act as heterogeneous ice nuclei at

lower RHw than required for homogeneous freezing.12,65 In

this case, the number concentration of ice nuclei required to

dominate ice formation in competition with homogeneous

freezing depends on the dynamics (updraft) driving cirrus

formation.66 For example, if mesoscale motions exceeding

25 cm s�1 are driving cirrus cloud formation, then ice nuclei

concentrations of 0.03 to 0.3 cm�3 ice nuclei are required in

order to prevent the onset of homogeneous freezing process in

the background aerosols;66 whereas typical ice nuclei number

concentrations may be only on the order of 0.01 cm�3.61 In

our study, hydrophobic TS, and hydrophilic TC1 and TOS

soot particles are examples of soots that were observed to

serve as heterogeneous IN. Below �40 1C and below RHKoop,

ice formation occurred within 1 in 10 000 to nearly 1 in 100

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(for TOS) soot particles. Considering previous parcel

modeling studies,67,68 background upper tropospheric soot

concentrations of 0.1 to 1 cm�3,69 would be predicted to have

no impact to very modest impacts on natural cirrus formation

if 10�4 to 10�2 fractions of the particles serve as freezing nuclei

at RHw o RHKoop. These impacts would be restricted to cloud

formation in the synoptic scale regime of vertical motions

(under several cm s�1), and are not expected for the stronger

mesoscale motions present in cirrus generating cells. However,

depending on the type of soot present or its transformation

through oxidative processing, the impact of heterogeneous

freezing nuclei on cirrus formation could be quite significant

for higher soot concentration scenarios, such as those that

occur when cirrus form in air masses affected by aged aircraft

exhaust plumes or biomass smoke plumes. Studies of aged

aircraft exhaust have not been performed, but we may note that

some biomass smoke plume particles have been found to be active

as ice nuclei in water saturated cloud conditions at�30 1C,70 witha range of activated fractions from 10�5 to 10�2.

Considering briefly the case of contrail formation conditions,

Karcher et al.17 suggest that ice nucleation happens at water

saturation conditions, and contrail visibility criteria14 assume

a few percent of soot particles acting as the source particles for

ice nucleation. We can compare the ice nucleation observations

for AEC soot particles to this understanding, recognizing that

our results are from a single aircraft soot type and that many

other types may be produced by other engines burning other

fuels. Nevertheless, in general agreement with atmospheric

observations, it was found that up to 3% of AEC soot

particles were capable of initiating ice formation at water

saturation at �40 1C. Furthermore, our experiments are

consistent with observations of up to 33% of all soot particles

being involved in contrail formation71 when water super-

saturation conditions are achieved in condensing plumes,

which is a typical situation. More quantitative implications

for contrail formation could be examined by incorporating

results from ice nucleation studies into numerical models of

contrail ice formation on aerosols.

5. Conclusions

CCN activity and ice nucleation behavior in relation to water

uptake were investigated for soots whose properties ranged

from hydrophobic to hygroscopic. Extremely hydrophobic

soot is only expected to be able to act as CCN under highly

supersaturated conditions, possibly restricted by a large

contact angle for condensation over the surface; critical super-

saturations were not achieved for most of the tested particle

types and sizes up to 2% Sw. Water adsorbability of hydro-

phobic and porous hydrophilic soots were also insufficient for

Kelvin activation, with only r5% activating at 102% RHw.

While an expected result for particles of low polarity and low

porosity, the results for particles of intermediate polarity and

high porosity are consistent with evidence that some of their

water uptake is due to filling the microporous structure.

A larger fraction of the hygroscopic particles (B35%) were

able to act as a CCN up to 102% RHw, but only smaller

fractions of the particles activated as efficiently as would be

expected based on previously reported subsaturated hygroscopic

growth measurements or consideration of the water soluble

fraction in bulk particle samples.36 Shape factor, chemical

heterogeneity among individual particles and the nature of

soluble species may all play a role in the relatively poor CCN

activity of the major fraction of hygroscopic particles studied

here despite their apparent hygroscopic growth below saturated

conditions.

Hydrophilic non-porous soot stands out as the only soot

that activated as a CCN at conditions very near the Kelvin

supersaturation limit. We attributed this observation to the

hydrophilic surface created by oxidation.

The water interactions reflected in the CCN behavior of this

set of soots had clear influences on ice nucleation behaviors at

low temperatures. Extremely hydrophobic soot does not act as

an ice nucleus prior to the point where water adsorption or

even CCN activation occurs at high water supersaturations;

supersaturations that may occur in some contrail situations,

but never in cirrus clouds. The presence of a few to moderate

numbers of polar sites on the soot particle surface and the

related water adsorption promote heterogeneous ice nucleating

ability at RHw below 100% water saturation, but only at

temperatures somewhat below �40 1C, documented thus far

only at �57 1C. At this temperature, 10�3 to 10�4 fractions of

these particles were capable of activating heterogeneous ice

nucleation prior to RHKoop, defined herein as the conditions

for homogeneous freezing of soluble atmospheric particles. An

apparent continuum of increasing heterogeneous ice nucleated

fraction extended to higher RHw values, with several percent

of these types of soot particles activating ice formation by

109% RHw.

A more polar and nonporous soot surface favoring higher

water adsorption was associated with the highest hetero-

geneous ice nucleating fraction found in this study, (up to 1%)

in the regime RHw o RHKoop at �51.5 1C. This ice nucleating

behavior appeared to continue to higher RHw, with most of

these particles freezing (heterogeneously or via homogeneous

freezing of water volumes) in the regime of RHw 4 100% at

all temperatures below �40 1C. Therefore, we conclude that

hydrophilic nucleation sites on a surface of intermediate

polarity are advantageous for nucleation of ice germs while

water molecules on a surface totally covered by polar sites are

likely to bound into the layers and lose their freedom for the

orientation needed for ice-like structure formation.39 The

freezing mechanism in this most favorable case is best

described as adsorption freezing. It is interesting to note that

similar conclusions were obtained for the surface properties of

the most efficient soot ice nuclei acting in the mode of

immersion freezing of water drops.72

Nevertheless, the hydrophilicity requirement for soot ice

nucleation does not necessarily extend to soots with large

soluble fraction on the basis of our results. Soluble coverage

changes the mechanism of water uptake from water cluster

growth on the active sites to formation of aqueous solutions in

the soluble surface material. The hygroscopic soot particles

examined here did not activate detectable ice formation in

advance of the homogeneous freezing of soluble aerosol

particles of similar sizes at any temperature examined, for

the relatively few experiments performed and within our

uncertainty in defining RHw. Ice formation by hygroscopic

7918 | Phys. Chem. Chem. Phys., 2009, 11, 7906–7920 This journal is �c the Owner Societies 2009

soot particles appeared quite similar to the response expected

for homogeneous freezing of the fraction of particles containing

soluble matter. Any role of the non-hygroscopic fraction of

particles could therefore not be discerned. The consequence is

that threshold contrail formation on the aerosols with properties

similar to AEC soot is predicted to occur in accord with the

water activity limit for homogeneous freezing. Ice formation in

contrails on all of the different soot types, even on extremely

hydrophobic, would be predicted by our results, depending on

the exact value of water supersaturation and temperature in

the cooling plume. Assuming that aircraft engines may generate

particles with a spectrum of hygroscopic properties we may

conclude that the most hydrophobic soot surface will activate

ice formation later, at water supersaturations, while the

particles with soluble coverage will form ice first, near water

saturation.

The implications of these types of soot for ice nucleation in

cirrus conditions depend on the comparative nature of

atmospheric soot to any of the types examined, the number

concentrations of soot, and cirrus dynamics, which controls

the competition between heterogeneous and homogeneous

freezing processes. The results suggest that the impact of soot

aging on altering the hydrophilicity of particles may be closely

related to heterogeneous ice nucleation efficiency. In the

relative humidity regime that is most critical for impacting

cirrus, RHw o RHKoop (where background hygroscopic

particles will freeze) as noted above, only modest potential

impacts on cirrus cloud properties are predicted based on the

simple consideration of soot concentrations in the background

upper troposphere. Nevertheless, much stronger impacts may

occur in regions where higher soot particle concentrations

exist, such as in aircraft corridors and forest fire plumes.

Numerical modeling studies will be required to explore the

full implications for cirrus clouds.

To what extent the results presented herein transfer to other

sizes and types of soot particles remains to be seen. It is clear

from this study that any change in combustion conditions or

other factors that alter the water uptake properties of the

particles will also alter their ice nucleation properties. Based

on our results, hydrophobic soot with low oxygen content may

not impact ice nucleation in atmosphere, while highly hygro-

scopic soot will mimic the ice nucleation behaviors of any

other hygroscopic particles in the upper troposphere. Surface

polarity of different soots from various combustion sources

should be analyzed with regard to the most effective extent of

hydrophilicity supporting the mechanism of ice germ growth

on hydrophilic nucleation sites. In particular, it will be useful

to further investigate the role of varying degrees of oxidation,

the impact of the addition or increase of water soluble

fraction, and the impact of previous cloud processing on

subsequent ice nucleation behavior.

Acknowledgements

This research was supported by the National Aeronautics and

Space Administration under Grant #NNG06GF00G, the U.S.

Department of Energy’s Office of Science (BER) through the

Western Regional Center of the National Institute for

Climatic Change Research (Contract #MPC35TA-A1) and

the National Science Foundation via Grant ATM-0521643.

This research was also partly funded by EC Project

QUANTIFY 003893, the grant of President of Russian

Federation SS- 133.2008.2 and Project ISTC 3097. We

specially thank Dr Margaret Tolbert for the use of her

fluidized bed generator for these studies.

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