Aerosol dynamics upon Terni basin (Central Italy): results of integrated vertical profile...

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Rendiconti LinceiSCIENZE FISICHE E NATURALI ISSN 2037-4631 Rend. Fis. Acc. LinceiDOI 10.1007/s12210-013-0230-8

Aerosol dynamics upon Terni basin(Central Italy): results of integratedvertical profile measurements and electronmicroscopy analyses

Beatrice Moroni, Luca Ferrero, StefanoCrocchianti, Maria Grazia Perrone,Giorgia Sangiorgi, Ezio Bolzacchini &David Cappelletti

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Aerosol dynamics upon Terni basin (Central Italy): resultsof integrated vertical profile measurements and electronmicroscopy analyses

Beatrice Moroni • Luca Ferrero • Stefano Crocchianti •

Maria Grazia Perrone • Giorgia Sangiorgi •

Ezio Bolzacchini • David Cappelletti

Received: 20 February 2013 / Accepted: 11 March 2013

� Accademia Nazionale dei Lincei 2013

Abstract In this work, aerosol size distribution mea-

surements along with individual particle analyses were

performed along the vertical profile in the atmosphere, to

shed some light on the dynamics of evolution of aerosol

properties upon a basin valley. The case study is the Terni

basin, one of the most polluted urban and industrial sites in

central Italy. Aerosol vertical profile measurements were

performed using a helium-filled tethered balloon equipped

with an optical particle counter (OPC), a miniaturized

cascade impactor with particle collection filter, and a por-

table meteorological station. Combined OPC number size

measurements and single particle analyses by scanning

electron microscopy were employed to reconstruct the

pattern and evolution of aerosol properties over the basin.

Moreover, the CHIMERE chemistry-transport model was

applied over a selected computing domain to obtain a

general overview of the driving forces of the aerosol

dynamics. Scanning electron microscopy methods along

with chemical transport modeling revealed distinct distri-

butions of number, size and geochemical properties of

different particles classes in the aerosols. These reflect

distinct behaviours and spatial/temporal evolution of the

constituent particles, along with the common occurrence of

dust inputs from regional to long range sources (e.g.,

Saharan dust inputs). All these features have to be taken

into consideration when approaching the modeling of

atmospheric processes, particularly in basin valleys located

in Central and Southern Italy where the influence of Sah-

aran dust outbreaks is more pronounced.

Keywords Size distribution � Particle morphochemistry �Optical particle counter � Scanning electron microscopy �CHIMERE chemistry-transport model

1 Introduction

Basin valleys are elective sites for human settlement and

industrial activities. Here, however, the peculiar environ-

mental conditions due to topography and meteo-climatic

variables along with the environmental impact of human

activities may affect atmospheric aerosol and its properties.

Therefore, studies on the origin, concentration, composi-

tion, distribution and dispersion of atmospheric pollutants

upon these sites (see for instance Seibert et al. 2000; Henne

et al. 2004) are necessary tools for environmental sus-

tainability and public health policies (ICSU 2011).

A variety of basin valleys of different size, location and

level of anthropogenic pressure are present in Italy. The Po

valley, in Northern Italy, is the largest and best known

among them. Owing to the significant pollutants emission

Electronic supplementary material The online version of thisarticle (doi:10.1007/s12210-013-0230-8) contains supplementarymaterial, which is available to authorized users.

B. Moroni (&)

Dipartimento di Ingegneria Civile ed Ambientale,

Universita di Perugia, Via G. Duranti 93, 06125 Perugia, Italy

e-mail: [email protected]

B. Moroni � D. Cappelletti

Centro di Eccellenza SMAArt, Universita di Perugia,

Via Elce di Sotto 8, 06123 Perugia, Italy

L. Ferrero � M. G. Perrone � G. Sangiorgi � E. Bolzacchini

Dipartimento di Scienze Ambientali, Centro Ricerca POLARIS,

Universita di Milano-Bicocca, Piazza della Scienza 1,

20126 Milan, Italy

S. Crocchianti � D. Cappelletti

Dipartimento di Chimica, Biologia e Biotecnologie,

Universita di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy

123

Rend. Fis. Acc. Lincei

DOI 10.1007/s12210-013-0230-8

Author's personal copy

http://dx.doi.org/10.1007/s12210-013-0230-8

rates and the high density of population, it has become in

the last years a hotspot in environmental research (see for

instance, Perrone et al. 2012). In Central Italy, the Tiber

valley and satellite basins, though very interesting both for

model complexity and also for the remarkable similarities

with continental basins in Eastern and Southern Europe

(Rodriguez et al. 2007), have received so far much less

attention.

The Terni basin is a small satellite basin of the Tiber

valley, Central Italy. The name is from the town of Terni

which is the main settlement in the plane. Terni is the

second chief town in Umbria region (approx. 110,000

inhabitants), and one of the largest stainless steel produc-

tion sites in Europe. Product range includes ferritic,

austenitic and special stainless steels (Cr–Mn–Ni grades).

This town has a long tradition of iron and steel activity

since the nineteenth century. A substantial overlap of urban

and industrial settlements occurs in the Terni area and this

fact along with local topography and meteorology issues

make particulate matter (PM) a main pollutant affecting the

air quality in both the town and the surrounding valley. The

aerosol chemistry at ground level in the Terni area has been

investigated in detail (Moroni et al. 2012), including an

apportionment of the local and long range sources of par-

ticulate pollutants.

Aerosol properties and effects depend on a number of

variables the most important of which are number size

distribution, structure and chemical composition of the

constituent particles. Possible synergistic effect of different

chemical substances and phases on the particle toxicology

is a key point (Moroni et al. 2013) and require an integrated

characterization of the bulk chemical composition and also

of the geochemical and morphological features at a single

particle level, as that proposed in the present paper.

Recently (Ferrero et al. 2012) we combined number size

measurements and individual particle analyses along ver-

tical profiles, inside and above the mixing layer, to study

the dynamics and evolution of aerosol properties over basin

valleys, including a comparison between the Po valley and

the Terni basin. Herein, we focus the attention on the Terni

basin by means of systematic comparison between aerosol

size distributions and single particle properties. In both

cases, samplings and measurements involved tethered

balloon soundings along the vertical profiles.

Distinct scenarios were evidenced in the period of

interest depending on atmospheric conditions. The results

for the urban area of Terni were compared with those

obtained at the regional scale by means of chemistry-

transport modeling in order to distinguish the origin (nat-

ural from anthropogenic), sources (local from long range),

and dynamics of dispersion and transformation of different

PM components in the atmosphere. A model is proposed

which takes into consideration the contribution of natural

and different anthropogenic sources to evaluate the effec-

tiveness of policies of local abatement strategies and

emission control.

2 Experimental

2.1 Balloon soundings

Vertical aerosol profile measurements were carried out by

means of a helium-filled tethered balloon equipped with an

optical particle counter (1.108 ‘‘Dustcheck’’ GRIMM, 15

size classes between 0.3 and 20 lm, 6 s time resolution), a

portable meteorological station (BABUCABC LSI-Lastem;

pressure, temperature and relative humidity measured, 6 s

time resolution), and a miniaturized cascade impactor

(Sioutas SKC, polycarbonate filters, Ø = 37 mm). Details

on the experimental set-up are reported in Ferrero et al.

(2012) and Moroni et al. (2012).

The balloon’s ascent and descent rates have been set at

30.0 ± 0.1 m min-1, resulting in a 3.0 m vertical resolu-

tion for OPC measurements, and the maximum height of

launching was 600 m AGL. Only vertical profiles carried

out at RH \65 % were taken into consideration, to avoid

the effects of aerosol hygroscopic growth on particle size

distribution measurements.

The balloon campaign took place from 12 to 25 of

January 2009. The balloon took off from Piazza Tacito

which is the main square in the town and shows typical

urban atmospheric conditions. Six to eight consecutive

launches were performed each day from the early morning

to the mid afternoon. The synoptic meteorological condi-

tions changed during the sampling campaign. Jan 16 and

Jan 17 were characterized by low winds, high daily tem-

perature excursions and a marked atmospheric stability

(Fig. ESM1, Online Resource 1). The following days reg-

istered a significant increase of wind speeds and tempera-

tures, a decrease of atmospheric pressure and a consequent

reduction in the PM values. A Saharan dust intrusion was

registered on Jan 20–21, and rains followed on Jan 21. Jan

22 and Jan 23 were clear sky days.

2.2 Particle size distribution and volume concentration

along vertical profiles

Optical particle counters (OPCs) provide real time direct

measurements of particle number concentration and size

distribution in the atmosphere based on the principle of

light scattering from particles.

Particle number measurements and size distributions

have been employed to calculate the volume size distri-

bution, dV/dlogDp (lm3 cm-3), of particles along the

vertical profile. In this case, particles are assumed to be


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spheres, whose diameter, D, is the mean value of the

extremes of the corresponding size interval. In a second

step data from the volume size distribution have been

conveniently re-arranged to obtain the pattern of the vol-

ume concentration of particles (lm3 cm-3) in the fine

(Dp \ 1.6 lm; VFF) and the coarse (Dp [ 1.6 lm; VCF)

fraction along the vertical profile:

MPVFF;CC ¼P

i NiViPi Ni

ð1Þ

where Ni is the number concentration of particles for the ith

aerosol size fraction of the OPC, and Vi is the mean volume

of the particles in the same size fraction. In the final step,

the mean particle volume (MPV) for the fine (FF) and the

coarse (CF) fraction have been calculated from the

following equation:

VFF;CC ¼X

i

NiVi ð2Þ

The MPV values can be used to visualize the time

evolution of the volume size distribution along the vertical

profiles.

Vertical aerosol profiles allow the determination of the

mixing height (MH), i.e., the height to which the lower

atmosphere will undergo mechanical or turbulent mixing,

producing a nearly homogeneous air mass. The same pro-

files also enable us in exploring changes in aerosol size

distribution as a function of height. In particular, the MPV

values can be used to visualize the time evolution of the

volume size distribution along the vertical profiles.

2.3 Scanning electron microscopy

Scanning electron microscopy (SEM) determinations were

performed at the Department of Chemistry and Industrial

Chemistry, University of Genoa, using a ZEISS SUPRA 40

VP field emission device equipped with an X-ray energy

dispersive spectrometer (Oxford link with Pentafet Si–Li,

coupled with INCA-Feature software for data treatment).

Single portions of the polycarbonate filters about

10 9 10 mm in size were cut from the central part of the

membranes and mounted on to SEM aluminium stubs using

double-sided carbon tape. Samples were then coated with a

100–150 A carbon film to provide electrical conductivity

and to prevent charge build-up during the exposure to the

electron beam.

Analyses were performed in automated mode on selec-

ted representative fields of the filter after evaluation of

sample uniformity, adequate greyscale calibration and

proper definition of the working parameters (Capannelli

et al. 2011). Magnification was chosen depending on par-

ticle size: 5,0009 was a sufficient magnification for par-

ticles [1 lm, while the use of high magnifying factors

(50,0009) was requested for nanoparticle identification. A

total number of about 700–1,500 particles in 20–300 fields

of view were counted for each sample.

The software scans fields one at a time, detecting the

single particles and recording morphology (digitalized

image) and chemical composition (spectrum) data. It can

measure several morphological parameters, e.g. length,

area, Feret diameter, aspect ratio, perimeter, etc. EDS

spectra (voltage 30 kV, focal distance 8.5 mm) were col-

lected for 30 s live time and the elemental composition

obtained after standardless matrix correction. Values lower

than 0.1 wt% (SEM detection limit) were omitted.

According to Kandler et al. (2007) approach, the elemental

atomic ratios were calculated and then compared with

those of pure minerals and phases to obtain distinct particle

classes.

2.4 Chemistry-transport modeling

The last available version (chimere2011a?) of the CHI-

MERE chemical and transport model (http://www.lmd.poly

technique.fr/chimere/chimere.php) was implemented on a

6?1 2 GHz quad-core Intel Xeon multicomputer with

4 GB/node main memory located at the Department of

Chemistry, Biology and Biotechnologies of the University

of Perugia. The computing domain extended approximately

from 9.50E, 41.08N to 15.75E, 44.43N (the domain

boundaries are not squared in latitude-longitude coordi-

nates since a WGS84-UTM32 coordinates system was

used). Horizontal resolution of the domain was 5 9 5 km,

dividing the domain in 8,000 cells for each of its eight

vertical levels (the vertical coordinate is of hybrid sigma P

type). The model was run with an hourly resolution starting

from 01/14/2009 0000 hours and ending to 01/25/2009

2400 hours. The run required 4.94 h, distributed on 25

processes (included the master one).

Meteorological input data provided by the local agency

for environmental protection (ARPA Umbria) were trans-

formed from their original GRIB format to the diagmet

CHIMERE preprocessor compatible netcdf format using

the grib_to_netcdf interface built to this end. The same

code provided also to interpolate the rotated latitude/lon-

gitude grid fields to make the projection onto the CHI-

MERE computing domain and also to derive the diagmet

needed variables from the meteorological model ones.

Biogenic emissions were calculated by the MEGAN

model, part of the CHIMERE package. Boundary and

initial conditions, courtesy of ARPA Umbria, were given

on a 0.5� 9 0.5� domain ranging from 8E,32.5N to

17E,46N and were produced by a coarser domain CHI-

MERE run. Anthropogenic emissions, estimated on the

same computing domain, were distributed on two vertical

levels (0–94 and 94–184 m). On the Umbria fields, detailed


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http://www.lmd.polytechnique.fr/chimere/chimere.php

http://www.lmd.polytechnique.fr/chimere/chimere.php

regional emissions, courtesy of ARPA Umbria, were used.

Outside the region, national emissions available from

ISPRA (Istituto Superiore per la Protezione e la Ricerca

Ambientale) database. Point emissions were not allowed in

this work to be treated directly by the CHIMERE prepemis

preprocessor.

Global dust underestimation of the input boundary

conditions of the model is a general lack of the process.

This leads to an overestimation of the other PM constitu-

ents, and to unexpected time dependence. In addition,

height of the vertical levels changes with pressure as they

are of hybrid sigma-p type (Jacobson 1999). To overcome

these problems, dust and PM10 concentrations at different

levels (1–7) in the atmosphere were normalized to the

corresponding values near ground (level 0). In the mean-

while, the height of levels was (reasonably) assumed to be

constant in the period of measurements.

3 Results and discussion

Representative aerosol volume concentration profiles on

Jan 17, 18, 22 and 23 are reported in Fig. 1. At strictly

stable atmospheric conditions (i.e., Jan 17; Fig. ESM1,

Online Resource 1) the MH is well below 200 m AGL; the

particle pollutants are, thus, compressed at the bottom of

the air column leading to the higher PM10 concentrations

found at ground level (46 lg m-3; Fig. 1a). For decreasing

atmospheric pressure and increasing wind speed (i.e., Jan

18; Fig. ESM1, Online Resource 1) some dispersion of

aerosol particulate occurs at ground level leading to lower

PM10 concentration, although further thinning of the MH

occurs (Fig. 1b). After the Saharan dust intrusion and

subsequent rainy period (i.e., Jan 22 and Jan 23;

Fig. ESM1, Online Resource 1) the MH tends to expand

well above 400 m AGL; the particle pollutants tend to be

0 m100m200m300m400m500m600m

0.1

1

10

100

dV/d

logD

p(µ

m3 cm

-3)

0.1 1 10 0.1 1 10 0.1 1 10 0.1 1 10

Diameter, Dp (µm)

(e) (f) (g) (h)

Volume concentration (µm3cm-3 )

0 10 20 300

100

200

300

400

500

600

Hei

ght (

m A

GL)

(a)

0 10 20 30

(b)

0 10 20 30

(c) (d)

0 10 20 30 40

Fig. 1 Vertical profiles of volume concentration of fine (thin solidline), coarse (dotted line), and total particles (thick solid line)

collected on a Jan 17, b Jan 18, c Jan 22, and d Jan 23 at 13:00 UTC,

with corresponding mean particle volume profiles (insets; lm3). Daily

PM10 concentration were 46 lg m-3 on Jan 17, 30 lg m-3 on Jan 18,

29 lg m-3 on Jan 22, and 28 lg m-3 on Jan 23. e–h Aerosol volume

size distribution along height on the same days and time


123


dispersed in the air column leading to a decrease of PM10

concentration at ground level. Ground level coarse particle

concentrations increase from Jan 22 to Jan 23, while fine

particle concentrations do, actually, decrease. This is

probably the consequence of decreasing wind speed

(Fig. ESM1, Online Resource 1), and also the reason for

the similar PM10 concentrations found on these days

(Fig. 1). Results of CHIMERE modeling of the vertical

profiles confirm the oscillating structure of dust/PM dis-

tribution along height at qualitative and, in some instance,

even quantitative level (Fig. ESM2, Online Resource 2).

However, unexpected time dependence clearly results

when comparing calculated and experimental PM10 ratios.

In this case the patterns show less structures and worse

agreement with experimental measurements.

Abrupt changes of volume concentration along height

are present in the profiles, typically ascribed to particle

pollutants stratification. Stratification mainly affects the

coarse particle (CF; particle diameter Dp [ 1.6 lm), while

it is nearly absent in the fine fraction (FF; Dp \ 1.6 lm).

This is particularly evident on Jan 23, less pronounced on

Jan 17 and 22, and almost absent on Jan 18 probably due to

different wind speed and turbulence of the air mass. The

corresponding MPV profiles (insets in Fig. 1a–d) reveal a

progressive increment of the FF volume size and a decre-

ment of the CF volume size along height on Jan 17 and 18,

and no significant change on Jan 22 and 23. In a previous

paper (Ferrero et al. 2012) stratification and MPV variation

along height have been attributed to coarse particle sedi-

mentation and fine particle ageing. These facts point at the

great influence of atmospheric conditions on the mode and

extent of mixing of atmospheric aerosols.

Particle volume size distributions at different height in

the atmosphere (Fig. 1e–h) reveal two main modes at the

extreme ends of the distribution at ground level. Pro-

gressive narrowing of the modes for increasing height is

evidenced on Jan 17 and 18. In particular, on Jan 17 two

marked narrow modes at *1 and *2 lm develop above

the MH following the great decrease of particle volume

concentration. On the other hand, particle volume size

distribution and concentration are very stable along

height on Jan 22 and 23. All these features suggest some

additional influence of the origin and composition of

distinct particle types on the mode and dynamics of

aerosol formation and evolution within and above the

mixing layer.

Fig. 2 SEM micrographs of particle types in the aerosol samples:

a dendritic calcite aggregates and Ca–Zn carbonate aggregates, along

with a Zn–Ca sulfate grain (sul), b clay mineral (illite–smectite) with

a gypsum crystal (gy), c mixed Fe-bearing octahedral particle,

d carbon soot particles and aggregates


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Individual particle aerosol characterization revealed five

different main particle classes; namely, carbonates, sili-

cates, sulfates, metal oxides, and carbonaceous particles.

Minor phases are chlorides, phosphates and distinguished

(Al-bearing, Cu, and Sb) metal particles. The carbonates

consist of calcite grains of variable size and shape, the

smallest of which are represented by well-sorted grains

arranged in dendritic aggregates; small amounts of simi-

larly shaped Ca–Zn carbonates and sulfates are also present

in this latter case (Fig. 2a). These features point at a

common origin for these particles from industrial processes

(e.g., slagging) of the local steel production. The silicates

consist of quartz, feldspar (alkali-feldspar and subordinate

plagioclase), and clay minerals (illite, smectite) of variable

Jan 17 Jan 23

GySulfateQz

SilicateMixedOther

CarbonaceousCalciteCa-Zn

FeMetalTi

(a) (b)

(c) (d)

Fig. 3 Relative abundance of

the particle types in the aerosol

samples on Jan 17 and Jan 23:

a, b overall data; c, d the same

after subtraction of

carbonaceous particles and

recalculation over the non-

carbonaceous ones. Ca–Zn,

Ca–Zn carbonate; Fe, iron

oxide; Metal, mixed metal

oxide; Ti, Ti-oxide; Gy,

gypsum; Sulfate, composite

sulfate; Qz, quartz

Table 1 Statistics (mean ± standard deviation) of Feret diameter (FD, lm) and shape factor (SF) for airborne particle classes at 600 m AGL on

Jan 17 and Jan 23, 2009

FD SF

Jan 17 Jan 23 Jan 17 Jan 23

Carbonaceous 0.20 ± 0.08 (34) 0.86 ± 0.76 (1,013) 1.28 ± 0.29 1.43 ± 0.62

Calcite 0.39 ± 0.32 (335) 1.16 ± 0.92 (13) 2.00 ± 1.47 1.49 ± 1.10

Ca–Zn 0.37 ± 0.22 (4) – 2.65 ± 1.53 –

Fe 0.44 ± 0.45 (124) 0.61 ± 0.40 (14) 2.04 ± 2.21 1.27 ± 0.67

Metal – 0.67 ± 0.40 (40) – 1.17 ± 0.40

Ti – 1.40 ± 1.61 (7) – 1.17 ± 0.38

Gy 0.16 ± 0.07 (2) 4.30 ± 5.46 (3) 1.42 ± 0.28 2.51 ± 2.02

Sulfate 0.68 ± 0.71 (8) 0.55 ± 0.14 (9) 3.77 ± 3.14 1.16 ± 0.17

Qz 0.33 ± 0.31 (5) 0.92 ± 0.47 (10) 1.64 ± 0.70 1.11 ± 0.06

Silicate 0.38 ± 0.22 (13) 1.70 ± 1.68 (17) 2.50 ± 2.60 1.70 ± 1.09

Mixed 0.49 ± 0.50 (131) 1.16 ± 0.64 (16) 2.06 ± 1.19 1.21 ± 0.34

Other 0.38 ± 0.29 (26) 1.07 ± 0.82 (17) 1.58 ± 0.77 1.17 ± 0.25

Number of particles analyzed are in parenthesis. Acronyms as in Fig. 3


123


size and shape. Coarse grained clay minerals, in particular,

are often sites for deposition/crystallization of different

phases such as gypsum (Fig. 2b). Gypsum and the com-

posite sulfates exhibit distinct morphological features, the

former being represented by fine grained idiomorphic

crystals (Fig. 2b), and the latter by fine to coarse grained

hypidiomorphic platy grains (Fig. 2a). According to Sinha

et al. (2008), secondary origin from in-cloud processes

involving airborne carbonate and metal particles of natural/

anthropogenic sources can be postulated for both particle

types. Metal oxides include iron particles, mixtures of iron

and different metals (mainly Cr, Mn, Ni, Zn), and titanium

particles. The Fe-bearing particles are spherical to irregular

shaped. Typically idiomorphic octahedral fine grained

crystals were also found in a very few cases (Fig. 2c). The

morphology and/or the mixed chemical composition of

spherical and octahedral particles suggest that they are

spinel-like oxides resulting from high temperature pro-

cesses such as those related to the steel plant activities

(Moroni and Viti 2009), while irregular shaped grains are

likely from soil erosion. Carbonaceous particles mostly

consist of carbon soot particles and aggregates (Fig. 2d).

Isolated particles represent the freshly emitted combustion

residue, while compact large aggregates represent the aged

material. A few spherical shaped smooth to porous fine

grains were also found in some of the samples, typically

the result of coal and oil combustion (e.g., Casuccio et al.

2002). Different particle types (mainly calcite and/or

silicates and/or quartz) may cluster around a single

grain giving rise to larger aggregates of intermediate

composition.

Relative abundance of the particle classes along the

vertical profiles is prone to significant changes over time

(Moroni et al. 2012). In particular, when considering the

composition of the atmosphere above the MH (600 m

AGL), calcite turns out to be the main constituent phase on

Jan 17, while carbonaceous particles prevail on Jan 23

(Fig. 3a, b). After subtraction of the carbonaceous phase

(Fig. 3c, d), remarkable increase of quartz and silicates

0.0 0.4 0.8 1.2 1.6 2.00

10

20

30

Rel

ativ

e ab

unda

nce

(% n

umbe

r)

0.0 0.4 0.8 1.2 1.6 2.00

10

20

30

Feret Diameter (µm)

Rel

ativ

e ab

unda

nce

(% n

umbe

r)

(a)

(b)

Fig. 4 Grain size (Feret diameter, lm) distributions of calcite (a) and

iron particles (b) at 600 m AGL on Jan 17, 2009

0.1 1

1

10

Sha

pe F

acto

r

(a)

0.1 1

1

10

Sha

pe F

acto

r

Feret Diameter (µm)

(b)

Fig. 5 Correlation diagrams Feret diameter (lm) versus shape factor

for calcite (a) and iron particles (b) at 600 m AGL on Jan 17 (solidsymbols) and Jan 23 (open symbols), 2009


123


along with occurrence of mixed metal and Ti particles, are

evidenced on Jan 23. Minor phases amount also increase on

Jan 23, while the relative abundance of the mixed particles

tends to decrease. All these features provide evidence of

the renewal and the general remix of the lower troposphere

induced by the Saharan dust outbreak and consequent rainy

period.

Grain size (e.g., Feret diameter, FD) and shape (e.g.,

shape factor, SF) of the particle types above the MH are

reported in Table 1. Particle mean value of FD is well

below the micron size on Jan 17, and above this value on

Jan 23. Ranges of variation of the grain size are generally

quite large; largest values are registered on Jan 17 and

belong to calcite, iron particles and sulfates. Grain size

distributions shown in Fig. 4 for calcite and iron particles,

on turn, reveal a main mode at *0.2 lm, and minor modes

in the range 0.7–1.0 lm. These latter fit well the mode of

the aerosol volume size distribution resulting from OPC

measurements (Fig. 1e). Therefore, not all the particle

types, but only calcite, iron particles and, in some instance,

the sulfates are involved in coagulation/aggregation pro-

cesses of ageing above the mixing height at stable atmo-

spheric conditions. The positive correlation between FD

and SF in the calcite and iron particles (Fig. 5) supports

particle aggregation, while SEM observations (Fig. 2a)

confirm calcite and sulfate particle aggregation above the

MH. All these features suggest the local steel plant emis-

sions be the common source for all these particle types

above Terni basin.

Abrupt increase in the grain size of carbonaceous, cal-

cite, gypsum, quartz and silicate particles occurs on Jan 23,

while the iron and sulfate particles have quite similar grain

size ranges (Table 1). Mixed iron and pure iron particles

exhibit the same grain size ranges; Ti oxides are much

larger, and similar in size to the silicates. Carbonaceous

and gypsum particles show increasing morphological

complexity (i.e., SF), while calcite, sulfates, quartz and the

silicates show decreasing SF values (Table 1). The mor-

phological features are clear evidences of turbulent re-

mixing in the lower troposphere induced by atmospheric

instability (i.e., the rainy period; Fig. ESM1, Online

Resource 1). In particular, the size and shape of carbona-

ceous and gypsum particles reveal some occurrence of

condensation/coagulation ageing processes at some height

in the troposphere prior to re-mixing. Further evidence of

ageing is the compaction of particles in the carbon soot

aggregates (Fig. 2d). On the other hand, increasing FD for

decreasing SF values of calcite, quartz and silicates show

that turbulent re-mixing affected the particles as soon as

they entered the air column. Calcite (and, in some instance,

sulfate) morphology shows the regular input from the local

steel plants. Quartz and silicates, on turn, reveal a common

origin from soils. The mineral chemistry of the silicates

shows the composition of illite–smectite mixed layer clay

minerals (Fig. 6a); however, the silicates found on Jan 23

are more illitic, whereas those present on Jan 17 are more

montmorillonitic in composition. The different chemical

compositions suggest distinct provenances for them. Back-

trajectory evaluations on those days (Moroni et al. 2012)

revealed a regional air mass circulation on Jan 17, and long

range transport phenomena from Northern Saharan regions

(northern Algeria to northern Libya) on Jan 23. Higher

amounts of illitic clay minerals do, actually, characterize

Saharan dusts from these sites (Coz et al. 2009). Therefore,

a significant part of the silicates found on Jan 23 are, likely,

the remnant of the Saharan dust intrusion passed over Terni

2 days before.

Further evidence of the variable influence of mineral

dust emissions transported by the air masses over Terni is

provided by the CHIMERE chemical and transport model

simulations. The concentration of fine (Dp \ 1.25 lm) and

coarse (1.25 \ Dp \ 10 lm) particle component at eight

(a)

0.2 0.4 0.6 0.8 1.0

0.0

0.5

3.0

3.5 K

(b)

0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

Ca

Al/Si

Fig. 6 Correlation diagrams Al/Si versus K (a) and Al/Si versus Ca

(b) for silicates at 600 m AGL on Jan 17 (solid circles) and Jan 23

(open circles), 2009. Solid stars are for pure illite, open stars are for

pure montmorillonite


123


distinct levels of increasing height from ground level were

evaluated over the time. For the same reason discussed

above, concentrations were then normalized to the con-

centration of the lowermost level to display the vertical

concentration gradients. The situation depicted in the

simulations (Fig. 7) show similar patterns of concentra-

tion of the fine and the coarse fraction at any time up to

*200 m AGL, and higher concentration of the coarse

fraction above this level starting from Jan 19. This is a

consequence of increasing air mass movement and dust

advection from remote areas. Starting from Jan 21 some

inversion of the concentration gradients (i.e., increasing

concentration) is observed above 300 m AGL. On Jan 23

inversions become apparent even at the highest level (i.e.,

*5,000 m AGL) in the troposphere. This fact supports

our hypothesis on the persistence of residual dust from

long range sources after the end of dust advections, with

important consequences on the properties and the phys-

ico-chemical behaviour of the aerosols (e.g., Poschl

2005).

4 Conclusion

Combined particle number and morphochemical charac-

terization of the airborne dust over Terni basin revealed the

main influence of the sources in the modulation of particle

concentration over space and time. Among them, the local

steel plants supply a regular contribution of calcite, metal

particles along with mixed metal carbonate and sulphate

compounds to the airborne dust. These phases are emitted

tens of metres above ground level within the MH, or just

above it during winter, when the flattening of the mixing

layer leaves the plume source in the free troposphere.

Metal accumulation and aerosol acidification by direct

sulfation of calcite in the presence of SO2 (Liu et al. 2005)

are, thus, to be expected at both local and regional scale.

Topsoils, on turn, are natural sources for quartz and sili-

cates from both local and remote areas. The dust supply is

intermittent in this case, depending on air mass movement

and circulation, but it can persist in the troposphere even

for a long time after the advection episode. Increased

Fig. 7 Ratios of calculated dust concentrations of fine (Dp \ 1.25 lm;

diagrams on the left) and coarse (1.25 \ Dp \ 10 lm; diagrams on theright) particles at levels 1–7 in respect to level 0 in the computing

domain air column obtained by means of CHIMERE modeling. Values

of approximately constant height AGL are assumed to be the following:

25 m (level 0), 71 m (level 1), 156 m (level 2), 318 m (level 3), 625 m

(level 4), 1,225 m (level 5), 2,473 m (level 6), 5,377 m (level 7)


123


particle nucleation and adsorption upon these phases (e.g.,

Seinfeld and Pandis 1996; Archuleta et al. 2005) are

driving processes in the modulation of the aerosol prop-

erties and physico-chemical behaviour. In the light of these

points, more stringent local abatement strategies for the

industrial emissions, and better definition of the contribu-

tion of the soil sources in terms of the constituent phases

and minerals, are to be set up to overcome the problems.

From a general point of view, the results from this study

suggest that the total mass of particulate matter (e.g., PM10)

cannot provide by itself the exact picture of the aerosol

properties without a comprehensive individual particle

morpho-chemical characterization. In this respect, SEM

methods provide simple, powerful tools to unravel the

questions concerning the nature, the properties and the

behaviour of aerosol particles.

Acknowledgments Prof. Giorgio Liuti died during the preparation

of the manuscript. This work and our future endeavours are dedicated

to him, whose intellectual honesty and genuine love of chemistry

continue to inform our scientific life. We thank Fondazione CARIT di

Terni e Narni for financial support to this research. We also thank

ARPA Umbria for providing detailed regional emission data.

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