ORIGINAL PAPER

Cloud condensation nuclei from biomass burning duringthe Amazonian dry-to-wet transition season

Jorge Alberto Martins Æ Fabio Luiz T. Goncalves Æ Carlos A. Morales Æ Gilberto F. Fisch ÆFrancisco Geraldo M. Pinheiro Æ Joao Bosco V. Leal Junior Æ Carlos J. Oliveira ÆEmerson M. Silva Æ Jose Carlos P. Oliveira Æ Alexandre A. Costa Æ Maria Assuncao F. Silva Dias

Received: 12 March 2007 / Accepted: 9 January 2009 / Published online: 5 February 2009

� Springer-Verlag 2009

Abstract Aircraft measurements of cloud condensation

nuclei (CCN) during the Large-Scale Biosphere–Atmo-

sphere Experiment in Amazonia (LBA) were conducted

over the Southwestern Amazon region in September–

October 2002, to emphasize the dry-to-wet transition sea-

son. The CCN concentrations were measured for values

within the range 0.1–1.0% of supersaturation. The CCN

concentration inside the boundary layer revealed a general

decreasing trend during the transition from the end of the

dry season to the onset of the wet season. Clean and pol-

luted areas showed large differences. The differences were

not so strong at high levels in the troposphere and there

was evidence supporting the semi-direct aerosol effect in

suppressing convection through the evaporation of clouds

by aerosol absorption. The measurements also showed a

diurnal cycle following biomass burning activity. Although

biomass burning was the most important source of CCN, it

was seen as a source of relatively efficient CCN, since the

increase was significant only at high supersaturations.

1 Introduction

From a global scale perspective, clouds and precipitation in

the Amazonian region are a tropical heat source strongly

coupled to other tropical heat sources (Gandu and Silva

Dias 1998). Together with dynamic and thermodynamic

processes, the cloud condensation nuclei (CCN) concen-

tration has an important role in the establishment of the

magnitude of this source. As an example, Cifelli et al.

(2002) shows that, during the wet season of the Amazonian

region, different cloud microphysics properties were asso-

ciated with the convection when different wind regimes

were observed. According to Silva Dias et al. (2002), the

picture may be seen as an interrelation of processes on

different scales, starting from a large-scale forcing. During

the dry season, the atmosphere evolves to a more complex

picture, since biomass burning injects a surprising amount

of aerosols and gases that strongly interact with cloud and

precipitation. The magnitude of this impact has not yet

been estimated, but there is enough evidence of its effect

(Crutzen and Andreae 1990; Rosenfeld 1999; Roberts et al.

2001; Hudson and Yum 2001, 2002; Yum and Hudson

2002; Sherwood 2002). The assessment of the biomass

burning impact requires measurements of those properties

that modify clouds and precipitation.

CCN concentration is probably the most important

factor in determining cloud microphysics properties, since

J. A. Martins (&)

Universidade Tecnologica Federal do Parana,

Londrina, PR, Brazil

e-mail: jmartins@utfpr.edu.br

F. L. T. Goncalves � C. A. Morales � M. A. F. Silva Dias

Universidade de Sao Paulo, Sao Paulo, SP, Brazil

G. F. Fisch

Centro Tecnico Aeroespacial, Sao Jose dos Campos, SP, Brazil

F. G. M. Pinheiro � J. B. V. Leal Junior �C. J. Oliveira � E. M. Silva � A. A. Costa

Universidade Estadual do Ceara, Fortaleza, CE, Brazil

J. C. P. Oliveira

Universidade Federal do Ceara, Fortaleza, CE, Brazil

A. A. Costa

Fundacao Cearense de Meterologia e Recursos Hıdricos,

Fortaleza, CE, Brazil

M. A. F. Silva Dias

Centro de Previsao de Tempo e Estudos Climaticos,

Cachoeira Paulista, SP, Brazil

123

Meteorol Atmos Phys (2009) 104:83–93

DOI 10.1007/s00703-009-0019-6

it is largely responsible for the establishment of cloud

droplet spectra (e.g., Hudson 1983; VanReken et al. 2003;

Sotiropoulou et al. 2006). Increases in CCN concentration

can likely have a great impact on the radiative properties of

clouds (Twomey 1977; Albrecht 1989; Feingold et al.

2003). It has also been demonstrated that variations in CCN

concentrations, even within maritime air masses, modulate

precipitation (Ackerman et al. 1993; Hudson and Yum

2001; Hudson and Mishra 2007). Consequently, measure-

ments of CCN are an important key in understanding the

role that clouds may have on climate, since the quantifi-

cation of their effects has, as yet, large uncertainties

(Hudson 1993; Lohmann and Feichter 2004; IPCC 2007)

and the process may be nonlinear (Feichter et al. 2004;

Kulmala et al. 2003). Further, the increase in the number of

measurements may contribute to a global inventory of

CCN properties and, thus, help to improve parameteriza-

tions used in the predictions of cloud droplet number in

general circulation models (Ghan et al. 1997) and cloud-

resolving models (Goncalves et al. 2008; Martins et al.

2009; Martins and Silva Dias 2009).

The aerosol size distribution that defines the CCN con-

centration is the result of a complex interaction between

natural forest (Claeys et al. 2004) and biomass burning

components (Rissler et al. 2006). Ambient organic parti-

cles, for example, contain a vast number of compounds,

many or most of which are unknown, not individually

measurable, and without available thermodynamic infor-

mation (e.g., their dissociation constants and activity

coefficients), which make it practically impossible to

derive a soluble fraction for such particles and decide about

their critical supersaturation, i.e., the values to become

activated (Andreae and Rosenfeld 2008). A recent study

conducted by Decesari et al. (2006) comparing carbona-

ceous aerosols collected during the period of most intense

biomass burning and the onset of the rainy period revealed

results that suggested a constant emission of coarse parti-

cles from the natural background.

In spite of the great number of studies related to CCN,

the study of their properties becomes more important when

related to tropical regions, where the number of measure-

ments is still low. Further, assuming the fact that, in remote

forested regions the background CCN concentration may

be very low, as suggested by Roberts et al. (2001), the

increase in CCN concentration may have a great impact on

the microphysics of clouds that develop in that region.

Therefore, the aim of this study is to analyze the charac-

teristics of CCN spectra obtained during the Large-Scale

Biosphere–Atmosphere Experiment in Amazonia (LBA),

which has been developed since 1999, as described by

Silva Dias et al. (2002). The data were collected during

September and October 2002. The period of measurements

was from the end of the dry season until the onset of the

wet season. Additionally, this paper compares CCN spectra

in clean and polluted conditions and relates them to the

prevailing meteorological and biomass burning conditions

during the experiment.

2 The LBA experiment

During the last few years, a large experiment called

the Large-Scale Biosphere–Atmosphere Experiment in

Amazonia (LBA) has been continuously held in the

southwest Amazon basin. The objective of the LBA Project

is to improve the current knowledge of the Amazon envi-

ronment. Since the sustainable use of rain forests in the

Amazon must be based on solid scientific understanding of

the environment, and its relationship with climate, it

is necessary to understand how the various ecosystems

respond to anthropogenic influences. During January and

February of 1999, the first period of intensive measure-

ments of the LBA took place in the southwest Amazonian

region, which was entitled WET-AMC/LBA, because of

the corresponding rainy season. The campaign was orga-

nized jointly with the Tropical Rainfall Measuring Mission

(TRMM) during its validation campaign in the Amazonian

region (known as TRMM/LBA). Some results of the first

LBA field campaign were reported by Silva Dias et al.

(2002).

During September and October of 2002, a second

intensive campaign took place in the southwest Amazon

region under the scope of the LBA Project. The major

objective of the campaign was to collect data between

tropical dry and rainy seasons; therefore, it was entitled

DRY-TO-WET/LBA. In the context of this campaign,

there were aggregated projects. For example, smoke,

aerosols, clouds, rainfall, and climate (SMOCC), and

radiation, cloud, and climate interactions (RACCI) were

two important LBA aggregated projects. The first results

from DRY-TO-WET/LBA have been published (Andreae

et al. 2004; Decesari et al. 2006; Rissler et al. 2006). The

CCN dataset used in this paper is part of the RACCI project

and the complete description of the methods is given

below. The RACCI campaign took place in the south-

western Amazonian region, with most of the CCN

measurements taken in the Brazilian state of Rondonia

(Fig. 1). The experimental area was selected due to fre-

quent biomass burning activity prevailing during the dry

season.

3 Field campaign and CCN data acquisition

The CCN measurements were obtained through aircraft

flights from 21 September to 13 October 2002 during the

84 J. A. Martins et al.

123

dry-to-wet transition season. The flights were carried out at

the southwestern part of the Amazonian Basin, including

the Brazilian states of Acre, Amazonas, Mato Grosso, and

Rondonia (Fig. 1). An additional flight from Ceara state

(close to the Atlantic coast) to Rondonia on 21 September

was included in the analysis. Clean atmospheric conditions

were observed in the states of Acre and Amazonas, while

intense biomass burning activities were reported in

Rondonia and Mato Grosso. The flight measurements were

conducted with an Embraer-Bandeirante aircraft, as

described by Almeida et al. (1992). The main instruments

used during the flights provided the following variables:

CCN spectrum, cloud and rain drop spectra, cloud liquid

water content, temperature, dew point temperature, pres-

sure, and GPS positioning.

The CCN concentration is the variable analyzed in this

study. A static thermal-gradient chamber was used to obtain

the CCN supersaturation spectra (Oliveira and Vali 1995).

The chamber operates at supersaturation values decided by

the user. During the campaign, four values of supersaturation

were selected for each air sample: 0.2, 0.3, 0.5, and 0.7%,

although not all of these values were available for all flights.

The system was controlled by a computer, which allowed the

storage of the data. A video camera was used to capture the

pictures of the growing droplets inside the chamber.

Calibration was performed comparing the detector

signals with visual counts of the number of droplets

developing in the chamber. The CCN concentration was

found to be proportional to the detector output. During the

activation and growth of droplets, the time of the peak

signal corresponds well to the time at which the droplet

count is a maximum within the sample volume. The use of

an average output signal over a period bracketing the mean

position of the peak is a slightly better measure of droplet

concentrations than the peak value. The calibration equa-

tions used to obtain the data were derived by Oliveira and

Vali (1995), where additional information about the cali-

bration procedure can be found.

Considering the aircraft velocity and the time necessary

for a full cycle, each measurement represents a typical

distance of 5 km. Table 1 shows a summary of the flights

conducted during the LBA campaign, from 21 September

to 13 October, with respective initial and final local times

(LT). When two or more flights were conducted on the

same day, different letters were used in order to identify

each one. About 40 flight hours were performed during this

campaign.

Although biomass burning emissions were present dur-

ing most of the flights, the average CCN concentrations

analyzed in this work were not made within the smoke

plumes but, rather, in the air that might go into the clouds.

The measurements associated with clouds that develop

above the fires would bias the data. Sometimes, the CCN

concentration within smoke plumes reach ten times the

background values. Therefore, the analysis includes only

CCN concentrations in clean and polluted background

atmospheric conditions prevailing in that region and is the

subject of the following discussion.

Fig. 1 Area in which the flights were conducted, with the main sites

of interest: Cruzeiro do Sul (CS), Rio Branco (RB), Porto Velho (PV),

Fazenda Nossa Senhora (FNS), Ji-Parana (JP), Vilhena (VL), Alta

Floresta (AF), Teresina (TE) and Fortaleza (FL). The image showing

part of the Amazon rainforest in western Brazil was acquired on

05 (right, 10:05 LT) and 06 (left, 10:50 LT) October 2002.

http://modis-atmos.gsfc.nasa.gov

Table 1 Flight summary of the flights conducted during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia (LBA) campaign,

from 21 September to 13 October 2002

Day 21 23 24 26 27 28 30a 30b 01a 01b 04a 04b

Time 13:24 14:07 14:43 14:02 14:27 13:37 11:14 13:49 10:16 16:47 11:06 13:54

(LT) 15:30 16:16 17:04 16:18 16:03 15:38 12:17 16:40 12:25 17:20 12:25 15:49

Day 04c 05a 05b 06a 06b 08 09 11a 11b 12 13

Time 16:48 12:13 15:24 11:12 14:19 14:53 13:32 10:43 13:21 11:45 13:20

(LT) 18:36 14:18 17:01 13:15 16:45 16:30 15:27 12:10 14:46 13:38 15:50

The letters indicate that more than one flight was performed during that day

Cloud condensation nuclei from biomass burning 85

123

4 Results and discussion

As will be shown, the atmospheric variables analyzed

during the transition period show strong variability, which

can be generally related to the large-scale circulation

characteristics, local rainfall events, and sampling in dif-

ferent areas. Figure 2 shows time series of the total rainfall

during this dry-to-wet transition from total daily rainfall

measurements at the Fazenda Nossa Senhora site (LAT

10�4504400S, LON 62�2102700W). This shows the expected,

general increase in the total rainfall. CCN concentration at

0.5% supersaturation are shown in Fig. 3. In order to

analyze the entire experimental period, we had to limit the

analysis to CCN concentrations measured from the surface

(altitude of about 200 m) to 1,800 m (approximately the

top of the boundary layer). It is important to note that

Fig. 3 does not include fire spots in the calculations,

because it takes into account only the atmospheric back-

ground conditions, as explained above.

4.1 Local- and large-scale circulation influences

Very smoky conditions prevailed on 21 September, the first

day of the aircraft campaign, between Alta Floresta (AF)

and Ji-Parana (JP), with poor visibility and low relative

humidity (40%) during the afternoon at Fazenda Nossa

Senhora (FNS) (see Fig. 1). A high number of fire spots

were observed in the area in previous days with no

occurrence of rainfall events. According to forest fire

monitoring provided by the National Institute of Space

Research (http://www.inpe.br) using satellite data, the

number of hot spots detected was about 500 per 106 km2 on

21 September in Rondonia and Mato Grosso states. This

number was higher in the previous days in the Rondonia

area, which might explain the strong smoke conditions

observed during the beginning of the campaign. In

addition, trends in the concentration of carbonaceous

compounds obtained from analysis of the aerosol particles

during the LBA-SMOCC experiment (Decesari et al. 2006)

can confirm the high aerosol concentrations during the

period.

From 21 September onwards, changes in the atmo-

spheric conditions were observed. Convection in general

started in late morning, with shallow cloud streets devel-

oping in the Rondonia area (Fig. 4a). These shallow cumuli

formed at about noon and rapidly evolved to a few deep

cells observed in mid afternoon (Fig. 4b). The develop-

ment of these cells was common inside the radar 150-km

range in the mid afternoon. An example of such cells (23

September) can also be seen in the radar images of Fig. 4c,

d, where a small circle indicates in Fig. 4a, b the area of the

flight at that moment. Although rain was not recorded at

FNS owing to the fact that the cell developed in the

vicinity, this particular cell developed into a strong thun-

derstorm in early evening with windstorm and lightning.

The occurrence of such isolated events nearby JP may

explain the observed slight decrease in CCN concentrations

during the days from the beginning of the campaign until

27 September. For example, the relative dispersion of the

CCN concentration at the boundary layer (BL) on 27

September (0, 32), higher than previous days (0, 20),

suggests that the decrease is associated with local effects,

i.e., the atmosphere continues to be polluted but with clean

localized domains as a consequence of local and isolated

cells. The visibility also improved as a consequence of

local rainfall occurrences. The behavior on 24 September is

an exception and it will be discussed in Sect. 4.3. At the

same time, it is necessary to recognize the aspects related

to the geometry of the deforestation process that implies

non-uniformity in the distribution of hot spots (see Fig. 5a).

The temporal evolution of the number of fire pixels on 23

September for an area of about 240,000 km2 (involving the

Rondonia state) is shown in Fig. 5b. The dataset was

Fig. 2 Time series of the daily accumulated precipitation near the

Fazenda Nossa Senhora (FNS) site. The horizontal rectangle identifies

the period of cloud condensation nuclei (CCN) measurements

Fig. 3 Time series of the CCN concentration obtained from aircraft

measurements at 0.5% supersaturation inside the boundary layer.

CCN measurements on 05 October are outside the Rondonia area

86 J. A. Martins et al.

123

provided by the automated biomass burning algorithm

(ABBA), developed to monitor fire activity using GOES

imagery (Prins et al. 1998).

On 28 September, the satellite images indicated con-

vective clouds developing in northern Rondonia, and

altocumulus and cirrus covered the rest of the area in the

beginning of the morning. These convective clouds rapidly

evolved, with a few deep cells observed in mid morning in

the northwest, central, and east regions. In the beginning of

the afternoon (about 14:30LT), one of these cells caused a

windstorm in JP and its vicinity, followed by heavy rain

and lightning activities until 16:00LT (38 mm of total

precipitation in JP). The prevailing winds from the north

could be advecting smoke plumes towards the south. As a

consequence of the reduced smoke, the visibility increased

in JP during this day and CCN concentrations were very

low (CCN concentrations decreased by more than 50%

compared to the previous day, see Fig. 3). In addition,

satellite products showed that fire spreads just to south-

western Rondonia until 1st October, which could explain

the fact that smoky conditions prevailed just to the south-

west of Vilhena (VL). These CCN measurements during 30

Fig. 4 Satellite visible image

and radar reflectivity on 23

September 2002. The upperimages show atmospheric

conditions before the flight start

(a) and at the end of the flight

(b). The bottom images show

radar reflectivity (dBZ) at the 3-

km level for all of the radar area

(c) and a vertical profile of radar

reflectivity for the main cell

observed during the flight (d).

The circle identifies the area in

which the flight was performed

and the arrow identifies the

convective cell shown in d

Fig. 5 Spatial distribution (a)

and temporal evolution (b) of

the number of fire pixels on 23

September 2002 for an area of

240,000 km2 in the southwest

region of the Amazonian forest.

The white triangle indicates the

site FNS. Data were provided

by Elaine M. Prins

(NOAA/NESDIS)

Cloud condensation nuclei from biomass burning 87

123

September and the first days of October were conducted far

from JP and their characteristics will be discussed in Sect.

4.2.

Two mesoscale convective systems developed on 07

October in western Mato Grosso and traveled westward,

reaching JP during the night, accumulating more than

50 mm of precipitation in some specific sites. The sys-

tems were followed by a large area of stratiform rain that

lasted throughout the morning and early afternoon in the

west part of Rondonia. Even though there was advection

of air masses, scavenging by rain might explain the low

CCN concentrations observed in this case. As a result of

rainfall activity, the lowest CCN concentration in the JP

area was observed since the beginning of the campaign

(note that CS showed values lower than JP when com-

pared with any day of the campaign). CCN concentrations

of about 300 cm-3 were found on 08 and 09 October (see

Fig. 3). The mesoscale convective systems observed on

07/08 October can be seen as the beginning of the tran-

sition to the wet season. A strong decrease of the CCN

concentration was observed to be associated with the

passage of these systems and low CCN concentra-

tions were predominant afterwards. Smoke advected to

Rondonia and, together with some local fires, can explain

the increase in CCN concentrations on 11 October (dou-

ble when compared to 09 October). During the following

days, the sky was clear with good visibility and shallow

cumulus developed around noon. These clouds subse-

quently evolved into deep convective cells in different

times between late afternoon and early evening.

4.2 Horizontal sampling

During some flights, the aircraft maintained the same

altitude for hundreds of kilometers, which provided

excellent horizontal averages of CCN concentrations dur-

ing the experiment. Likewise, CCN spectra obtained from

these flights revealed interesting time and space variability

features, as shown in Fig. 6. The flight from Fortaleza (FL)

to JP on 21 September passed through Teresina (TE) and

AF and constitutes a good example of this variability. It

demonstrates the transitions among different regions, as

well as different air mass characteristics. During this flight,

the aircraft flew across maritime (close to FL), continental

(TE area), and polluted air masses (nearby AF). The first

part of the flight (FL–TE) was very close to the Atlantic

Ocean and was influenced by easterly maritime air masses

associated with the trade winds. For example, the average

CCN concentration at 0.5% supersaturation was 131 ±

42 cm-3. In the second part of the flight (TE–AF), the

results were completely different and a variety of air

masses were associated with the measurements. The

average CCN concentration was 418 ± 206 cm-3 and the

high variance shows a transition toward a more polluted

region. The high CCN concentrations (744 ± 107 cm-3)

associated with the final part of the flight (AF–JP) suggest

that the flight occurred under intense biomass burning

activity, as discussed above. The altitude in each part of the

flight was very similar (around 2,760 m), with almost no

vertical variations. The CCN spectra for this flight can be

observed in Fig. 6a.

The flights on 30 September were performed in the

region between JP and VL. Clouds on 29 and 30 Sep-

tember developed in a relatively clean atmosphere with

just a few hot spots at the JP area. The flight JP–VL

(1,600 m high, 11:14–12:17 LT, flight a in Table 1) was

conducted at the top of the boundary layer (BL), when the

emission started, while the flight VL–JP (2,600 m, 13:49–

16:40 LT, flight b in Table 1) was performed above the

BL after prolonged emissions. Despite the similar spectra

observed in both flights (Fig. 6b), it should be noted,

however, that differences in flight altitudes (the CCN

concentration decreases with altitude above BL) could

mask some possible association with a diurnal cycle in

CCN concentrations as a consequence of the biomass

burning.

CCN measurements were performed between JP and AF

on 01 October. There were no thunderstorms in the area

between the two cities during the afternoon and atmo-

spheric conditions were clear, with only light smoke. Fires

were observed from satellite images just to the south of the

JP–AF line. The airplane crossed a Pyro-cumulus at the

south of AF, which developed in wet soil conditions,

probably as a result of the intense convection observed

during the previous day. Flight JP–AF (3,100 m high,

approximately 11:00 LT, flight a in Table 1) performed on

this day showed lower CCN concentrations than flight AF–

JP (3,400 m high, 5 h later, flight b in Table 1), in spite of

the fact that this later flight was performed at a higher

altitude (Fig. 6c). However, the first flight occurred during

the morning (when biomass burning activity was starting)

and the latter occurred during the afternoon (after pro-

longed aerosol emissions). Both flights had similar

trajectories.

Flights were performed in the western Amazon region

(see Fig. 1) near Cruzeiro do Sul (CS), passing through

Porto Velho (PV) and Rio Branco (RB) during 04, 05, and

06 October. Light to moderate smoky conditions were

observed during the flights to western Amazonia on 04

October (see Fig. 6d, all three flights around 2,700 m). The

three flights on 04 October were conducted at approxi-

mately 11:06 LT (flight a in Table 1), 13:54 LT (flight b in

Table 1), and 16:48 LT (flight c in Table 1). During the

flight CS–JP on 06 October, many fires were detected

randomly distributed in the Rondonia area, but, unfortu-

nately, there is no CCN data available on this flight.

88 J. A. Martins et al.

123

4.3 Vertical profiles

The CCN vertical profiles associated with the flights in

which a minimum of vertical data was available are shown

in Fig. 7 at 0.5% supersaturation (date and LT is shown in

the figure). The high values of CCN concentration in low

levels are expected due to the fact that the main source (the

biomass burning) is located at the surface. The comparison

between averaged spectra approximately within the BL

(from surface to 1,800 m) and at the free atmosphere (from

3 to 5 km) is shown in Fig. 8. Although the data avail-

ability is lower in the free atmosphere, the measurements

show that the CCN spectra have different behavior for each

atmospheric layer. While the spectrum for the BL has a

slope parameter of 1.18, using a power law to parameterize

(N = 1,260S1.18), for the free atmosphere, this value is 0.81

(N = 450S0.81), where S is supersaturation and N is the

CCN concentration (for a more complete discussion about

the parameterization of CCN spectra, see Cohard et al.

1998). This means that dN/dS increases inside the BL

(slope k greater than 1.0) and decreases in the free

atmosphere (slope k between 0 and 1.0) with increasing

supersaturation. The consequence is that the role of the

biomass burning increases towards the small particles or at

least hygroscopically small particles. In other words, the

observed spectra indicate that the fires are a significant

source of only relatively efficient CCN, as suggested by

Hobbs and Radke (1969). A comparison between averaged

Fig. 6 The CCN concentration

as a function of supersaturation

measured during horizontal

flights between the cities:

Fortaleza (FL), Teresina (TE),

Alta Floresta (AF), and

Ji-Parana (JP)—21/09/2002;

Ji-Parana and Vilhena (VL)—

30/09/2002; Ji-Parana and Alta

Floresta—01/10/2002;

Ji-Parana, Porto Velho (PV),

Rio Branco (RB), and Cruzeiro

do Sul (CS)—04/10/2002

Fig. 7 Vertical profiles of CCN concentrations at 0.5% super-

saturation

Cloud condensation nuclei from biomass burning 89

123

spectra in polluted (23/09/2002) and clean (05/10/2002)

conditions is also shown in the Fig. 8.

On 24 September, the 08:00 LT sounding at the FNS site

showed southeasterly winds stronger than 12 m/s in the

lowest levels (Fig. 9a). The occurrence of this low-level jet

can probably explain the differences observed among the

vertical layers, as well as the CCN concentration decrease

observed from 23 to 24 September (see Figs. 3 and 7). A

decrease of 50% in the CCN concentration within the BL

can be observed. This is a particularly interesting case

related to the vertical structure of the atmosphere and

might be associated with a cleaner air mass behind a cold

front extending from the southeastern coast of Brazil and

reaching the southwest of Amazonia (including Rondonia).

The frontal passage caused the lowest temperature of the

campaign (18.6�C at night to early morning of 25 Sep-

tember), lowest atmospheric irradiance, strong winds at the

FNS site, and a mechanically induced mixed layer at night.

The impact of this low-level jet on the vertical distribution

of CCN is discussed in the subsequent paragraphs.

On 24 September, it was still smoky, with cleaner air

mixed with hazy layers in high levels and clouds were

difficult to see from the ground. During early afternoon, the

flight was performed around FNS below 3,500 m and under

very heavy smoke. Active fires were present in many of the

convective clouds and visibility was very limited. The

small, almost absent, turbulence in the sub-cloud layer

might be a consequence of the reduced solar heating by the

smoke. The cloud base was not very well defined. This

suggests that the smoke was blocking part of the solar

radiation, inhibiting the cloud triggering from surface heat

flux and maybe heating the layer around cloud base level,

which became not well defined. In addition, the fires could

be playing a role in the triggering of the observed con-

vection cells.

Despite the fact that the flight on 24 September was

confined to low levels (altitude lower than 3,500 m), the

CCN profiles measured for this day were able to confirm

the accumulation of particles around the cloud base level

(Fig. 9b). Moderate CCN concentration can be observed in

the two bottom layers (at 668 and 1,504 m), high con-

centration for the intermediate layer (at 2,096 m), and low

to moderate concentration at the higher layer (at 3,077 m).

Each level was averaged over 800 m thickness. Again, it is

possible to conclude that the CCN concentration increasing

with supersaturation has different behavior for each layer

(which means different aerosol types) or different slope

parameter, as discussed before.

Besides the accumulation of CCN around the cloud base

level, there is an increase in temperature (DT) between 8:00

and 14:00 LT strongly correlated with the CCN profile

(Fig. 9c). This suggests that the accumulation of CCN

could be coincident with the accumulation of radiation-

absorbing particles, probably black carbon. Otherwise, the

variation in vapor mixing rate (Dq) between 8:00 and 14:00

LT shows a significant drying of the lowest levels

(Fig. 9d). This is coincident with the layers in which the

southeasterly winds were blowing and the advection of dry

air could be responsible for the observed picture, i.e., the

vertical profiles may also be influenced by temperature

inversion.

The day of 24 September is an interesting example,

since there are studies showing that absorbing aerosol

particles layers have been associated with changes in low

cloud cover and liquid water path. This effect, as suggested

by Hansen et al. (1997), is characterized by the absorption

of solar radiation by soot and the subsequent evaporation of

cloud particles, leading to a positive or negative radiative

forcing at the top of the atmosphere (TOA), depending on

where the absorbing aerosol layer is situated (Johnson et al.

2004). The discussion about this effect is beyond the scope

of this paper, but is necessary to reinforce its importance.

4.4 Power law associated parameters

Numerical models traditionally make use of the Twomey

formula to obtain the number of cloud droplets (CCN-acti-

vated) as a function of the supersaturation. Therefore, it is

remarkable to compare the c and k parameters found in the

present studies with those found in the literature. The data are

summarized in Table 2. Firstly, according to Twomey and

Wojciechowski (1969), the values of c and k are related to

important characteristics of the nuclei distribution. For

example, a maritime distribution has a lower concentration

of active nuclei than a continental distribution for a given

Fig. 8 Average CCN spectra for the boundary layer, free atmosphere,

and a comparison between clean and polluted conditions

90 J. A. Martins et al.

123

supersaturation. Twomey (1959) found c = 2,000 cm-3 and

k = 0.4 for continental air masses and c = 125 cm-3 and

k = 0.3 for maritime air masses in the Australia region.

Kocmond (1965) has observed c = 3,500 cm-3 and k = 0.9

for the Buffalo area, New York, USA. Jiusto (1967), based

on data from Kocmond (1965), obtained c = 53 cm-3 and

k = 0.46 (ocean edge) in Hilo, Hawaii Island (the two-mile

inland sampled values were c = 105 cm-3 and k = 0.63).

Measurements of cloud condensation nuclei were made at

several locations along the US Pacific Coast by Hudson

(1980). The values obtained were c = 230 cm-3 and k =

0.20 (maritime), c = 1,400 cm-3 and k = 0.70 (continen-

tal), c = 2,500 cm-3 and k = 0.70 (polluted), and c =

2,000 cm-3 and k = 2.00 (continental maritime modified).

Raga and Jonas (1995) obtained CCN vertical profiles

from flights performed around the British Isles in different

seasons and synoptic conditions. The observations suggest

that CCN distribution as a function of supersaturation agree

with Twomey’s empirical formula for the measurements

performed inside the boundary layer (c = 700 cm-3 and

k = 0.7). On the other hand, in the high levels (above the

boundary layer), they observed that the CCN concentration

does not agree with Twomey’s formula, exhibiting a

different type of functional relation between CCN con-

centration and supersaturation. According to Cohard et al.

(1998), since the formula established by Twomey is

approximated, more realistic equations based in physical–

Fig. 9 Vertical profiles of wind

speed at the FNS site (a);

average CCN concentrations

(b); vertical profiles of changing

temperature (c) and vapor

mixing rate (d) between 8:00

and 14:00 LT on 24 September

2002. The dashed line in c is

just a speculation about heating

if the aerosol particles were not

present

Table 2 Comparison of parameters c and k for different atmosphericregimes

Author c k Type

Twomey (1959) 2,000 0.40 Continental

Twomey (1959) 125 0.30 Maritime

Kocmond (1965) 53 0.46 Maritime

Kocmond (1965) 3,500 0.90 Urban

Jiusto (1967) 53 0.50 Maritime

Jiusto (1967) 105 0.63 Maritime/continental

Twomey andWojciechowski (1969)

100 0.70 Maritime

Twomey andWojciechowski (1969)

600 0.50 Continental

Raga and Jonas (1995) 700 0.70 Continental

Hudson (1980) 230 0.20 Maritime

Hudson (1980) 1,400 0.70 Continental

Hudson (1980) 2,500 0.70 Polluted

Hudson (1980) 2,000 2.00 Continental maritimemodified

Roberts et al. (2001) 259 0.97 Continental

This work 450 0.81 Average on all data—freeatmosphere

This work 1,260 1.18 Average on all data—boundary layer

This work 438 1.08 Clean (05/10/2002)a

This work 2,220 1.28 Polluted (23/09/2002)a

a Average on boundary layer

Cloud condensation nuclei from biomass burning 91

123

chemical properties of the aerosol distribution should be

used to fit the measured CCN spectra. However, this

requires information about these properties that are not

frequently available.

Roberts et al. (2001) found CCN concentrations for the

rainy season to be very close to that found in clean con-

ditions of the LBA-SMOCC campaign. Their results

correspond to the first measurements of CCN in the

Amazon Basin and showed values more typical of near-

marine locations (182 and 267 cm-3, respectively, at

supersaturations of 0.5 and 1.0%). The data presented in

this work show CCN concentrations of about 300 cm-3

characterizing the 08 and 09 October clean days, and even

lower values for 05 October (204 cm-3 at 0.5% supersat-

uration). In this case, the measurements were performed in

the clean air at the western part of the Amazon, defined by

Andreae et al. (2004) as green ocean in their conceptual

model about precipitation regimes.

5 Conclusions

In this paper, we have shown the characteristics of cloud

condensation nuclei (CCN) spectra observed in the Ama-

zonian region during the dry-to-wet transition period, which

is linked to biomass burning emissions. Measurements were

performed during a field campaign of the Large-Scale Bio-

sphere–Atmosphere Experiment in Amazonia (LBA)

Project in September and October of 2002.

It is possible to verify that there is a general decrease of

the CCN concentrations from the end of the dry season to

the onset of the wet season. In addition, the comparison

between clean and polluted areas shows at least five times

larger CCN concentrations over polluted areas. During the

dry period, the results also suggested a CCN concentration

diurnal cycle, following the biomass burning, which

accumulates high concentrations mainly inside the atmo-

spheric boundary layer.

There was some evidence suggesting the accumulation

of aerosol particles around the cloud base level, at least on

some days during the biomass burning season. This rein-

forces the fact that an increase in CCN concentration from

biomass burning may be directly associated with the

simultaneous increase in black carbon particles, which

could directly alter the vertical distribution of heating. If

black carbon and CCN are spatially correlated, this means

that there would be a competition between effects associ-

ated with absorbing particles and CCN in the context of

biomass burning.

Differences between clean and polluted supersaturation

spectra can be seen as an indicator that biomass burning is

more efficient at producing mainly small particles or large

particles, but with small soluble fraction.

Further investigation on the role of CCN under conditions

of intense biomass burning needs to incorporate physically

based parameterizations able to represent the aerosol system

as a whole. Besides the fact that this procedure can reduce the

uncertainties associated with the modeling predictions, it is

necessary to recognize that these first studies were performed

in the Amazonian region during the dry season, where

burning activities prevailed. There is, however, no infor-

mation available about the CCN and cloud properties of a

forested region during the dry season not influenced by

anthropogenic emissions. Besides the documentation of the

characteristics associated with an environment highly

influenced by human activities, it is necessary to rediscover

the previously predominant conditions.

The results concerning the spatial and temporal vari-

ability in the c and k parameters allow us to conclude that

clouds cannot be definitively parameterized in a simplified

way. If the activation spectra contain all of the necessary

information on the ability of an aerosol population to

nucleate cloud droplets, as pointed by Cohard et al. (1998),

the knowledge of the c and k parameters on a global scale

would be very useful to reduce the current uncertainties in

the global models. Although a mechanism of the activation

of cloud droplets based on the power law could be seen as

very simple, it could be able to represent important

regional characteristics not captured by the current global

circulation models.

Acknowledgments This study was supported by the Conselho

Nacional de Desenvolvimento Cientıfico e Tecnologico (CNPq), Fi-

nanciadora de Estudos e Projetos (FINEP), and Fundacao de Amparo

a Pesquisa do Estado de Sao Paulo (FAPESP).

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