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ATMOSPHERIC SCIENCE LETTERS
Atmos. Sci. Let. 6: 148–151 (2005)
Published online 27 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/asl.108
Scavenging efficiency of rainfall on black carbon aerosols
over an urban environment
K. Madhavi Latha,1 K. V. S. Badarinath1 * and P. Manikya Reddy2
1 Forestry and
2 Department
Ecology Division, National Remote Sensing Agency, Department of Space-Government of India, Hyderabad, India
of Environmental Science, Osmania University, Hyderabad, India
*Correspondence to:
K. V. S. Badarinath, Forestry and
Ecology Division, National
Remote Sensing Agency,
Department of
Space-Government of India,
Hyderabad, India.
E-mail:
badrinath kvs@nrsa.gov.in.
Received: 22 September 2004
Revised: 5 May 2005
Accepted: 26 May 2005
Abstract
Black carbon (BC) aerosols are the optically absorbing part of carbonaceous aerosols that
have significantly different optical and radiative properties. The present study addresses
the estimation of black carbon aerosol scavenging coefficient by using ground-based
measurements over an urban environment of India, namely, Hyderabad. Extensive ground
measurements of black carbon have been carried out during January to December 2004 over
a tropical urban environment of Hyderabad. Seasonal variations of black carbon aerosol
mass concentration showed high values during dry season and low values during monsoon
season. The diurnal variations of BC suggest that the concentrations increased by a factor
of ∼2 during morning and evening hours compared to afternoon hours. Drastic reduction in
black carbon aerosol loading has been observed during rainy days. The statistical fit between
black carbon aerosol mass concentration and rainfall suggests the reduction of ∼3.6 µg/m3 in
atmospheric black carbon aerosol loading for every 1-mm increase in rainfall intensity over
the study area. The scavenging coefficient of black carbon aerosols is found to be 1.64 × 10−5
s−1 .  Crown Copyright 2005. Reproduced with the permission of Her Majesty’s Stationery
Office. Published by John Wiley & Sons, Ltd.
Keywords:
deposition
black carbon; scavenging coefficient; precipitation; diurnal variation; wet
1. Introduction
Black carbon (BC) has become a subject of interest in
recent years for a variety of reasons. BC aerosol may
cause environmental as well as harmful health effects
in densely inhabited regions (Agrawal and Narain,
1999). BC is a strong absorber of radiation in the
visible and near-infrared part of the spectrum, where
most of the solar energy is distributed. Black carbon is emitted into the atmosphere as a by-product
of all combustion processes viz., vegetation burning,
industrial effluents and motor vehicle exhausts etc.
Aerosols are scavenged in the surface layer by dry
deposition and at greater heights by precipitation. In
both of these deposition processes, pollutant release
heights and precipitation intensities are quite significant (Samara and Tsitouridou, 2000). Deposition processes limit aerosol lifetimes in the atmosphere, control the distance traveled before deposition and thus
affect their atmospheric concentrations. Dry deposition is capable of filtering the larger particles from
the atmosphere in 2 or 3 days and would require several weeks to remove the more harmful sub-micron
fraction, which is, however, removed through various
wet deposition processes. Wet deposition processes
assume immense significance from the human health
and ecosystem health point of view. Wet scavenging
means the attachment of gaseous and aerosol pollutants to cloud droplets, ice crystals and raindrops
followed by droplet removal from the atmosphere to
the earth’s surface by rain or snow. For aerosols and
gases that are irreversibly captured by hydrometers,
wet deposition can be considered as an exponential decay process (Pandey et al., 2002). Usually, a
layer average wet scavenging coefficient is used in
long-range transport and deposition models. Numerous
researchers have sought to determine wet scavenging
coefficients, particularly for SO2 and SO4 −2 (Okita
et al., 1996). These data are generally based on model
calculations or estimated from measurements of the
scavenging ratio. Few field studies have been conducted to determine the wet scavenging coefficient
directly. The authors attempted to derive the scavenging coefficient for black carbon aerosols based on
Aethalometer measurements during January to December 2004 over a semi-arid urban environment, namely,
Hyderabad, India.
2. Study area and instrumentation
Hyderabad (Figure 1) is the fifth largest city in India.
It has twin cities, viz. Hyderabad and Secunderabad,
with its suburbs extending up to 16 km. Hyderabad
is situated 17◦ 10 to 17◦ 50 N latitude and 78◦ 10
to 78◦ 50 E of the longitude. The measurements have
been carried out in the premises of National Remote
Sensing Agency at Balanagar (17◦ 28 N and 78◦ 26 E),
 Crown Copyright 2005. Reproduced with the permission of Her Majesty’s Stationery Office. Published by John Wiley & Sons, Ltd.
Scavenging efficiency of rainfall on black carbon aerosols over an urban environment
149
India
Hyderabad
Figure 1. Location map showing study area
located well within the urban center. Black carbon
aerosol measurements have been carried out using
Aethalometer (Model AE-21) of Magee Scientific,
USA. The Aethalometer makes measurements of mass
concentration of aerosol black carbon by measuring
the attenuation of light transmitted through a quartz
filter tape on to which the ambient particles are
made to impinge. The attenuation of the intensity (I )
transmitted through the collecting part of the filter
relative to the intensity (I0 ) through the reference
part is A = 100 ln (I0 /I ) and is proportional to the
surface concentration of black carbon. More details are
available elsewhere (Liousse et al., 1993) (http://www.
mageesci.com/Aethalometer abook 2009.pdf).
3. Methodology
Airborne pollutants in the atmosphere are scavenged
by dry deposition and precipitation. In these deposition processes, pollutant mixing heights and precipitation intensity play an important role. Dry deposition
 Crown Copyright 2005. Reproduced with the permission of Her
Majesty’s Stationery Office. Published by John Wiley & Sons, Ltd.
is capable of filtering the larger particles from the
atmosphere in 2 to 3 days and would require several weeks to remove the more harmful sub-micronfraction, which is removed through various wet deposition processes (Okita et al., 1996). Ws is the wet
scavenging coefficient (s−1 ) and is dependent on particle size and rainfall intensity. From the regression
analysis in Figure 2, it has also been represented as a
function of the following form:
Ws = a(P )b
where P is the rainfall rate (mm/h) and values for
parameters ‘a’ and ‘b’ have been obtained from
the regression relation between BC concentration and
rainfall (Figure 3). The estimated values of ‘a’ and
‘b’ are 3.4091 and −0.0797 respectively in the present
study.
4. Results and discussion
Figure 4 shows seasonal variation of black carbon
aerosol mass concentration over the study area. Large
Atmos. Sci. Let. 6: 148–151 (2005)
K. M. Latha et al.
100000
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
Normal day
22:40
21:20
20:00
18:40
17:20
16:00
14:40
13:20
12:00
9:20
10:40
8:00
6:40
5:20
4:00
2:40
1:20
Rainy day
0:00
BC(ng m−3)
150
Time (h)
Figure 2. Diurnal variation of black carbon aerosol concentration during normal day (10 April 2004) and rainy day (15 April
2004)
18
16
BC(ug/m−3)
14
12
10
8
6
4
2
0
0
5
10
15
Rainfall (mm)
Figure 3. Black carbon aerosol concentration vs rainfall
intensity along with the trend line
concentrations of BC (∼15 µg/m3 ) during the dry
months (November to April) and low BC concentrations (∼6 µg/m3 ) during the monsoon months (June
to October). The mean BC levels over the study area
are higher than values reported for suburban sites in
the literature (Allen et al., 1999; Chen et al., 2001;
Bhugwant et al., 2001; Babu et al., 2002). The study
area under the influence of a continental air mass
from the north during November to April and the
air mass changes to marine from south-west during
May to September. The study area is purely urban
and is subjected to seasonal anthropogenic activities
and the observed seasonal changes in BC have bearing
on the synoptic meteorology and long-range transport.
Figure 4 indicates that during rainy days considerable
washout of BC takes place. High differential heating of land coupled with air masses transported from
continental regions enhances the BC loading during
dry months (Chen et al., 2001; Latha and Badarinath,
2003; Latha et al., 2004). Average atmospheric residence time of BC is about 7 to 10 days during dry
periods compared to ∼5 days or less during wet periods (Reddy and Venkataraman, 2000). The BC levels
for a normal day without rain (10 April 2004) and a
rainy day (15 April 2004) over the study area have
 Crown Copyright 2005. Reproduced with the permission of Her
Majesty’s Stationery Office. Published by John Wiley & Sons, Ltd.
been shown in Figure 2. The diurnal variations of BC
suggest that BC concentrations increased by a factor
of ∼2 during morning (6:00 to 9:00 h) and evening
hours (19:00 to 23:00 h) compared to afternoon hours
during both rainy days and normal days. During early
morning hours, high values of BC have been attributed
to the turbulence resulting from solar heating, which
breaks the nighttime stable layer and aerosols in the
nocturnal residual layer are mixed up with those near
the surface. Low values of BC during afternoon hours
have been attributed to the dispersion of aerosols due
to increases in boundary layer height in addition to
low traffic density. BC peaks during morning and
evening hours have been attributed to the increase in
traffic density. BC concentrations are found to be in
the range of 500 to 12 000 ng/m3 during a rainy day,
whereas, during a normal day, it ranges from 4000
to 88 200 ng/m3 . The normal BC concentrations are
observed to be high over the study area compared
to other studies (Babu et al., 2002; Bhugwant et al.,
2000; Jacobson, 2001; Offenberg and Baker, 2000).
Comparison of BC diurnal variations with the traffic
density suggests that BC concentrations over the study
area clearly related to traffic density patterns (Latha
et al., 2004; Latha and Badarinath, 2003). Both normal
and rainy days showed similar diurnal patterns of the
influence of vehicular exhausts on BC concentrations.
Drastic reduction in BC loading has been observed as a
result of the scavenging effect on rainy days (23 mm)
compared to normal days (no rainfall). Figure 3 shows
a statistical fit between black carbon aerosols and rainfall intensity, suggesting an inverse relation. A statistical fit through the data points shows a negative correlation and the estimated slope is ∼−0.36. Thus, every
1-mm increase in rainfall causes a ∼3.6 µg/m3 reduction in atmospheric black carbon aerosol flux over the
study area. This is the average reduction of aerosol
black carbon flux due to rainfall over the study area.
Black carbon is more hydrophobic than many inorganic aerosols such as sulfates and nitrates and is thus
expected to have a longer lifetime in the atmosphere
(Andronache, 2004). Estimation of wet deposition of
black carbon aerosol is important in studies related
to eco-environmental impacts. The scavenging coefficient of black carbon aerosols estimated from Figure 3
was found to be 1.64 × 10−5 s−1 .
5. Conclusions
The simultaneous measurements of black carbon
aerosol mass concentration and rainfall over an urban
environment suggests the following:
1. Seasonal variations of black carbon aerosol mass
concentration showed high concentrations during
dry season and low concentrations during monsoon
season.
2. Drastic reduction in black carbon aerosol loading
has been found during a rainy day compared to a
normal day.
Atmos. Sci. Let. 6: 148–151 (2005)
Scavenging efficiency of rainfall on black carbon aerosols over an urban environment
151
25
Winter
Monsoon
Summer
Winter
BC(ug/m^3)
20
15
10
5
364
355
347
339
331
317
309
298
290
279
268
258
249
235
226
169
161
153
139
126
117
105
97
74
63
55
25
14
1
0
Julian day
Figure 4. Julian day variations of black carbon aerosols during 2003. The thin, solid line shows the daily average black carbon
aerosol concentration and the thick, dotted line shows the five-day moving average
3. The statistical fit between black carbon aerosol
mass concentration and rainfall suggests ∼3.6
µg/m3 reduction in atmospheric black carbon
aerosol flux in the atmosphere for every 1-mm
increase in rainfall.
4. The scavenging coefficient of black carbon aerosols
is estimated to be 1.64 × 10−5 /s.
Acknowledgements
Authors are grateful to Director, NRSA and Dy Director
(RS&GIS), NRSA for their help and encouragement and to
ISRO-GBP for funding support.
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 Crown Copyright 2005. Reproduced with the permission of Her
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Atmos. Sci. Let. 6: 148–151 (2005)
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