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1. ORIGINAL PAPER
Aerosol pollution and its impact on regional climate
during Holi festival inferred from ground-based
and satellite remote sensing observations
C. P. Simha • P. C. S. Devara • S. K. Saha
Received: 22 May 2012 / Accepted: 23 May 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract In this paper, we report some salient features from a suit of special experiments
that have been conducted over a coastal site (Mumbai) during February 23–March 03, 2010,
encompassing an Indian festival, namely Holi, using solar radiometers and pyranometer. The
results of the analysis of observations at the experimental site show higher (more than
double) aerosol optical depth, water vapor, and lower down-welling short-wave radiative
flux during the festival period. This is considered to be due to anthropogenic activities and
associated meteorological conditions at the experimental location. To illustrate further,
Angstrom parameters (alpha, denoting the aerosol size distribution, and beta, representing
the loading) are examined. These parameters are found to be greater on Holi day as compared
to those on the normal (control, pre-, and post-Holi) days, suggesting an increase in accu-
mulation mode (smaller size) particle loading. The aerosol size spectra exhibited bimodal/
power-law distribution with a dominant peak, modulated by anthropogenic activities,
involving local and long-range transport of dust and smoke (emanated from biomass-
burning) aerosols, which is consistent with MODIS satellite observations. The aerosol direct
radiative forcing estimation indicated cooling at the bottom of the atmosphere.
Keywords Aerosols Ozone Precipitable water content Angstrom exponent
Holi festival MODIS Surface aerosol radiative forcing Meteorological parameters
Climate change
1 Introduction
Anthropogenic emissions are major sources of atmospheric aerosols. Aerosol pollutants
can adversely affect human health and also have impacts on climate and precipitation on a
regional scale (Penner and Novakov 1996). Natural sources such as volcanoes are also a
large source of aerosols and have been linked to changes in the earth’s climate, often with
C. P. Simha P. C. S. Devara () S. K. Saha
Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pune 411 008, India
e-mail: devara@tropmet.res.in
123
Nat Hazards
DOI 10.1007/s11069-013-0743-6

2. consequences for the human population. The increased levels of fine particles in the air as a
result of anthropogenic activities are consistently and independently related to the most
serious effects, including lung cancer and other cardiopulmonary mortality. The effects of
inhaling fine particles that have been widely studied in humans and animals now include
asthma, lung cancer, cardiovascular issues, respiratory deceases, birth defects, and pre-
mature deaths (Pope et al. 2002). Biomass-burning, which is widely prevalent in the
tropics, serves to clear land for shifting cultivation and expanding population (Dwyer et al.
1998). Anthropogenic activities of this kind during festival periods over urban/metropol-
itan cities deteriorate local/regional air pollution, visibility, and human health issues. It
produces large amounts of trace gases and aerosol particles, which play a pivotal role in
tropospheric chemistry and climate (Arola et al. 2007, Crutzen and Andreae 1990). Aerosol
particles emitted from biomass-burning will also act as major source of cloud condensation
nuclei (CCN), which affect the microphysics of boundary-layer clouds and hence the
radiation budget of the earth by increasing the albedo (Penner and Novakov 1996). Smoke
particles and generation from heat from biomass-burning may have a significant impact on
(modulate) local meteorology (for example, Devara et al. 1994) and in turn on climate by
altering the global radiation balance. Each year, more than 100 million tons of smoke
aerosols are released into the atmosphere as a result of biomass-burning (Hao and Liu
1994). More than 80% of this burning activity in the tropical regions produces submicron
smoke aerosols that play a major role in the radiation balance of the earth–atmosphere
system (Balis et al. 2004; Kaufman and Nobre 1998). Because of this, they reflect
incoming solar radiation back to space, thereby reducing the amount of sunlight reaching
the earth’s surface (Christopher et al. 2000; Reid et al. 2005a, b).
2 Holi festival
Festival activities in every country perturb the radiation balance between the earth and
atmosphere due to additional anthropogenic activities in different ways. ‘Holi,’ the festival
of colors, is one such major festival in India. It is celebrated usually in the months of
February and March. It is mainly associated with the burning of Holika, and people
celebrate the festival by smearing each other with paint, and throwing colored powder and
dye around in an atmosphere of great good humor. On this occasion, the firewood is
arranged in a huge pile at a clearing in the locality. In the evening, the fire is lit. Then, high
concentrations of anthropogenic aerosols and toxic material are injected into the atmo-
sphere due to colored powder and fireworks, especially in urban regions. Thus, vast
amounts of various chemicals and particulate matter enter into the atmosphere. Some
chemicals which are in the Aitken nuclei range (0.01–0.1 lm) convert into the accumu-
lation mode (0.1–1 lm) particles by means of gas-to-particle conversion processes and
affect the atmospheric temperature and incoming solar flux.
Some researchers (Martin 2007; Lelieveld et al. 2001) pointed out that the Bombay
plumes during the winter monsoon transport black carbon-rich air from Western/Northwest
India over the Arabian Sea (AS). Thus, Mumbai is a source of anthropogenic aerosols such as
CO and VOCs (black carbon) as well as NOx and O3 (Phadnis et al. 2002). Ramachandran
(2004) reported that columnar AOD increases with increase in the marine boundary-layer
aerosol concentrations over coastal India and Arabian Sea, while an opposite trend is seen
over tropical Indian Ocean. The differences in the surface and columnar measurements could
also occur due to changes in the meteorological conditions in addition to changes in pro-
duction and subsequently the transport of aerosols. Aerosol extinction and mass measured
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3. over coastal India, the Arabian Sea, and the tropical Indian Ocean are found to show large
spatial variations during the winter monsoon (Moorthy et al. 2001; Ramanathan et al. 2001;
Ramachandran and Jayaraman 2002; Ramachandran 2004). The 5-year mean (1996–2000)
variations show that the aerosol mass concentrations over coastal India in the coarse,
accumulation, and nucleation modes are higher than those measured over the Arabian Sea
and the tropical Indian Ocean (Ramachandran and Jayaraman 2003). Studies by Krish-
namurti et al. (1997) have identified the dominant flow in the region at lower levels as
northeasterly during winter monsoon months. Due to the limited knowledge of aerosol
sources, composition, properties, and processes, the actual effects of aerosols on climate and
health are still far from being fully understood. In this paper, we report some of the char-
acteristic features such as relative changes in aerosol loading, size distribution, and radiative
forcing exerted by these modified aerosols at the earth’s surface over the coastal region of the
metropolitan city (Mumbai, formerly known as Bombay). Some of these features have been
compared with concurrent space-borne (MODIS) measurements during the study period.
3 Experimental site
The experimental location (Mumbai) is situated about midway on the western coast of
India (19°230
N, 72°500
E) and is a peninsular city joining the mainland at its northern end
(Fig. 1). Large petrochemical, fertilizer, and power plants are located to the eastern and
Fig. 1 Map showing the location (Star) of the experimental site
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4. southeastern sectors of the city. Several thousand medium and small-scale industries are
located in the city including chemical, textile and dyeing, pharmaceutical, paint and pig-
ment, and metal-working industries. The land-use pattern is industrial-cum-residential with
a total population of over 10 million and a population density of 16,500 persons Km-2
.
Sources of particulate matter in the city include vehicular emissions, power plants,
industrial oil burning, and refuse burning plants (Larssen et al. 1997). Observations were
collected from the roof of a five-stored building, near Versosva, West Andheri, Mumbai.
This represents a background urban site, over 3 km inland from the coast, and is likely to
receive both marine and continental aerosols. The site is located sufficiently away from the
transportation or industrial sources, with the traffic roadway about 1 km to the east and
industrial cluster about 3 km to the southeast. However, burning of refuse and biomass
(leaves, garden waste) is an intermittent, local source throughout Mumbai, especially at
nighttime in winter (December–January).
The daily mean air temperature and relative humidity during the pre-Holi, Holi, and
post-Holi periods over the experimental site are plotted in Fig. 2. Temperature was in the
range of 28.5–30° C, and the relative humidity (RH) varied from a low of 54 % to a high of
about 70 %. The variations in relative humidity play an important role in the coagulation
and growth processes of aerosol particles, while the temperature variations influence the
aerosol size distribution. Higher humidity values also favor the gas-to-particle conversion
(secondary aerosols) process, and thereby, they affect the number density. The daily
average higher temperatures might lead several chemical reactions and cause production of
the most of the aerosols in the Aitken and accumulation mode.
4 Experimental setup, principle of operation, and analysis of data
We used two compact, online, multi-wavelength solar radiometers (MICROTOPS-II,
manufactured by M/s Solar Light Co., USA) in the present experiment. These instruments
yield instantaneous estimates of aerosol and gaseous optical depths (extinction) as opposed
to conventional ones (Devara et al. 2001). One of these radiometers (sun photometer)
provides height-integrated AOD at six wavelengths covering from UV to NIR and hence
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72
Experimental days
Temperature
(°C)
Temperature (°C)
Relative
Humidity
(%)
R. H (%)
Fig. 2 Time variation of
surface-level temperature and
humidity on experimental days
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5. the size distribution of aerosols, and the other radiometer (ozone and perceptible water
content monitor) determines the total column ozone (using the UV band) and perceptible
water content (using the NIR band) simultaneously. Both the radiometers are mounted on a
single wooden platform which in turn fixed to a tripod for achieving high stability, time
synchronization between observations and easy focusing of radiometers to the sun’s disk.
Each radiometer is equipped with internal barometer/altimeter for monitoring the atmo-
spheric pressure and altitude of the experimental location. The global positioning system
(GPS) receiver provides the geographical coordinates of the site, which are used for
estimating the local air mass. The experiments have been conducted on the terrace of the
building to make the radiometers free from nearby topographic targets like tall buildings,
trees, etc. The main parameters of these radiometers are presented in Table 1.
Both the radiometers are tailored with optical collimators accurately aligned with their
fields-of-view (FOVs) and baffles for eliminating internal reflection. Each channel is fitted
with a narrow-band filter and a gallium phosphide (GaP) detector suitable for the particular
wavelength range. A sun target and a pointing assembly are permanently attached to the
optical block and laser aligned to ensure accurate alignment with the optical channels.
When the image of the sun is centered at the cross-hairs of sun target, then all channels are
looking directly at the solar disk. The radiation captured by the collimator and band-passed
by the filters falls onto the photodiode that produces an electrical output proportional to the
radiant power (irradiance). These outputs measured at each optical filter are amplified and
analog-to-digital converted, and finally stored, together with the time of observation pro-
vided by the built-in master clock, in the memory for further analysis. The LCD on sun
photometer provides instantaneous display of AOD, record number and several other
information like power density, etc., recorded at each wavelength. Similarly, the LCD on
the monitor provides instantaneous display of total column ozone and perceptible water
content together with other parameters such as received solar power density at each filter.
Moreover, these radiometers are equipped with built-in algorithms for computing the
ozone, precipitable water content and AOD from the output of the amplifier recorded for
each filter. The radiometers are operated initially by keeping the cover closed for the
optical blocks (consisting of windows, filters, etc.). During this period, the instrument
stores the background values for all the filters. In the next few seconds, on removing the
cover, it collects a set of over 25 observations for each filter. The average value, thus
obtained for each filter, is used to compute the spectral variation of columnar AOD, ozone,
and perceptible water content instantaneously and is depicted on the display for a quick
Table 1 Main specifications of
the portable, online, aerosol,
ozone, and precipitable water
content monitor
Parameter Value/description
Sun photometer
Filter wavelengths 380, 440, 500, 675, 870, and
1,020 nm (5–10 nm FWHM)
Ozone monitor
305.5, 312.5, and 320 nm
(5 nm FWHM)
Precipitable water
content monitor
940 and 1,020 nm (10 nm FWHM)
Field-of-view 2.5°
Dynamic range 3 9 104
Data storage 800 records
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6. look and stored in the memory. The ozone monitor used in the present experiment utilizes
this relationship to derive the total ozone column (the equivalent thickness of pure ozone
layer at standard temperature and pressure) from the measurements of two wavelengths in
the UV region (305.5 and 320 nm). The measurement at additional third wavelength
(312.5 nm) enables correction for particulate scattering and stray light. The columnar
precipitable water content is determined based on the measurements at 940 nm (H2O
absorption peak) and at 1,020 nm (no absorption by water content). The AOD at 1,020 nm
is also calculated based on the extra-terrestrial radiation at that wavelength, corrected for
sun–earth distance, and the ground-level measurements of the radiation at 1,020 nm. This
optical depth at NIR in conjunction with those from the sun photometer at UV and V
regions is utilized to retrieve columnar aerosol size distribution by applying the constrained
linear inversion method (King et al. 1978; King 1982). This method for retrieving the
aerosol size distribution from AOD data has been carried out mainly in two steps. The first
step is for finding out the optimum particle radius range. Having found this, the next step is
for finding out the number concentration [dNc(r)/dlog r] at these radii and also for finding
out a factor Rei
2
that monitors the deviations between the observed and back-calculated
optical depth for a number of iterations. The size distribution corresponds to the iteration
for which the minimum Rei
2
value is considered as the best. The error involved in such
method is less than ±10%. The observed ASDs exhibit a power-law distribution with
different exponents changing at an intermediate size. Hence, this modified Junge power-
law distribution can be approximated by combining the distribution factors with different
components with a switching radius. These exponents and switching radius can be
determined by fitting the size distribution function to the inverted mean size at each step.
More details of the exact procedure can be found in Pandithurai et al. (1997).
5 Results and discussion
5.1 AOD during Holi
The daily mean aerosol optical depth (AOD), estimated for the entire Holi campaign
period, is displayed in Fig. 3. The standard error of the mean aerosol optical depth,
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Experimental days
Aerosol
optical
depth
AOD 380 nm
AOD 440 nm
AOD 500 nm
AOD 675 nm
AOD 870 nm
AOD 1020 nm
Fig. 3 Day-to-day variation of
wavelength dependency of
aerosol optical depth on
experimental days. The Holi
festival period is indicated with a
dashed circle
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7. represented by the vertical bar on each day, is a measure of the scatter of individual optical
depth that has been used to obtain the diurnal averages. The plot shows significant day-to-
day variations of AOD and its dependence on wavelength. The daily mean AOD values at
380, 440, 500, 675, 870, 1,020 nm indicate maximum (more than 0.85) during the Holi
day. These variations in spectral dependence are found to have influence on the derived
aerosol size distributions which are explained in the forthcoming section. This increase is
largely due to biomass-burning and smearing of some colors (in both dry and wet form)
such as red and royal pink (mercury), golden powder (lead, mercury, and cadmium), and
metallic green powder (lead and cadmium) during the Holi festival. It is also interesting to
note a decrease in AOD before and after the Holi period due to the dust-free clean air
condition due to scavenging and wet removal processes during this campaign. AODs were
initially in the range of 0.4–0.5 up to February 28, 2010, but due to the Holi affect, it
increased to 0.85, and thereafter, AOD slowly decreased to the initial value on completion
of the festival on March 3, 2010 onward.
The spectral dependence of aerosol optical depth is typically approximated using
Angstrom’s formula that proposes that extinction of solar radiation by aerosols is a con-
tinuous function of wavelength, without selective bands or lines for scattering or absorp-
tion. Thus, the Angstrom turbidity formula is given as
s ¼ b ka
ð1Þ
where s is the optical depth, k is the wavelength, b is the turbidity coefficient, and a is the
wavelength coefficient (Angstrom 1961). Cachorro et al. (2000) showed that Angstrom
formula provides a good spectral representation of atmospheric aerosol attenuation. From
Eq. 1, b known to vary from 0 to 0.5 or even higher is an index representing the amount of
aerosols present in the atmosphere in the vertical direction. This parameter characterizes
the spectral features of aerosols and it relates to the size of the particles (Shifrin 1995).
Large values of a indicate a relatively high ratio of smaller to larger particles. It is expected
that when the aerosol particles are very small of the order of air molecules, a should
approach 4 and it should approach 0 for very large particles (Holben et al. 2001; Pinker
et al. 2001). The Angstrom exponents in the present study have been computed using the
AODs measured at all the six wavelengths (380, 440, 500, 675, 870, 1,020 nm) of the
radiometer. The mean wavelength exponent (a380-1,020 nm), thus obtained, for the exper-
imental site was 0.671 ± 0.334 as shown in Fig. 4. This parameter was very low (0.2) on
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Experimental days
Beta
Alpha
Alpha
Beta
Fig. 4 Day-to-day variation of
Angstrom parameters (Alpha and
Beta) during the study period
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8. and February 24 and 25, 2010, which indicates the presence of coarse-mode particle, but it
increased to 0.6 due to anthropogenic activity and it reached to very high around 1.0 due to
Holi effect. Aerosols in the nucleation size range of 0.01–0.1 lm, undergo condensational
growth or coagulate to form bigger particles, and contribute to accumulation mode
(0.1–1 lm). The precursor gases for the formation of particles in the smaller size range
come mostly from the continents, such as sulfur dioxide, which through oxidation and
condensation get transformed into sulfate aerosol and get transported over long distances in
the marine atmosphere. Such particles are found in abundance near the coastal regions (Qiu
1998). The slightly larger size aerosols in the accumulation mode are relatively longer
lived when compared to the smaller size aerosols, and they also can get dispersed over
longer distances. The larger particles dominating the Indian Ocean region indicate that they
could be mainly sea-spray particles, but the contribution from the windblown mineral dust
from the surrounding continents cannot be ruled out. The mean turbidity parameter or the
Angstrom coefficient b which represents the total aerosol loading in the atmosphere over
the coastal site is found to be 0.10. The mean b values over coastal site, for the month of
February and March 2010, are estimated to be about 0.318 ± 0.114 (Fig 4). The loading is
low (*0.15) during initial period of the campaign (February 25, 2010), but loading sud-
denly increased to higher value on March 1, 2010 (0.55), March 2, 2010 (0.66), and again
decreased on March 3, 2010 (0.26).
5.2 Comparison with MODIS satellite observations
In the comparison of sun photometer- and MODIS-derived AOD at the experimental site,
we need to maintain the time difference between MODIS overpass and the ground-based
AOD measurement as minimum as possible. In order to maintain the spatial coherence
better, we included MODIS data only when the distance from the center of MODIS pixel to
the experimental station was less than 0.2°, both in latitude and in longitude. Figure 5
depicts the comparison between MODIS and MICROTOPS measurements at 500 nm. The
data period covered is from February 24 to March 3, 2010. The correspondence is very
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Experimental days
AOD
at
550
nm
(MODIS)
AOD
at
550
nm
(MICROTOPS)
AOD at 550 nm (MICROTOPS)
AOD at 550 nm (MODIS)
R = 0.86
Fig. 5 Comparison between aerosol optical depths observed at characteristic wavelength of 550 nm
between MICROTOPS and MODIS during the study period
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9. good with a linear correlation coefficient of 0.88, which is partly due to the fact that these
periods exhibited a clear range of AOD values, including both very high AOD values and
significantly lower values on some days, when the air masses arrived from outside bio-
mass-burning areas. The linear fit applied between the AODs from MICROTOPS and
MODIS showed correlation of 0.865 ± 0.087 (Fig. 6).
5.3 Short-term variations in aerosol size distribution
Figure 7 displays daily variation of aerosol size distribution (ASD), inverted from the
spectral distribution of AOD following the constrained linear numerical inversion scheme
as suggested by King et al. (1978) and King (1982). Aerosol turbidity shows accumulation
mode type of aerosol during the observation campaign. The columnar ASD, thus evaluated
from the above methodology, for the experimental days is shown in Fig. 7. Most of the
time, either power-law or bimodal distribution was noticed. A bimodal distribution was
observed on February 24, 25, and 27 and March 3, 2010, and power-law distribution was
noticed during the remaining period. The b value is very low and a value is high on
February 28, 2010, indicating the fine mode dominance in the aerosol size distribution
spectrum. This loading, dominated by the fine mode particles, continued to increase for
some time even after the Holi because of their lighter mass.
5.4 Daily mean variations of PWC and ozone
The daily averages of PWC (Fig. 8) varied from 0.85 to 1.6 cm during the campaign
period. At the commencement of the experiment (February 24, 2010), PWC was very low
(*0.85 cm); thereafter, the AOD and PWC almost followed each other, which clearly
signifies the hydrophilic nature of the particles on the eve of Holi festival. This aspect
implies that the higher water content helped the growth of existing particles and also the
formation of new particles, both resulted in higher AOD. The daily mean ozone indicated
somewhat different trend, but in the beginning of the campaign, it was low (*215 DU) but
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
AOD
at
550
nm
(MODIS)
AOD at 550 nm (MICROTOPS)
R=0.867+/- 0.087
N=8
Fig. 6 Correlation between AODs observed from MICROTOPS and MODIS
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10. rose to 260 DU on 1 March and decreased (*250 DB) on the festive day (Fig. 9). This
opposite relationship between AOD and ozone variations on March 2, 2010, clearly hints
the role of heterogeneous chemistry, similar to the result reported in the previous works by
Devara et al. (2001).
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24
th
Feb 10
25
th
Feb 10
26
th
Feb 10
27
th
Feb 10
dN
c
/d
(Log
R)
(cm
-2
µ
−1
)
Radius (µm)
28
th
Feb 10
1
st
March 10
2
nd
March 10
3
rd
March 10
Fig. 7 Aerosol size distributions observed on different experimetnal days
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Experimental days
Precipitable
water
content
(cm)
Fig. 8 Time variation of
precipitable water content on
experimental days
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11. 5.5 Long-range transport of aerosols
Figure 10 shows one-, three-, five-, and seven-day back trajectories [at three characteristic
altitudes of 500 m (red color), 1,500 m (blue color), and 2,000 m (green color)] with
reference to March 3, 2010 (post-Holi day) over the region, obtained from NOAA HY-
SPLIT (National Oceanic and Atmospheric Administration Hybrid Single-Particle
Lagrangian Integrated Trajectory) version 4 model (Draxler and Hess 1998). The plots
clearly display that different air masses (originating mainly from the Arabian Sea and
partly from the Indian subcontinent) influenced the AOD over the observational site. It is
also evident from the figure that in all cases, the higher altitude air masses originate from
Arabian Sea and Indian subcontinent without traveling over longer distance, while at lower
altitudes, air masses travel more distance and affect significantly the trajectory direction on
certain days.
5.6 Surface radiative forcing
The pyranometer-measured global short-wave (SW) flux in the wavelength region between
0.3 and 3.0 lm is correlated with instantaneous AOD at 500 nm after correcting to the air
mass factor (1/l) so as to calculate the radiative forcing (Jayaraman et al. 1998). Nor-
malization of the AOD with l (=cos h) is found to be necessary as the slant air column
length increases with increasing solar zenith angle, h. The observed solar flux represents
the solar flux at the surface, normal to the angle of incidence, with a cone of about 2.5°
around the Sun. The data for solar zenith angles greater than 60° are excluded (to avoid
earth’s curvature effect) and the AOD/l values are restricted to within 0.8. Figure 11
shows daily variation of the surface SW aerosol radiative forcing, which is derived from
the scatter plot of the measured-normalized SW flux with AOD for the experimental days
during the festival eve. A straight line could be fitted to the data with a negative slope
(radiative efficiency) of about 781.701, 318.88, 284.05, 295.795, and 451.477 W/m2
per
unit AOD for February 24, 25, 26, and 28 and March 3, 2010 days, respectively. This
2
4
-
F
e
b
-
1
0
2
5
-
F
e
b
-
1
0
2
6
-
F
e
b
-
1
0
2
7
-
F
e
b
-
1
0
2
8
-
F
e
b
-
1
0
1
-
M
a
r
-
1
0
2
-
M
a
r
-
1
0
3
-
M
a
r
-
1
0
-
-
160
180
200
220
240
260
280
300
Experimental days
Ozone
(DU)
Fig. 9 Day-to-day variation of
columnar ozone during the study
period
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12. implies that for a 0.1 increase in the prescribed columnar AOD, the direct visible solar flux
at surface decreases by about 78.1, 31.8, 28.4, 29.5, and 45.1W/m2
for the above days.
Such a range of forcing values have been reported over the Arabian Sea and higher values
during volcanic episodes in the literature (for example, Suresh Babu et al. 2007; Young
et al. 2012). The greater aerosol forcing at the bottom of the atmosphere on clear-sky days
Fig. 10 1-day (a), 3-day (b), 5-day (c), and 7-day (d) back trajectories at 500 m, 1,500 m, and 2,000 m
levels observed with respect to March 3, 2010
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13. over a coastal site during the Holi festival, as expected, could be due to higher aerosol load,
which attenuates the surface-reaching solar irradiance.
6 Summary and conclusions
The atmosphere over the coastal site (West Andheri, Mumbai) is found to be relatively
clean from aerosols before the Holi period (between March 1 and 2, 2010). During that
time, biomass-burning and toxic aerosols from Indian subcontinent are noticed to transport
to the experimental site and to the other parts of India. The present paper reports a compact
study, exploiting AOD measurements on such occasions. Moreover, concurrent MODIS
satellite AOD measurements obtained at the above site were also used for monitoring the
spatial and temporal evolution of particulate matter spread from these fires. In the first part
of our study, we compared MICROTOPS and MODIS AOD at coastal site, West Andheri.
In this comparison, we found a very good correlation, although it is emphasized that this
eight-day period (from February 24 to March 3, 2010) is rather short to be compared
against previous MODIS validation studies with longer-term data sets. However, the
present results clearly indicate the potential MODIS AOD data offer in AOD monitoring.
During this event of biomass-burning aerosols, clear AOD variability was apparent in the
measurements. This large range in AOD obviously partly explains the high correlation
(0.86) between MODIS and MICROTOPS measurements. The day-to-day AOD variability
was also investigated using HYSPLIT generated trajectories. The backward trajectories up
to seven days (in steps of two days) from the post-festive day were used to assess the origin
of the air masses arriving at experimental site. The air mass mostly from the Arabian Sea
and Indian subcontinent contributes to the aerosols over the experimental sire. The surface
aerosol radiative forcing over the observational (coastal) site showed variation between
28.4 and 78.1 W/m2
, which is consistent with the results reported by earlier investigators.
24-Feb-10
25-Feb-10
26-Feb-10
28-Feb-10
3-March-10
20
30
40
50
60
70
80
Aerosol
radiative
forcing
(wm
-2
)
Experimental days
Surface radiative forcing
Fig. 11 Day-to-day variation of aerosol surface radiative forcing during the study period
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14. Acknowledgments This work was supported by the ISRO-SAC, OCEAN SAT-II project. The authors are
grateful to the Editor and anonymous Reviewers for their critical comments and valuable suggestions.
Authors are thankful to Latha Shenoy of the Central Institute for Fisheries and Education (CIFE), West
Andheri; and DRDO, Mumbai, for providing infrastructure support for observations. Thanks are also due to
Director, IITM, for infrastructure support. The authors gratefully acknowledge the NOAA Air Resources
Laboratory (ARL) for providing the HYSPLIT transport and dispersion model and/or READY website
(http://www.arl.noaa.gov/ready.html). We are thankful to GES-DISC DAAC On-line Visualization and
Analysis System: Web-based interface for the visualization and analysis of the MODIS data
(http://g0dup05u.ecs.nasa.gov/Giovanni/).
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