Bhuyan, P. K.

Gogoi, M. M.

Krishna Moorthy, K.

English

Publications of the IAS Fellows

SCIENTIFIC CORRESPONDENCE 
 
CURRENT SCIENCE, VOL. 92, NO. 8, 25 APRIL 2007 1047 
Possible impact of a major oil-well fire on aerosol optical depth at  
Dibrugarh 
 
The role of atmospheric aerosols in modi-
fying the radiation budget of the earth–
atmosphere system with implications on 
regional to global climate is being in-
creasingly understood and recognized1–5. 
Due to the short residence times, aerosols 
with varying optical properties are not 
uniformly mixed around the globe and 
consequently, their chemical and physical 
characteristics including radiative proper-
ties are subject to significant regional 
variations6. Over land, this arises primar-
ily from the wide variety of source proc-
esses, while the transport of aerosols from 
different source regions on the continents 
to the remote oceans could play a signi-
ficant role in the global radiative forcing. 
Among the various natural and anthro-
pogenic sources of aerosols, man’s do-
mestic and industrial activities contribute 
a dominant share of the tropospheric 
aerosol burden. The major categories in-
clude combustion of fuels and incinera-
tion, industrial processing and production. 
Forest, and oil and gas fires continue to 
contribute to aerosol loading all around 
the globe. 
 A devastating fire broke out at oil well 
no. 15 of the Oil India Limited (OIL) at 
Dikom, Upper Assam on 15 September 
2005 and continued to blaze for about 
three weeks, often reaching a height of 
about 10 m. There was gas leakage in the 
area two days ahead of the day of fire at 
Dikom, which resulted in the major in-
ferno. The sky in the area was black with 
thick smoke, ash and debris scattered 
everywhere and a film of crude oil cov-
ered the ground. The site of the blaze is 
about 15 km east of Dibrugarh (27.3°N, 
94.6°E). The area around Dibrugarh is 
rich in mineral deposits and has large 
deposits of crude oil and coal. Majority 
of the oil fields are located to the east 
and southeast of Dibrugarh town. 
 Aerosol optical depth (AOD) is being 
measured over Dibrugarh since October 
2001, at ten narrow wavelength bands 
centred at 380, 400, 450, 500, 600, 650, 
750, 850, 935 and 1025 nm using a Multi 
Wavelength Radiometer (MWR). The 
MWR works on the principle of filter 
wheel radiometers7,8 and is operated from 
the rooftop of the building of the Physics 
Department, Dibrugarh University (~ 8 m 
above ground level) on all days when 
unobscured solar visibility is available 
for at least 2–3 h, and AOD is retrieved 
following the Langley technique. Details 
of application of this technique to ana-
lyse MWR data and the errors involved 
are described by Moorthy et al.9,10. The 
experiment conducted during September 
2005 is considered to analyse the impact 
of the fire on AOD over the area.  A total 
of eight days (2, 10, 12, 14, 15, 18, 26 
and 28 September 2005) of MWR data 
during the period of observation have 
been analysed and the results are re-
ported here. 
 Variations of daily mean AOD values 
at 500 nm along with the MODIS (Terra 
and Aqua satellites)-derived AOD11 at 
550 nm are shown in Figure 1 a. Com-
parison for the observational period shows 
a good agreement between the ground-
based and satellite-derived AOD. AOD 
observed by MWR in the pre-fire days, 
i.e. first half of September was low and 
lies between a minimum of 0.06 at 935 
and 1025 nm and a maximum of 0.81 at 
450 nm, which is about the average Sep-
tember AOD observed in previous years, 
2002–04. After two days of fire, on 18 
September, AOD increased by 69 to 
86%, within the wavelength range. No 
MWR observation was possible due to 
cloudy and rainy weather on 16, 17, 19 to 
25 and 27 September. Light to moderate 
rainfall occurred on 19–24 September, 
which may have washed out the suspended 
aerosols. Towards the end of the month, 
the AOD was low as in the pre-fire days. 
However, on days in which the MWR 
observations were not available, the MODIS 
satellite-derived AOD data are available. 
AOD as observed by Aqua satellite 
reached a high value (~2.86) just after two 
days of fire and then gradually decreased 
towards the end of the month. On 22 Sep-
tember also, the MODIS AOD remained 
much above the September average at 
1.69. Such abnormal increase in AOD is 
possible only when some external source 
 
 
Figure 1. a, Variation of daily mean AOD values measured using MWR at 500 nm and 
MODIS satellite at 550 nm. b, Daily total rainfall during September 2005. 
SCIENTIFIC CORRESPONDENCE 
 
CURRENT SCIENCE, VOL. 92, NO. 8, 25 APRIL 2007 1048 
mechanism contributes significantly to 
aerosol concentration in the atmosphere. 
The Terra satellite observation was not 
available on 18 September. We shall now 
investigate the possible impact of the fire 
on the total aerosol loading in the atmos-
phere over this location and on the distri-
bution of finer and coarse-mode aerosols. 
 The spectral variation of AOD bears 
the signature of the columnar size distri-
bution of aerosols. Inferences on the size 
spectra of aerosols can be obtained read-
ily from the corresponding AOD spectra 
[τp(λ)] by estimating the Ångström pa-
rameters in the expression12 
 
 
τp(λ) = βλ–α, (1) 
 
where wavelength exponent α is related 
to aerosol size distribution and is a measure 
of the relative dominance of fine, sub-
micron aerosols over coarse aerosols, 
while β represents the total aerosols pre-
sent in the atmosphere in the vertical di-
rection, and λ is the wavelength (in µm). 
A higher value of α indicates a sharper 
aerosol size spectrum, dominated by smaller 
aerosols. Estimates of α and β were 
made by least square fitting eq. (1) to the 
measured AOD spectra on a log–log 
scale. The values of α and β having cor-
relation coefficient ρ lying in the range  
–0.60 to –0.99 on the respective days of 
observation are given in Table 1, along 
with the corresponding values of AOD at 
500 nm. Values of α range from 0.86 to 
1.38 during the pre-fire days. During 
post-fire days, the highest value of α was 
1.66 along with the highest value of β 
(~0.39). This indicates increased abun-
dance of fine as well as coarse-mode par-
ticles on a post-fire day. The aspect is 
critically examined by estimating colum-
nar size distribution of aerosols. 
 From the aerosol spectral optical depths, 
the columnar size distribution (CSD) 
functions of aerosols have been deduced 
by numerical inversion of the Mie inte-
gral equation 
 
 
2
p ext c( , , ) ( ) d ,
b
a
r
r
r Q m r n r rλτ pi λ= ∫  (2) 
 
where Qext is the aerosol extinction effi-
ciency factor which depends on the aero-
sol refractive index (m), radius (r) and 
wavelength of incident radiation (λ), 
nc(r) is the columnar size distribution 
function of aerosols, and ra and rb are re-
spectively, the lower and upper radii lim-
its for integration. Equation (2) is solved 
following the iterative inversion proce-
dure described by King13 and as applied 
by Moorthy et al.9. 
 The size distributions for all days 
showed unimodal characteristics, except 
on 28 September in the radius range used 
in the present study. To reproduce the 
gross features of the size distribution in a 
quantitative manner, the observed changes 
in the retrieved CSD are represented in 
terms of appropriate analytical functions, 
and the physical parameters of the aero-
sol size distributions such as geometric 
mean (mode) radii (rmi), geometric mean 
standard deviations (σi), effective radius 
(reff), columnar mass loading (mL), colum-
nar concentration of small particles (Na) 
and coarse particles (Nc) are deduced 
from CSD. The various parameters thus 
determined are given in Table 2.  
 The columnar mass loading mL was 
highest on 18 September during the post-
fire period and then decreased towards the 
end of the month. Higher value of reff 
(~0.40 µm) was seen towards the end of 
the month. Mass loading is sensitive to 
the large particle end of the distribution, 
and small changes in the number of large 
particles can influence mass loading signi-
ficantly, whereas reff, the ratio of the to-
tal volume to area of aerosols depends 
more on the relative dominance of large 
aerosols over smaller ones. Changes in the 
number of aerosols in the small particle 
end of the distribution do not affect mass 
loading significantly, whereas effective 
radius is affected. Increase in reff towards 
the end of the month is indicative of an 
increase in the relative concentration of 
larger particles.  
 This aspect is examined further by esti-
mating the columnar concentration of 
small particles (Na) and coarse particles 
(Nc). Table 2 gives the relative dominance 
of the number concentration of coarse 
particles Nc with reference to Na. It is clear 
that the relative abundance of large parti-
cles (Nc/Na) was highest at the end of the 
month. However, concentration of both 
fine-mode (Na) and coarse-mode (Nc) 
aerosols was highest on 18 September. 
 Rainfall and wind can change the rela-
tive abundance of aerosols at any location. 
As already mentioned, light to moderate 
rainfall occurred from 19 to 24 Septem-
ber, which may be the reason for de-
creased concentration of both fine- and 
coarse-mode aerosols. The higher values 
of α and β (Table 1) as well as those of 
Nc and Na (Table 2) on 18 September indi- 
 
 
Table 1. Daily mean aerosol optical depth  
 (AOD) at 500 nm and Ångström parameters 
Dates in 
September Mean AOD 
2005 at 500 nm α β 
 2 0.35 0.86 0.17 
10 0.18 1.38 0.06 
12 0.53 1.34 0.17 
14 0.64 1.30 0.25 
15 0.24 0.87 0.11 
18 0.97 1.17 0.39 
26 0.22 1.66 0.06 
28 0.34 0.72 0.22 
 
 
Table 2. Parameters deduced from aerosol size-distribution analysis 
Dates in 
September 
2005 rm1 (µm) σ1 rm2 (µm) σ2 mL (mg m–2) reff (µm) Nc  (1010 m–2) Na  (1013 m–2) Nc/Na  (10–3 m–2) 
 
 2 0.06 0.77   116.9 0.25 0.98 0.27 3.58 
10 0.11 0.54   67.6 0.27 0.53 0.13 3.82 
12 0.08 0.59   158.1 0.18 0.56 0.72 0.76 
14 0.04 0.78   236.1 0.16 1.24 1.62 0.76 
15 0.1 0.64   85.6 0.19 0.59 0.41 1.42 
18 0.12 0.56   328.2 0.18 1.97 1.69 1.16 
26 0.16 0.39   65.8 0.16 0.14 0.38 0.38 
28 0.17 0.40 1.15 0.25 176.7 0.40 0.65 0.12 5.49 
SCIENTIFIC CORRESPONDENCE 
 
CURRENT SCIENCE, VOL. 92, NO. 8, 25 APRIL 2007 1049 
 
 
Figure 2. Plots showing forward wind trajectories obtained from NOAA HYSPLIT on selected post-fire days. Trajectories originate from the site 
of fire. 
 
 
cate that the fire released higher than 
normal aerosols into the atmosphere. De-
crease in AOD towards the end of the 
month may also be the result of wind 
blowing away the aerosols from over the 
area of observation in addition to rain 
wash. Surface winds over Dibrugarh are 
generally ~2–3 ms–1 and easterly for most 
of the year. Analyses of surface winds as 
well as forward trajectory analysis using 
the NOAA HYSPLIT model show that 
till 18 September, winds at the surface 
and higher levels were northeasterly and 
southeasterly that brought forth aerosols 
generated by the fire towards the obser-
vational site. As seen in Figure 2, after 18 
September, winds at all levels changed 
direction by about 180° or became south-
westerly, thus carrying away the aerosols 
in the opposite direction from the obser-
vational site. This accentuated by rain 
wash may be the reason for gradual de-
crease of AOD from 18 September onwards 
and low AOD on 26 and 28 September. 
 
 
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ACKNOWLEDGEMENTS. Dibrugarh Uni-
versity is a part of ACE (Aerosol Characteri-
zation and Experiment) network stations in 
India and the project is sponsored by the Indian 
Space Research Organization under its Geo-
sphere Biosphere Programme. M.M.G. is 
grateful to ISRO–GBP for providing a senior 
research fellowship. 
 
 
Received 22 March 2006; revised accepted 18 
November 2006 
 
M. M. GOGOI1 
P. K. BHUYAN1,* 
K. KRISHNA MOORTHY2 
 
1Department of Physics, 
Dibrugarh University,  
Dibrugarh 786 004, India 
2Space Physics Laboratory, 
Vikram Sarabhai Space Centre, 
Thiruvananthapuram 695 022, India 
*For correspondence. 
e-mail: pbhuyan@gmail.com 
 
 
 
 
 
 
 
 


Possible impact of a major oil-well fire on aerosol optical depth at Dibrugarh

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Possible impact of a major oil-well fire on aerosol optical depth at Dibrugarh

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