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Climate implications of atmospheric
aerosols and trace gases: Indian scenario
K. Krishna Moorthy, S. Suresh Babu, Space Physics Laboratory, Vikram Sarabahi Space Centre;
S. K. Satheesh, Centre for Atmospheric and Oceanic Sciences, Indian Institute of Sciences;
Shyam Lal, M M Sarin, S. Ramachandran, Physical Research Laboratory; C.B.S. Dutt, ISRO HQ
H
aving identified the climate implications of atmospheric
aerosols and trace gases over the complex, heteroge-
neous and densely populated south Asian region, the
Indian Space Research Organization (ISRO) has been pursuing
focused and thematic research to quantify the regional climate
implications through a series of projects.
Atmospheric aerosols are a suspension of tiny solid particles or liquid
droplets in the air, of a natural or anthropogenic origin. Though the
global aerosol abundance is dominantly natural (sea salt, dust), regionally
the anthropogenic species (sulphates, nitrates, soot, organics) dominate
in areas of high population density, industrialization and urbanization,
or regions of extensive biomass burning. The most important parameter
to represent the impact of aerosols is aerosol optical depth (AOD). This
is the vertical integral through the entire height of the atmosphere of the
fraction of incident light at any wavelength, scattered and/or absorbed
by aerosols. Aerosols affect the energy budget of the Earth-atmosphere
systemby scattering and absorbing radiation (direct effect) and by modi-
fying the amounts, and microphysical and radiative properties of clouds
(indirect effects). The resulting change in the energy budget
is termed aerosol radiative forcing (ARF).
ARF of the atmosphere refers to the energy absorbed by
aerosols and dispensed in heating the atmosphere. Such
heating, combined with surface cooling caused by the
reduction in solar energy reaching the surface, is shown
to increase atmospheric stability and adversely affect the
hydrological cycle, rainfall and crop yield.
1
Though ARF
due to anthropogenic species attracts wide research inter-
est, forcing due to natural aerosols is equally significant
for climate change.
2
Moreover, no proven methods exist
for quantitatively measuring the regional anthropogenic
aerosol component; satellite retrievals are only qualita-
tive and the best estimates attribute ~20 per cent of AOD
over the oceans to human activities.
3
ARF exhibits large heterogeneity in spatial, temporal
and spectral domains, primarily due to the spatio-temporal
heterogeneity of aerosols (diverse sources and sinks and
their heterogeneous distribution), the short and varying
atmospheric lifetime, as well as chemical andmicrophysical
properties. This heterogeneity makes the climate-impact
assessment of aerosols a challenging task, despite the
concerted efforts of the global scientific community over
the past decade or more.
4
The key to reducing this lies in
accurate characterization of aerosol properties from space,
integrated field campaigns, establishing ground-based
networks, determining size dependent chemistry on short
timescales, accurately determining aerosol single scatter-
ing albedo, the vertical distribution of all these properties
and incorporation in models.
The Indian scenario
Climate change has great significance in South Asia in
general, and India in particular. It has diverse and contrast-
ing geographical features, high population density, rapid
urbanization and industrialization, and above all the synoptic
meteorological conditions that produce the Asian monsoon.
Though Indian research to characterize regional
aerosol properties began in the forties,
5
systematic spec-
trally and spatially resolved measurements began during
the IndianMiddle Atmosphere Programme. ISRO took up
the development of suitable ground-based instruments,
formed a mini-network for measurements of spectral
AOD on a long-term basis,
6
and carried out rocket and
O
bserving
, P
redicting
and
P
rOjecting
c
limate
c
OnditiOns
The ARFI network of aerosol observatories
Source: ARFI, I-GBP