Climate Exchange

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ealth

ently high BC concentrations prevailing over the IGP and

the north-eastern regions of India, compared to the south-

ern and western parts. In addition, BC concentrations

show temporal variations on both diurnal and seasonal

scales. While the diurnal variations are pronounced over

the plains, and are mostly associated with atmospheric

boundary layer (ABL) dynamics, the seasonal variations

(winter high and summer low) are influenced by synoptic

monsoon circulation modulated by the seasonal changes

in the convective mixing and the resulting enhanced verti-

cal dispersal of BC in the ABL during summer. Additional

contributions arise from the heterogeneous spatial distri-

bution of the sources. The higher organic carbon (OC) to

BC ratio (typically >5 except in cities) seen over Indian

regions indicates the significant contribution of biomass

burning and also that total carbon over India is more scat-

tering (less absorbing) in nature.

Unlike over the landmass, variations over the oceans

are more controlled by transport and driven by meteorol-

ogy. Extensive measurement of BC and its fraction of total

aerosol mass made over the Arabian Sea during the Arabian

Sea Monsoon Experiment (ARMEX) have shown a seven-

fold decrease in BC concentration (~700 ng m

-3

to ~100 ng

-3

), associated with changes in synoptic winds between

continental and marine environments.

Radiative trans-

fer calculations show that such large decrease in BC leads

to significant decrease in atmospheric forcing (heating)

efficiency (from 70 W m

-2

-1

to 15 W m

-2

-1

). Based on

the most exhaustive shipboard measurements under the

Integrated Campaign for Aerosols, gases and Radiation

Budget (ICARB) and Winter_ICARB (W_ICARB), the

spatial distribution of BC has been shown with associated

atmospheric warming over the oceanic regions around

India and a strong seasonality associated with synoptic

meteorology and forest fires.

The concentration of BC

from Trivandrum, a coastal station at the southern tip of the Indian

Peninsula

using a dual-channel aethalometer. Synthesizing aerosol

absorption with other optical and physical properties of aerosols, from

collocated and concurrent measurements, a quantification of the effect

of BC on aerosol forcing was done over an urban site for the first time

in India,

which showed that large BC absorption led to a change in the

sign of the forcing at the top of the atmosphere. This was perhaps the

first regional effort in this direction.

Taking a cue from this, and realizing the highly heterogene-

ous sources of carbonaceous aerosols over this region arising from

varying living habits, long-term measurements of BC using similar

and inter-compared instruments following well-laid protocols have

been established under ISRO-GBP, in a phased manner, involving a

large number of universities and research institutions. These were

also supported by thematic field campaigns (fixed stations and mobile

platforms) over the mainland, shipboard measurements over oceans,

and aircraft and balloon experiments.

Spatio-temporal changes in BC concentrations over the landmass

arise due to changes in the synoptic and regional meteorology as well

as changes in human activities (agricultural waste burning, forest fires

etc). While these are quite significant, it is difficult to characterize them

because of the large inter-annual variations in these processes. A land

campaign (LC-I) was conducted in February-March 2004, to under-

stand the spatial distribution of aerosol including BC over central and

peninsular India, using identical instruments over spatially separated

locations, covering an area of more than 1 million square kilometres.

This was followed up with Land Campaign II (LC-II) in December

2004,

during which BC and composite aerosol measurements were

made with atmospheric boundary layer parameters along a chain of

stations in the Indo-Gangectic plain (IGP).

The ARFINET network of BC observatories was established under the

ARFI project of ISRO-GBP. It has data from 32 observatories, including

the two island stations to study long-range transport and spatial gradi-

ents. This forms the only long-term and exhaustive source of primary

data over the region. Spatial distribution measurements show consist-

Source: ARFI

The Himalayan aerosol observatory at Hanle (left); the mean picture of diurnal and monthly temporal evolution of BC over the area (right)