Climate Exchange

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ealth

over south Asia. Millions of its population are directly or

indirectly dependent on the water flowing through the

rivers fed by these glaciers. Even though there are several

geological and glaciological reasons for glacier retreat or

advance, BC deposition on Himalayan glaciers is increas-

ingly projected as a major factor contributing to faster

retreat,

and several caveats have been put forward. Model

simulations have predicted increases in Himalayan glacier

retreat due to BC-induced snow surface darkening. The

long-term measurements of BC being made over a high

altitude location, Hanle (32.5°N, 78.5°E and 4520 mmsl)

in the western trans-Himalayas, have shown significantly

higher concentrations during spring and low ones during

summer.

These also showed that the BC concentrations

over Hanle are, in general, significantly lower than those

observed over other eastern Himalayan regions such as

Nepal. It would be premature to draw strong conclusions

on the impact of BC on snow/glaciers and more efforts

are needed.

Outstanding issues

Extensive measurements of BC and assessment of its

impact on air quality and regional climate are being made

through the ARFINET network of 35 observatories estab-

lished as a part of ISRO- GBP. In addition campaigns are

conducted to address to specific questions on BC and its

effects. These efforts brought out the large spatio-temporal

heterogeneity as a result of changes in local BC emis-

sions, meteorology and long-range transport. The most

important outcomes include a first-cut spatial map of BC

showing high concentrations over the IGP and north-east

India, and spatial as well as seasonal distinctiveness over

the oceans. Significant impacts of long-range transport are

noted from the east on the Bay of Bengal and from the

west over the Arabian Sea. Altitude profiles of BC aero-

sols using aircraft and high altitude balloons have led to

discovery of the presence of elevated aerosol layers (with

substantial BC fractions) and a strong meridional gradient

in BC-induced atmospheric warming. On examination of

our current knowledge on BC-climate impact, it appears

that a number of outstanding issues remain.

More information is needed on the vertical distribution

of BC over the entire Indian region, while information on

the state of mixing of BC with other species is virtually

unavailable over the region, and this needs to be addressed.

In addition, while addressing the indirect effect of BC, it

is important to consider the Twomy effect versus ‘cloud

burning’ by BC. Simultaneous measurements of BC and

cloud microphysical properties are essential.

There is a tendency to project the mitigation of BC as

a rapid solution to slow down global warming. It is very

important to consider the large values of OC/BC ratios

reported over India before attempting such strategies,

especially because in most cases the sources of OC and

BC are the same. The construction of regional maps of

OC/BC ratios should be a target in future field campaigns.

Accurate assessment of any such strategies is required

before implementation, as they could play a significant

role in shaping international policy.

over the Bay of Bengal is around three to four times higher in winter

compared to that during pre-monsoon, and in the given season it is

about three to four times higher than that over Arabian Sea.

Altitude profiles of BC

The climate impact of BC depends on its altitude distribution in the

atmosphere. The higher the altitude of a BC particle, the higher its

direct radiative forcing, which will be highest when it is placed over

cloud layers. Despite being produced primarily near the surface, signifi-

cant amounts of BC occur at higher altitudes, being lofted by turbulent

motions, thermal convections and general circulation. Being located in

the tropics and with significantly strong sources of BC on the ground,

the altitude profile over the Indian region assumes great significance

from a climate perspective.

The first in-situ measurement of the altitude profile of BC over India

was made using an instrumented aircraft in 2004. This showed a rapid

decrease in its concentration in the ABL, while above the ABL the BC

concentration decreases much more slowly so a significant amount of

BC (as large as 1 μg m

-3

) is present within and above low-level clouds.

Subsequently, several airborne measurements have been made for the

altitude distribution of BC over India during the pre-monsoon season of

2006 and winter 2009 under the air segment of ICARB and W_ICARB.

Measurements during ICARB have shown that during summer and pre-

monsoon seasons, India is characterized by the presence of elevated

layers of enhanced BC above the boundary layer, in the region of 2-4

km over different locations.

Such layers of enhanced aerosol absorp-

tion during the summer and pre-monsoon seasons, when the region

receives high solar insolation and experience increased cloudiness, lead

to significant atmospheric warming which could reach peak values of

3-5 K during local noon.

A synthesis of the ship-borne, ground-based

network and aircraft data along with a radiative transfer simulation has

led to the discovery of a strong meridional gradient in aerosol-induced

atmospheric warming: from about a degree at ~2 km over the ocean,

gradually increasing to about 5 K at around 4 km over central India.

This aerosol layer persists over the entire Indian region during the

summer season, as revealed by the ARFI network observations and by

the examination of CALIPSO data, which showed the layer rising to ~5

km at the Himalayan region.

In view of the importance of the above finding, the Regional Aerosol

Warming Experiment (RAWEX) was formulated to quantify the

atmospheric effects of elevated BC layers. Under RAWEX extensive

measurements of BC altitude distribution were made concurrently

with the atmospheric thermodynamics, onboard a high-altitude balloon

from the central part of India. During the pre-monsoon season, when

convection is also strong over the region, the altitude distribution of

BC showed multiple peaks; two surprisingly large ones at ~ 4.5 km

and another above 8 km. Associated with these, a rapid decrease in the

environmental lapse rate and a sharp increase in atmosphere stability

were observed, probably caused by the atmospheric warming of the BC

layers. The change in environmental lapse rate and increase in atmos-

pheric stability lead to the further trapping of BC aerosols at higher

altitudes, raising the question of whether BC layers build ‘their own

homes’ up in the atmostphere.

BC over the Himalayas

The Himalayan region assumes enormous significance both in the

climate change scenario and in societal implications. The Himalayan

glaciers are believed to be the largest source of fresh water outside the

Polar region and play a very important role in the hydrological cycle