Climate Sense

[

] 158

bserving

, P

redicting

and

rOjecting

limate

OnditiOns

Strong signatures of marine aerosols are even

reported in the interiors during summer.

Over

central and northern India, dust, produced locally or

transported from the arid regions of Africa, Arabia and

Asia, contributes significantly to aerosol abundance and

AOD throughout the spring and summer. A synergy

of ground-based and satellite infrared measurements

shows that the absorption efficiency of dust is higher

over Asia than Africa

during winter – when the dust

is aged and mixed with soot – while the more nascent

summer dust has lower absorption efficiency. Chemical

reactions involving this dust result in neutralization of

acidic and maintain alkaline rainwater.

Urbanization has resulted in the evolution of urban

conglomerations in India. An analysis over four

regional mega cities – Chennai, Mumbai, Kolkata and

New Delhi – showed high AOD with strong seasonal

variations. Of these, Chennai had the least AOD, while

Kolkatta (eastern coast) had the most fine mode aero-

sols throughout the year. In the other two cities, the fine

mode contribution varied as a function of season; being

very high in winter. During pre-monsoon and summer,

coarse mode aerosols associated with dust and sea salt

dominated the distribution.

Thus, even the mega

cities show temporal heterogeneity, making it difficult

to model for regional ARF estimation.

Modulation by atmospheric dynamics

Aerosol abundance, composition and vertical distribution

over India is strongly modulated by the natural dynamical

features of the atmosphere. Based on multi-year, multi-

station data on AOD, it has been shown

that AOD over

the peninsula tends to be higher in the years when the

southward excursion of the intertropical convergence

zone is smaller. Long-range transport of aerosols leads to

significant modulations in aerosol properties, and corre-

sponding changes in ARF and in forcing efficiency, which

implies changes in the aerosol types and impacts. While

the advection of mineral dust-richWest Asian and Arabian

desert aerosols are important over the Arabian Sea and

western parts of the peninsula during March to May,

advection of fine and accumulation mode aerosols from

the East Asian regions cause large perturbations in spectral

AOD and BC concentrations over the Bay of Bengal during

January to April.

Over the Ganga basin, the dynamics of

the shallow atmospheric boundary layer (ABL) strongly

influences the build-up and spatial confinement of aero-

sols and trace gases from local emissions.

While the

deepening ABL in spring and summer flushes the pollut-

ants further aloft resulting in a dilution in their surface

concentration,

stronger long-range transport leads to

increase in AOD and ARF over the entire north-Indian

Plains.

This leads to large space-time heterogeneity in

AOD and its spectral variation over India.

The tropical atmosphere supports a variety of plan-

etary scale wave motions, the propagation of which

causes significant modulations to the abundance and

type of aerosols resulting in corresponding modula-

tions in ARF. The eastward propagating Madden Julian

balloon borne measurements of the altitude distribution of aerosols

and trace gases.

As a result, ISRO, while formulating its Geosphere

Biosphere Programme (I_GBP), identified Aerosol Climatology and

Effects (ACE) and characterization of trace gases as central activities.

ACE activity proliferated under I_GBP to become a national effort.

A network of ground-based observatories were established for regional

aerosol characterization. As the programme matured, it became the

Aerosol Radiative Forcing over India project (ARFI), which is now a

network spanning the entire country and surrounding oceans. Some of

its stations have databases of more than two decades for AOD and several

years for black carbon (BC), enabling quantifying of long-term trends.

Data from Trivandrum, a remote coastal location in the southern

peninsula, shows the monthly AOD at 500nm. While the increase in

AOD of ~2.4 per cent per year from its base value in 1985 is of concern,

it is equally interesting that the concentration of BC, normally consid-

ered as a tracer for human impact, shows a decreasing trend of ~250

ng m-3 y-1. This has several implications. The reduction of surface BC

could be considered an indicator of emission control strategies, while

the increase in AOD indicates an overall increase in columnar abun-

dance. It remains to be seen whether this increase is occurring at higher

levels, above the atmospheric boundary layer.

Natural and anthropogenic

Despite the need to focus on anthropogenic aerosols from the climate

changemitigation perspective, the role of natural aerosols in ARF should

not be underestimated over the Indian region. Campaign mode obser-

vations over the oceans

and long-termmeasurements from islands and

the mainland have shown dramatic change in aerosol characteristics

(abundance, vertical distribution, composition, and size distribution)

due to large influx of marine aerosols, leading to large reduction in

AOD, and flattening of its spectral response. This caused a significant

reduction (by more than a factor of >2) in the ‘top of the atmosphere’

and atmospheric ARF.

Chemical speciation analysis shows an increase

in the concentration of water-soluble salts from 29 per cent in winter

to 44 per cent in summer.

Trends in aerosol optical depth and black carbon mass

Source: ARFI, I-GBP

Trends of AOD at 500nm (bottom) and black carbon mass concentration (top)

over Trivandrum. The points are monthly means for AOD and daily means for

black carbon. Mean trend lines and 95 per cent confidence bands are also shown