Atmospheric aerosols assume greater importance due to their influence on human health and their potential to perturb the Earth's climate both by directly and indirectly altering the radiation reaching the Earth's surface and to some extent alter the properties of the clouds etc. During the last two decades ISRO as part of Indian Middle Atmospheric Programme (IMAP) has undertaken atmospheric aerosol investigations, which has resulted in the aerosol climatology over Trivandrum region. This has also provided an opportunity to indigenously develop a Multi Wavelength Radiometer (MWR) by Space Physics Laboratory, VSSC. This instrument provides the spectral aerosol optical depths in 10 discrete wavelength regions.

From 1991 onwards the IMAP programme towards its culmination formed the basis for undertaking Geosphere Biosphere Programmes (ISRO-GBP) related to climate change aspects, As part of this ISRO undertakes several projects covering broad project areas of :

1. Palaeoclimate Studies
2. Regional Climate Modeling
3. Atmospheric Aerosols, Radiation and Climate effects
4. Atmospheric Trace Gases and Aerosol Chemistry
5. Land Use, Land Cover Dynamics and Ecosystem Studies
6. Terrestrial Carbon and Biospheric Modeling
7. Ocean primary productivity and nutrient cycling.

ISRO has conceived the above broad project goals considering the inherent strength of ISRO / DOS institutions and other national peer institutions.

In the light of above major goals, ISRO-GBP has taken initiatives to undertake major experimental campaign based projects through the large participation of Indian institutions for understanding specific weather / climate related phenomena. Towards this one of the major initiative is to understand the sources and composition of atmospheric aerosols and their role in modifying the climate of Indian region. The extremely limited quantifiable data on this important & less understood atmospheric aerosol information has been realized through debates of various technical reviews. It has been identified to undertake major campaigns on atmospheric aerosols to characterize the natural and anthropogenic sources for bringing out national level AOD maps of the whole country on a periodic basis. It has been realized to achieve such a goal through a network of MWR observations by instituting across various biogeographical regions in the country.

The MWR network is planned in about 22 locations across the country and so far ISRO in support of various national laboratories have already operationalized in about 11 locations. The remaining 11 MWR instruments would be commissioned in the next one-year including ISRO's proposed ISRO Laboratory for Aerosol, Radiation and Chemistry (I-LARC) at Port Blair. With the commissioning of all the 22 MWR network stations, it is possible to provide India's Spectral AOD maps on a monthly basis for suitably assimilating into the climate models. However, the satellite-based retrieval of AOD maps are also being underway through Indian as well as US based satellite platforms. The ground observed information would become inevitable for validating and improving the spectral AOD maps retrieval algorithms generated from satellite data.

In addition to physical properties of atmospheric aerosols, the optical properties of aerosols related to their scattering coefficient and chemical composition are the important elements for suitably understanding the absorbing and scattering aspects of aerosols for their positive and negative forcing. Towards this the understanding on aerosol chemistry becomes very vital.

In addition to aerosol chemistry, the trace gases and their composition assumes another dimension as there could be potential multiplying effect on the photo chemical reactions from the source regions to the impact regions wherein the composition of atmosphere would altogether different. The combined understanding of physical, optical and chemical properties of atmospheric aerosols and with respect to natural and anthropogenic sources is important for understanding the radiative forcing of atmospheric aerosols.

In addition to the local sources the atmospheric aerosols by virtue of their atmospheric dwell time of 7 - 10 days and transport from the marine, continental sources as well as inter continental transport would make altogether complicated in understanding the aerosol transport and composition over the Indian region.

In this regard the wind trajectories across the nation plays a vital role on the transport whereas the transformation of atmospheric aerosols due to local meteorology would change the composition of aerosols and make the entire chemistry of aerosols and their radiation impact becomes complicated.

The transport, transformation and natural and anthropogenic sources over our Indian region hence is little understood comprehensively though there are limited observations by various national institutions. In order to generate a long term climatology of atmospheric aerosols and their radiation effect we need minimum of 10 years continuous observations from any location. Towards this the MWR network is a first step towards making such long term climatology on the physical properties of atmospheric aerosols through measurement of spectral AODs.

ISRO-GBP also has taken initiatives to augment the existing network with continuous measurements of trace gases and vertical profiles through Lidar observations. In this aspect of special observations the efforts of PRL, IISc, IITM etc are underway.

In the above direction, ISRO-GBP has undertaken a mobile pilot land aerosol campaign during 1st -28th Feb. 2004 covering 15,000 km road length in southern India through participation of 10 national institutions. As a follow up of this pilot land campaign ISRO-GBP now is planning to undertake the entire northern corridor covering west to east of India covering entire Indo-gangetic plains through continuous measurements of aerosols for a period of entire 1st - 31st Dec. 2004.

The Land Aerosol Campaign is proposed in the 8 specified locations of Hissar, Delhi, Agra, Kanpur, Allahabad, Nainital, Jaduguda and Kharagpur. This campaign would be complemented through vertical measurements of atmospheric boundary layer through Micro Pulse Lidar (MPL) and tethered balloon. In addition NRSA aircraft also would be making observations through on board mounting of some atmospheric measuring instruments from various altitudinal levels up to 10 km in the troposphere.

The primary objectives of Land Aerosol Campaign are to characterize the atmospheric aerosols during the winter period and to identify the possibilities of transport and transformation across west to eastern Indian region. This would also give opportunity to understand the dynamics of fog formation during the winter period and the possible role of atmospheric aerosols. IMD - New Delhi also would be participating in a major way through support of UV and Global Radiation Data, Radiosondes and Ozonesonde data from various locations in the northern corridor.

There are 28 national institutions participating in this national endeavour involving nearly 70 scientists specialized in the areas of atmospheric sciences.