at several places in the country are coming up, with spatiotemporal variations and long-term trends in aerosol burden over India can be better understood. Such observations, both at single location by individual organizations and also at multiple-sites/multi- parameter/multi-institutional collaborative programs through campaign-mode experiments/modeling efforts will help to reduce the existing uncertainties due to aerosol and pre-cursor gas emissions. Such aerosol climatology will also help to reduce the existing uncertainties due to aerosol emissions.
Albeit the complete list is very exhaustive, some of the high-prioritized areas in which the aerosol studies are of immediate need are indicated in the paragraphs to follow:
i) Land and Marine Aerosol Climatology
In India, studies on the characteristics of aerosols up to stratospheric altitudes, using the data over longer period, have been carried out, so far, at Pune, Trivandrum, Visakhapatnam, Bangalore and at Background Air Pollution MoNitoring (BAPMoN) stations of India Meteorological Department (IMD). These data sets have been archived using ground-based lidar and solar radiometric methods and they have been utilized for studying recent long-term changes and trends in the boundary-layer aerosol loading, air quality and turbidity. The results show significant inter-annual and intra-seasonal variability and increasing trend in aerosol loading, but the year-to-year variability shows a strong dependency on local influences like monsoon precipitation etc.
As most part of India is surrounded by oceans, development of marine aerosol climatology, particularly over the Bay of Bengal, Arabian Sea and Indian Ocean needs to be strengthened, particularly to understand their impact on Indian monsoons. Albeit some data have been or being collected during the major research programs in India such as MONSOON- 77, MONEX-79, INDOEX-India, BOBMEX, ARMEX, ARFI, ICARB, more data covering different aspects of aerosols, are to be acquired by utilizing the cruises available for future research programs.
ii) Aerosol-Precipitation Relationship
The correspondence between atmospheric aerosol distributions and precipitation on different spatiotemporal scales needs further examination, understanding and development. These studies primarily require database for longer period, and more details about the aerosol (particularly hydrophilic or hydrophobic in nature) physico-chemical properties, dynamical and aerosol-cloud coupled processes. Studies undertaken using long-term data sets of boundary layer aerosol archived at Pune and Trivandrum suggest a fair agreement between aerosol loading and monsoon precipitation. Such information over other places in the country will play a vital role in understanding the role of aerosols in the Indian monsoon, and the ways to forecast its behavior.
iii) Effect of Absorbing and Non- or Less-Absorbing Aerosols
Information on absorbing and non-absorbing aerosols is essential for evaluating their impact on Earth-atmosphere radiation balance. Most of the observational data sets available, so far, in India limited to non-absorbing or less-absorbing aerosols. Hence, suitable experimental methods need to be developed for the measurement of absorbing aerosols such as carbon (elemental or black, organic, brown and yellow) and biomass aerosols. In this context, the black and organic carbon aerosol observations initiated at SPL, Trivandrum; PRL, Ahmedabad and IITM,
Pune followed by some more institutes would supplement this information to some extent, but regular measurements at other key carbon-rich locations in India, in a network mode, are highly essential to understand the regional effects of absorbing aerosols in regional/global climate change.
iv) Establishment of Network with Existing Lidars and Setting-up of Aerosol Monitoring Facilities in the North-Eastern (NE) Region
There is an immediate need for the establishment of a network with active remote sensing facilities such as lidars
available in the country for better understanding of the site-time- height variations in aerosol characteristics, on regular basis, and eventually for obtaining aerosol climatology for the study of aerosol properties under different environments and meteorological conditions, and related large-scale aerosol dynamics and wave activity. In this context, the programs such as I-LINK (Indian LIdar NetworK), proposed by PRL should be promoted. There is also a need for setting-up of aerosol measurement facilities including lidar in the north-eastern region where such observations are very sparse. The proposed, well planned, observational program at Mayapuri, Darjeeling would be a good beginning and it is expected that this program would also cover a few more stations in the NE region in the near future.
v) Effects of Aerosol Shape
It is realized in the recent investigations that non-sphericity of aerosols, particularly dust aerosols over oceans would contribute an uncertainty of about 10 per cent in the aerosol radiative forcing on climate. These effects were more noticed in the retrieval of climate forcing parameters due to aerosols from satellite observations as compared to land observations of aerosol extinction. Hence, retrieval algorithms involving better computer code that can correct the Mie scattering properties for non-sphericity of aerosol particles need to be developed in order to minimize this error among many other unknowns. Polarization properties or phase of aerosol particles at different altitudes using "polarization diversified lidars" would help such estimations.
vi) Development of Aerosol Multiple Scattering and Fluorescence Techniques
The aerosol lidar and radiometric data archival and retrieval techniques are more straight forward when the sky is clear or cloud-free. In the presence of turbid atmosphere (like cloud, fog, haze, snow etc.) and vegetation cover, the received signals suffer from multiple and fluorescence scattering processes. These effects have been realized recently even with moderate to high pollution episodes.
