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Aerosol Measurements and Impacts

CAOS faculty initiated aerosol research and established an aerosol-climate observatory at IISc,with a number of sophisticated instruments for the measurements of various parameters related to aerosols as well as radiation with an objective to characterize aerosols over this region and to assess its radiative impact. This laboratory has a complete set of instruments to conduct aerosol research. A similar observatory was set up at Minicoy (Lakshadweep) for marine aerosol characterization and delineating long-range transport of aerosols. Aerosol measurements at these sites are being carried out under identical or nearly identical conditions using instrumented mobile platforms for characterizing aerosol spatial heterogeneity. Aerosol measurements over the ocean are being carried out onboard R/V SagarKanya. Vertical distribution of aerosol properties are being routinely characterised by mounting a micro pulse lidar (MPL) onboard a research aircraft (belonging to the National Remote Sensing Agency, NRSA,Department of Space) and by making measurements over continental and coastal India, as well as over oceanic regions adjacent to the Indian sub-continent. These sites are equipped with facilities such as scanning electron microscope (SEM) to study microscopic features of aerosol samples. The Aerosol laboratory at IISc has now grown to a reputed lab for aerosol research at the international level. The laboratory is now engaged in designing satellite sensors jointly with ISRO, and developing laboratory-based instruments to study aerosols. More than 100 research papers in international journals have come out of this laboratory.

There have been sensational reports regarding the build-up of aerosols over the Indian region, and its consequences world-wide. Our synthesis using long-term, multi-decadal (back to approximately 25 years at some stations) of primary data from direct measurements of aerosol optical depth and black carbon from the ARFINET (Aerosol Radiative Forcing Network) have shown a statistically significant increasing trend in total aerosol abundance over India, while the data revealed a significant decreasing trend in black carbon (BC) aerosols. Aerosol levels varied seasonally, and the rate of increase was high only during the dry months of December through March, but there was no strong trend during the pre-monsoon (April to May) and summer monsoon periods (June through September). Aerosol optical depth was found to be increasing at a rate of 2.3% (of its value in 1985) per year. The rate of increase is more rapid (≈ 4%) during the last decade, while BC is de- creasing at a higher rate of 5.2%. The question of whether aerosols cool or warm the planet depends on the relative contribution of various chemical species which constitute the aerosols. An aerosol with significant BC content can have a net warming effect and complement greenhouse warming. However, an increasing trend in composite (total) aerosols simultaneously with a decreasing trend in BC aerosols suggest a decreasing warming potential of aerosols.

For accurate assessment of the regional climate impacts of aerosols, it is essential to have accurate aerosol optical depth (AOD) data with a fairly homogeneous spatial and temporal representation. Such a database does not exist for India. With a view to accomplishing this, highly accurate and long-term (12 years) ground-based AOD measurements from 44 ground-based aerosol observatories over Indian land (under ARFINET and AERONET) are being assimilated with satellite-derived AODs (which are spatial snapshots) to generate monthly and seasonal mean gridded AOD data over the Indian region, employing a weighted mean interpolation technique which is a modified form of the Successive Correction Method (SCM), to guarantee the assimilated data to be a convex combi- nation of its parent datasets as well as to account for the spatial heterogeneity of the domain and the seasonality of lower atmospheric dynamics. The assimilated product shows all the characteristic temporal and spatial features of AOD across India with statistical uncertainties that are lower than those in the widely used satellite data and at times, even lesser than its parents.