Status Report Carbonaceous Aerosols Over Northern India: Sources and Spatio-temporal Variability

Atmospheric aerosols, an important component of the atmosphere, are derived from a variety of sources (natural and anthropogenic) and comprise of wide-range of particles having different chemical composition, size (0.001 μm to 100 μm), shape and optical properties. They are mainly confined to the lower troposphere wherein intense vertical and horizontal mixing takes place. As a consequence, largescale temporal and spatial heterogeneity in aerosol loadings of the atmosphere is observed. The abundance of aerosols in the atmosphere is either quantified by their total mass concentration (μg m–3) or by an optical measurement referred to as aerosol optical depth (AOD). Atmospheric aerosols have short residence time (few days to a week) and are projected to have a regional to global impact on the radiation budget. Atmospheric aerosols produce a net cooling effect (negative forcing), however these estimates are associated with large degree of uncertainty and the present level of scientific understanding still remains in the range-medium to low (IPCC, 2007). The large uncertainty in the estimation of radiative forcing is partially attributed to the lack of knowledge on the spatio-temporal variability of chemical composition and optical properties of absorbing and scattering aerosols on a regional scale. A recent study suggests that uncertainty in the estimation of radiative forcing is due to relative increase in the mass concentrations of absorbing black carbon (BC) which is considered to be larger than the overall increase in the abundance of scattering anthropogenic aerosols (Myhre, 2009).