Wintertime aerosol characteristics over the Indo-Gangetic Plain (IGP): Impacts of local boundary layer processes and long-range transport

[1] The Indo-Gangetic Plain (IGP) encompasses a vast area, (accounting for similar to 21% of the land area of India), which is densely populated (accommodating similar to 40% of the Indian population). Highly growing economy and population over this region results in a wide range of anthropogenic activities. A large number of thermal power plants (most of them coal fed) are clustered along this region. Despite its importance, detailed investigation of aerosols over this region is sparse. During an intense field campaign of winter 2004, extensive aerosol and atmospheric boundary layer measurements were made from three locations: Kharagpur (KGP), Allahabad (ALB), and Kanpur (KNP), within the IGP. These data are used (1) to understand the regional features of aerosols and BC over the IGP and their interdependencies, (2) to compare it with features at locations lying at far away from the IGP where the conditions are totally different, (3) to delineate the effects of mesoscale processes associated with changes in the local atmospheric boundary layer (ABL), (4) to investigate the effects of long-range transport or moving weather phenomena in modulating the aerosol properties as well as the ABL characteristics, and (5) to examine the changes as the season changes over to spring and summer. Our investigations have revealed very high concentrations of aerosols along the IGP, the average mass concentrations (M-T) of total aerosols being in the range 260 to 300 mu g m(-3) and BC mass concentrations (M-B) in the range 20 to 30 mu g m(-3) (both similar to 5 to 8 times higher than the values observed at off-IGP stations) during December 2004. Despite, BC constituted about 10% to the total aerosol mass concentration, a value quite comparable to those observed elsewhere over India for this season. The dynamics of the local atmospheric boundary layer (ABL) as well as changes in local emissions strongly influence the diurnal variations of M-T and M-B, both being inversely correlated with the mixed layer height (Z(i)) and the ventilation coefficient (V-c). The share of BC to total aerosols is highest (similar to 12%) during early night and lowest (similar to 4%) in the early morning hours. While an increase in the V-c results in a reduction in the concentration almost simultaneously, an increase in Z(imax) has its most impact on the concentration after similar to 1 day. Accumulation mode aerosols contributed similar to 90% to the aerosol concentration at ALB, similar to 77% at KGP and 74% at KNP. The BC mass mixing ratio was similar to 10% over all three locations and is comparable to the value reported for Trivandrum, a tropical coastal location in southern India. This indicates presence of submicron aerosols species other than BC (such as sulfate) over KGP and KNP. A cross-correlation analysis showed that the changes in M-B at KGP is significantly correlated with those at KNP, located similar to 850 km upwind, and ALB after a delay of similar to 7 days, while no such delay was seen between ALB and KNP. Back trajectory analyses show an enhancement in M-B associated with trajectories arriving from west, the farther from to the west they arrive, the more is the increase. This, along with the ABL characteristics, indicate two possibilities: (1) advection of aerosols from the west Asia and northwest India and (2) movement of a weather phenomena (such as cold air mass) conducive for build up of aerosols from the west to east. As the winter gives way to summer, the change in the wind direction and increasedconvective mixing lead to a rapid decrease in M-B.