Seasonal Inhomogeneity of Soot Particles over the Central Indo-Gangetic Plains, India: Influence of Meteorology

Black carbon (BC) particles play a unique and important role in earth's climate system. BC was measured (in-situ) in the central part of the Indo-Gangetic Plains (IGP) at Varanasi, which is a highly populated and polluted region due to its topography and extensive emission sources. The annual mean BC mass concentration was 8.92 +/- 7.0 mu g m(-3), with 34% of samples exceeding the average value. Seasonally, BC was highest during the post-monsoon and winter periods (approximately 18 mu g m(-3)) and lower in the pre-monsoon/monsoon seasons (approximately 6 mu g m(-3)). The highest frequency (approximately 46%) observed for BC mass was in the interval from 5 to 10 mu g m(-3). However, during the post-monsoon season, the most common values (approximately 23%) were between 20 and 25 mu g m(-3). The nighttime concentrations of BC were approximately twice as much as the daytime values because of lower boundary layer heights at nighttime. The Angstrom exponent was significantly positively correlated (0.55) with ground-level BC concentrations, indicating the impact of BC on the columnar aerosol properties. The estimated mean absorption Angstrom exponent was 1.02 +/- 0.08 mu g m(-3), indicating that the major source of BC was from fossil fuel combustion. Significant negative correlations between BC mass and meteorological parameters indicate a pronounced effect of atmospheric dynamics on the BC mass in this region. The highest mean BC mass concentration (18.1 +/- 6.9 mu g m(-3)) as a function of wind speed was under calm wind conditions (38% of the time).