Variation of Black Carbon Concentration Associated with Rain Events at a Tropical Urban Location

Mass concentration of black carbon (BC) near the surface (within the planetary atmospheric boundary layer) was measured using a seven-channel aethalometer at Kolkata, a metropolitan city in the Indian tropical region, during the period from June 2012 to May 2013. The diurnal variation of BC concentration shows a prominent increase in the morning and evening hours, an usual feature seen over continents. However, an anomalous feature of the BC variation is observed subsequent to rain events. On normal days, the BC mass concentration during noon and early afternoon hours remains around 8000 ng/m³ at Kolkata. However, after the occurrence of isolated thundershowers, interestingly, the BC concentration increases (rather than decreasing due to washout) and at times reaches above 20,000 ng/m³ during noon and early afternoon hours. This increase is found to be associated with the formation of local temperature inversion within the atmospheric boundary layer during and after the occurrence of rain, which would suppress or inhibit vertical mixing and dispersion in contrast to non-rainy days. Results are presented to indicate the above-mentioned behaviour of BC concentration.