Sources of poor air quality and long-term variability over Kolkata

Abstract A 15-year long (2004–18) study was conducted to investigate the temporal and interannual variability, trend, and sources of major gaseous (NO2 and SO2) and particulate (PM2.5) air pollutants over a tropical metro city, Kolkata in Eastern India. The study was conducted in two phases: Phase-I (2004–09) and Phase-II (2012–18). PM2.5 showed a gradual rise in aerosol loading from 2004 to 2018 (5 μg m− 3 year− 1). SO2 (NO2) built up at the rate of 2.2 (~ 4) μg m− 3 year− 1 during 2004–11 (2004–12) and then declined at the rate of 3.2 (~ 7) μg m− 3 year− 1. The seasonal variations of all the pollutants were found to be similar; winter > postmonsoon > premonsoon > monsoon. Source apportionment of PM2.5 shows a noticeable shift of the highest contributing source; vehicular emissions in Phase-I to solid fuel burning in Phase-II. The banning of old vehicles, upgradation of fuel, and introduction of BS-III and BS-IV could reduce the vehicular emission contribution in Phase-II. In addition, the reduction in coal consumption in power sectors, low sulfur fuel in diesel vehicles, etc., in Kolkata could be the reason for the decrease in SO2 and NO2. Concentration weighted trajectory (CWT) analysis identified major source regions such as central and western IGP during winter and postmonsoon, arid/semiarid regions of Western India and biomass burning over Eastern Ghat during premonsoon. It was observed that the light rain with a longer duration scavenged more aerosols (55%–60%) than the shorter duration heavy rain (20%–22%). Overall, winter and postmonsoon were the worst for PM2.5 and NO2 pollution over Kolkata. However, the city remained clean in terms of SO2 pollution. Long-term variability in air quality index showed moderate to poor air quality in winter and postmonsoon. The air quality during premonsoon has been approaching toward poor air quality in Phase-II. Monsoon has been found to be the cleanest season with the air quality of good to satisfactory level.