Publications

Rawat, P; Sarkar, S; Jia, SG; Khillare, PS; Sharma, B (2019). Regional sulfate drives long-term rise in AOD over megacity Kolkata, India. ATMOSPHERIC ENVIRONMENT, 209, 167-181.

Abstract
We present here the first long-term (2001-2017) record of aerosol optical depth (AOD; interchangeably, AOT) and contributions of aerosol chemical components to light extinction over Kolkata, the second-most polluted metropolis in India. For this purpose, we use daily gridded MODIS AOD measurements coupled with modeled AOT for 5 major aerosol constituents - sulfate, sea-salt, dust, organic carbon (OC) and black carbon (BC), from the MERRA-2 reanalysis system. We find that AOD over Kolkata has increased significantly (32% overall, 0.010 y(-1); Mann-Kendall test with Thiel-Sen slope estimation; p < 0.01) over the past 17 y to a mean of 0.74 +/- 0.10 in 2017, and an overwhelming majority of this increase is driven by a concurrent rise (70% overall, 0.007 y(-1), p < 0.01) in sulfate AOT. This is followed by smaller but significant enhancements in OC (40% overall, 0.002 y(-1), p < 0.01) and BC (20% overall, 0.0004 y(-1), p < 0.01) AOT. Although a non-trivial fraction (24%) of AOT is associated with natural emissions (sea-salt and dust), these do not exhibit significant trends over time and therefore are not instrumental in driving the observed AOD rise. A concentration-weighted trajectory (CWT) analysis shows that sulfate aerosol over Kolkata is mostly regional (within similar to 250 km) during winter and post-monsoon with significant footprints over thermal power plant clusters in neighboring states and areas with considerable residential biofuel use (eastern Indo-Gangetic Plain (IGP) and the Ganges-Brahmaputra Basin). In comparison, BC and OC AOT show a significant long-range component (similar to 1800 km) originating from northwestern and central IGP, especially during post-monsoon, when open agricultural residue burning is rampant. Overall, these results provide a preliminary indication that a considerable fraction of aerosol light extinction over Kolkata might be related to non-local, atmospherically transported aerosol. Once validated by ground-based measurements, these results would reinforce the need to bring regional and long-range emissions, in addition to within-the-city emissions, under the purview of air quality management policies.

DOI:
10.1016/j.atmosenv.2019.04.031

ISSN:
1352-2310