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Leveraging the COVID-19 India-wide lockdown situation, the present study attempts to quantify the reduction in the ambient fine particulate matter concentrations during the lockdown (compared with that of the pre-lockdown period), owing to the highly reduced specific anthropogenic activities and thereby pollutant emissions. The study was conducted over Bengaluru (India), using PM 2.5 (mass concentration of particulate matter having size less than or equal to 2.5 µm) and Black Carbon mass concentration (BC) data. Open-access datasets from pollution control board (PCB) were also utilised to understand the spatial variability and region-specific reduction in PM 2.5 across the city. The highest percentage reduction was observed in BC ff (black carbon attributable to fossil fuel combustion), followed by total BC and PM 2.5 . No decrease in BC bb (black carbon attributable to wood/biomass burning) was observed, suggesting unaltered wood-based cooking activities and biomass-burning (local/regional) throughout the study period. Results support the general understanding of multi-source (natural and anthropogenic) nature of PM 2.5 in contrast to limited-source (combustion based) nature of BC. The diurnal amplitudes in BC and BC ff were reduced, while they remained almost the same for PM 2.5 and BC bb . Analysis of PCB data reveal the highest reduction in PM 2.5 in an industrial cluster area. The current lockdown situation acted as a natural model to understand the role of a few major anthropogenic activities (viz., traffic, construction, industries related to non-essential goods, etc.) in enhancing the background fine particulate matter levels. Contemporary studies reporting reduction in surface fine particulate matter and satellite retrieved columnar Aerosol Optical Depth (AOD) during COVID-19 lockdown period are discussed.

[1] Collocated measurements on optical and chemical properties made at a coastal urban location Visakhapatnam on the east coast of India were used to assess the relative contribution of different chemical species to composite aerosol radiative forcing. At such a location, the dominant species that decide the atmospheric forcing are the relative mass fractions of Black Carbon (BC) and sulfate. It is observed that the composite forcing at top of the atmosphere follows the BC mass concentration during all the seasons except for some days. In such cases the hypothesis on the role of mixing state of aerosol in deciding the net aerosol radiative forcing is examined to conclude that the BC either independently or in the internal mixture state during winter months would decide the aerosol composite forcing over this coastal urban location. Though the conditions for the formation of such mixtures and their seasonal dependence however remain unclear, drier weather conditions with abundance of sulfate seem to favor the formation of well mixed aerosol.Citation: Niranjan, K., T. Anjana Devi, B. Spandana, V. Sreekanth, and B. L. Madhavan (2012), Evidence for control of black carbon and sulfate relative mass concentrations on composite aerosol radiative forcing: Case of a coastal urban area, J. Geophys.

Abstract. Information on the aerosol intensive properties like Single Scattering Albedo (SSA) and asymmetry parameter are very limited, particularly over the peninsular India, though extensive reports are available on the aerosol bulk properties. In view of the importance of these parameters in evaluating the aerosol radiative forcing, we present for the first time the temporal variation in SSA with measurements on aerosol absorption and scattering coefficients over Visakhapatnam (17.72 • N, 83.32 • E; located on the east coast of India) for the year 2007. The inferred SSA was in the range of 0.65 and 0.9 with an annual mean of 0.76 ± 0.013 and with a probable value of 0.80, indicating a marginal atmospheric warming over the region. The mixed layer contribution to column Aerosol Optical depth is found to be 35 % in summer while it is well above 35 % in winter, indicating the confinement of aerosol within the boundary layer during winter. The asymmetry parameter which represents the angular scattering in radiative forcing estimation is found to be around 0.65 ± 0.1 for the location. The day to day variability in SSA is found to be well correlated with the variations in surface BC mass concentrations and/or the relative dominance of the fine/coarse mode aerosol. The results are discussed in light of the aerosol physical and optical properties and the asymmetry parameter.