On the Contribution of Particulate Matter (PM2.5) to Direct Radiative Forcing over Two Urban Environments in India

Radiative forcing by particulate matter (PM2.5) has been estimated for a period of one year (January-December 2015) over Delhi and Pune (polluted urban metro cities in India) In situ observations of PM2.5 and black carbon (BC) over both the cities were obtained from the ground-based System of Air Quality Forecasting and Research (SAFAR) network of stations. Observations have shown that PM concentrations over Pune had a strong diurnal cycle as compared to Delhi in all the seasons. Also, comparisons of the mode values and seasonal frequency distributions (FDs) over Pune and Delhi showed that pollution levels over Delhi were consistently above National Ambient Air Quality Standards (NAAQS). The mean monthly PM2.5 values ranged from 61.5 to 162.9 over Delhi and from 17.4 to 74.05 over Pune. The BC mass contribution to PM2.5 was found to be 10% to 25% over Pune. However, the contribution of BC to PM2.5 was up to 35% over Delhi. Radiative forcing due to PM2.5 (PRF) over both the sites was estimated using the Optical Properties of Aerosols and Clouds (OPAC) model along with the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The PRF in the atmosphere was between +7.73 Wm(-2) and +14.51 Wm(-2) over Delhi and between +3.12 Wm(-2) and +12.15 Wm(-2) over Pune. Sensitivity experiments showed that the impact of the increase in the hygroscopicity of the aerosols on the PRF was overshadowed by the net changes in albedo.