Abstract
ABSTRACT 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.
Highlights
Atmospheric aerosols controls Earth’s climate by influencing the radiation budget at regional and global scales
We have modeled the aerosol optical properties (AOD, single scattering albedo (SSA), and asymmetric parameter (ASP)) at different RH levels corresponding to the ambient RH values obtained from the India Meteorological Department (IMD) observations for the entire year 2015 for individual cities
The results show that the monthly variations of PM2.5 were consistently high over Delhi as compared to Pune
Summary
Atmospheric aerosols controls Earth’s climate by influencing the radiation budget at regional and global scales. Over South Asia, the extensive study of aerosol direct RF has been made using the single column as well as 3D models and field experiments to understand the climate implications (Ramanathan et al, 2005; Lau et al, 2006; Moorthy et al, 2009; Lawrence and Lelieveld, 2010; Bollasina et al, 2011; Vinoj et al, 2014) Experiments such as Indian Ocean Experiment (INDOEX) (Jayaraman et al, 1998; Satheesh et al, 1999) reported clear-sky aerosol surface forcing of about –29 Wm–2 over the tropical Indian Ocean.
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