Characteristics and Sources of Carbonaceous Aerosols in PM2.5 during Wintertime in Agra, India

PM2.5 samples were collected at traffic, rural and campus sites in Agra during Nov 2010 to Feb 2011 and characterized for carbonaceous aerosols. The average mass concentrations of PM2.5 were 308.3 ± 51.8 μg/m3, 91.2 ± 17.3 μg/m3 and 140.8 ± 22.3 μg/m3 at the traffic, rural and campus sites, respectively. The 24-h mass concentrations of PM2.5 were significantly higher than the limit of 60 μg/m3 prescribed in the National Ambient Air Quality Standards (Indian NAAQS) and 25 μg/m3 of those of the WHO (World Health Organization). The average concentrations of OC (organic carbon) and EC (elemental carbon) were 86.1 ± 5.2 and 19.4 ± 2.4 at the traffic site, 30.3 ± 12.9 and 4.0 ± 1.5 at the rural site and 44.5 ± 18.5 μg/m3 5.0 ± 1.4 μg/m3 at the campus one. The contributions of TCA (Total Carbonaceous Aerosol) at the traffic, campus and rural sites were found to be 52, 54 and 58% of PM2.5 mass, respectively. A significant correlation was observed between water soluble K+ and OC at the rural (R2 = 0.63) and campus (R2 = 0.53) sites compared to the traffic one (R2 = 0.35). This may be attributed to increased biomass burning emissions at the rural and campus sites. The concentrations of SOC (Secondary Organic Carbon) were estimated based on the minimum OC/EC ratio, and were found to be 15.3 ± 6.3, 8.2 ± 5.8 and 28.8 ± 15.8 μg/m3, accounting for 18, 24.7 and 60.7% of total OC at the traffic, rural and campus sites, respectively. The surface morphology of the particles was analyzed by scanning electron microscopy and energy- dispersive X-ray spectroscopy (SEM/EDX). The results indicated branched chain-like aggregates of carbon bearing spheres at the traffic and rural sites, while at the campus site carbon-rich and minerogenic (mineral dust) particles were the dominant ones.