Geochemistry of PM_(2.5) aerosols at an urban site, Varanasi, in the Eastern Indo-Gangetic Plain during pre-monsoon season

摘要

The knowledge of actual morphological features and composition of aerosols are very important to understand atmospheric chemistry, mixing state and radiative forcing on a global scale. Daily and weekly PM2.5 samples were collected during pre-monsoon season (March-May 2015) from an urban site, Varanasi, located in the eastern Indo-Gangetic Plain to study geochemical and morphological features of aerosols. PM2.5 samples were analyzed for their elemental concentration by means of an Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) whereas particle morphology and composition were ascertained by means of a Scanning Electron Microscope (SEM) coupled with an Energy Dispersive X-ray (EDX) spectroscopy. PM2.5 concentrations ranged between 22.2 and 70.5 mu gm(-3) in daily samples, whereas it varied between 52.0 and 106.2 mu gm(-3) in weekly samples. PM2.5 concentrations, except in one daily sample, were found to be higher than the 24-h threshold limit of World Health Organization (WHO) standards (25 mu gm(-3)). Elemental concentration of Pb and Zn were as high as similar to 2000 ppm and 3700 ppm, respectively highlighting impact of heavy metal pollution in Varanasi. The mass concentration of Al was the highest amongst all the measured elements followed by K, Fe, Zn, Ti, Pb, Mn, As, Cd. Enrichment Factors (EF) < 5 were observed for Fe, Ti and Mn pointing towards their crustal origin. However, an elevated Fe/Al ratio, with a mean value of 0.82, suggests an enrichment of Fe due to anthropogenic emissions. In contrast, EF values were > 5 for K, Zn, As, Pb and Cd suggesting anthropogenic sources as a major contributor to these elements at the study site. The cluster analysis suggests that biomass burning emissions and air mass traversing through northern and western parts of India majorly contributed to high PM2.5 concentration at Varanasi. In contrast, wind patterns did show lower velocities and different directions (from NW India to Eastern parts of India) during the days with relatively lower PM2.5 concentrations. Morphological features of individual particles reveal an irregular, aggregated and flaky morphology whereas the elemental composition revealed the dominance of aluminosilicates, soot and tarball with inclusion of anthropogenic elements (e.g. Pb) at Varanasi.