Mass-size distribution of PM10 and its characterization of ionic species in fine (PM2.5) and coarse (PM10−2.5) mode, New Delhi, India

Abstract

Size distribution of PM10 mass aerosols and its ionic characteristics were studied for 2 years from January 2006 to December 2007 at central Delhi by employing an 8-stage Andersen Cascade Impactor sampler. The mass of fine (PM2.5) and coarse (PM10−2.5) mode particles were integrated from particle mass determined in different stages. Average concentrations of mass PM10 and PM2.5 were observed to be 306 ± 182 and 136 ± 84 μg m−3, respectively, which are far in excess of annual averages stipulated by the Indian National Ambient Air Quality Standards (PM10: 60 μg m−3 and PM2.5: 40 μg m−3). The highest concentrations of PM10−2.5 (coarse) and PM2.5 (fine) were observed 505 ± 44 and 368 ± 61 μg m−3, respectively, during summer (June 2006) period, whereas the lower concentrations of PM10−2.5 (35 ± 9 μg m−3) and PM2.5 (29 ± 13 μg m−3) were observed during monsoon (September 2007). In summer, because of frequent dust storms, coarse particles are more dominant than fine particles during study period. However, during winter, the PM2.5 contribution became more pronounced as compared to summer probably due to enhanced emissions from anthropogenic activities, burning of biofuels/biomass and other human activities. A high ratio (0.58) of PM2.5/PM10 was observed during winter and low (0.24) during monsoon. A strong correlation between PM10 and PM2.5 (r2 = 0.93) was observed, indicating that variation in PM10 mass is governed by the variation in PM2.5. Major cations (NH4+, Na+, K+, Ca2+ and Mg2+) and anions (F−, Cl−, SO42− and NO3−) were analyzed along with pH. Average concentrations of SO42− and NO3− were observed to be 12.93 ± 0.98 and 10.33 ± 1.10 μg m−3, respectively. Significant correlation between SO42− and NO3− in PM1.0 was observed indicating the major sources of secondary aerosol which may be from thermal power plants located in the southeast and incomplete combustion by vehicular exhaust. A good correlation among secondary species (NH+, NO3− and SO42−) suggests that most of NH4+ is in the form of ammonium sulfate and ammonium nitrate in the atmosphere. During winter, the concentration of Ca2+ was also higher; it may be due to entrainment of roadside dust particles, traffic activities and low temperature. The molar ratio (1.39) between Cl− and Na+ was observed to be close to that of seawater (1.16). The presence of higher Cl− during winter is due to western disturbances and probably local emission of Cl− due to fabric bleaching activity in a number of export garment factories in the proximity of the sampling site.