Seasonal variations in water soluble inorganic ions, OC and EC in PM10 and PM>10 aerosols over Delhi: Influence of sources and meteorological factors

Abstract

The PM10 and particles of greater than 10 μm aerodynamic diameter (hence forth referred as PM>10) collected over Delhi exceeded the National Ambient Air Quality Standards (100 μg m-3) with an annual average of 215 μg m-3 and 495 μg m-3, respectively. The water soluble inorganic ions (WSII) were higher throughout the year in PM10 (13.5%) than in PM>10 (5%). SO42dominated over NO3-ions and contributed 54% to the total WSII in PM10. NO3-and Ca2+ were dominant ions in PM>10. NH4+ during winter and autumn, and Ca2+ in summer PM10 samples were the major acid neutralizing species. The WSII showed monthly and seasonal changes. The total carbon (TC = EC + OC) constituted 8.8%-47.8% of total PM10 mass, and OC and EC varied from 9.91 to 37.06 μg m-3 and 5.42 to 22.23 μg m-3 during the year long sampling period. The SOC contributed more to OC in summers and the char (EC1) dominated over soot (EC2 + EC3) in EC fractions throughout the year. In summer samples, secondary OC (SOC) contributed 81% to OC and the OC/EC ratio was 3.61 suggesting the possible role of mineral dust and high photochemical activity in SOC production. For the eight different fractions, three dominant sources were identified, coal combustion, biomass burning, and motor vehicle exhaust for OC1, OC2 and OC3; vehicle exhaust for OC4, OP, EC2 and EC3; and biomass and coal combustion for EC1. The seasonal changes in the WSII and carbon concentrations in PM10 and PM>10 are attributed to both the sources and the meteorological conditions in and around the study area.