Study of chemical species associated with fine particles and their secondary particle formation at semi-arid region of India

The fine particles (PM2.5) were collected for 24 h from Agra atmosphere and chemically characterized during the one year study period. The average concentration of PM2.5 was 90.21 ± 20.62 μg m−3 which is higher than NAAQS and WHO annual standards and their seasonal trend was winter > summer > monsoon. Ionic study indicates that cations were dominant in comparison to anions. Seasonally, F−, Ca2+ and Mg2+ were higher during summer period and Cl−, NO3−, NH4+ and K+ were higher during monsoon period while SO42− was higher during winter season. The ratios Ca2+/Na+, Mg2+/Na+ and SO42−/Na+ were higher, while Cl−/Na ratio was lower than sea water ratio indicating the incorporation of non marine constituents in aerosols. The observed Cl−/Na+ (average 0.71) deviate considerable from the Sea water ratio suggesting either a fractionation of Cl− or enrichment of Na+ as there was significant deviation from marine sources. Linear correlation was observable between nss-Ca2+ and nss-Mg2+ with HCO3− throughout the annual seasonal cycle (HCO3−: Ca = 0.59, HCO3−: Mg2+ = 0.53) suggesting the origination of these species from soil dust. The calculated conversion ratio of ‘S’ (CRS) was lower than ‘N’ (CRN) which suggests that the secondary formation of NO3− aerosol from NO2 was more predominant than SO42−. Trajectory analysis indicates that long range transportation also contributes to PM2.5 mass over the measurement site.