Investigating Day and Nighttime Variability of Major Water-Soluble Inorganic Species and Role of Reactive Nitrogen Species in PM2.5 and PM10 - A Two Year Study

The study reports day and nighttime temporal variation of prominent water-soluble inorganic species (WSIS) of fine and coarse mode particulate matter at a site in central Delhi for 2012 and 2013. The two years’ concentrations of major ionic species (NH4+, K+, Ca2+ and Mg2+ and NO3-, SO42-) showed that mean levels of both ∑cations and ∑anions were higher in nighttime than daytime in both PM2.5 and PM10. Moreover, the difference of mean levels in daytime and nighttime was noticed higher in PM2.5 compared to PM10. Among anions, more variation was observed for NO3-, i.e., ~49% higher level in nighttime compared to daytime, while~9% higher values in the nighttime for SO42-. Higher variation of NO3- level might be due to active participation of NOx in daytime photo-oxidation process. Among cations,Ca2+ and Mg2+ showed higher concentration in daytime compared to nighttime. Higher day and nighttime variation of Ca2+ and Mg2+ in coarse particulate matter suggested their dominance in PM10 compared to PM2.5. In PM10, the highest Ca2+ concentration was observed in daytime summer and least in nighttime monsoon, indicating the role of loose soil and meteorological conditions. The higher daytime SO42- concentration in monsoon compared to summer show edits significant formation as secondary aerosols in fine mode. The reactive nitrogen species NO3- and NH4+ were correlated during the day and nighttime in PM2.5 and PM10.Moreover, NH4+ was highly correlated to SO42- and NO3-, mainly in the form of (NH4)2SO4 and NH4NO3.