Characteristics of gaseous and particulate ammonia and their role in the formation of secondary inorganic particulate matter at Delhi, India

Abstract

Chemical characteristics of ambient ammonia (NH3), other trace gases (NO, NO2, SO2, and HNO3) and ionic species (NH4+, SO42−, NO3− and Cl− etc.,) of PM2.5 were estimated from January 2013 to December 2015 at an urban site of Delhi, India to evaluate the role of ambient NH3 in the formation of secondary inorganic aerosols over Delhi. The average mixing ratios of ambient NH3, NO, NO2, SO2 and HNO3 were recorded as 19.6 ± 3.5 (ppb), 20.4 ± 6.2 (ppb), 19.7 ± 5.3 (ppb), 1.7 ± 0.5 (ppb) and 1.2 ± 0.3 (ppb), respectively during the entire study period. The mixing ratios of NH3, other trace gases (SO2, NO and NO2) and ionic species of PM2.5 were recorded higher during winter season (except HNO3). The result reveals that the increased relative humidity (RH) during winter season plays a major role in the formation of NH4+ aerosol over the observational site of Delhi. The annual average concentration of total water soluble inorganic ionic components (WSIC) in PM2.5 was 69.1 ± 38.1 μg m−3 accounting for ~60% of PM2.5 concentration. The secondary aerosol components i.e. NH4+, SO42−, NO3− and Cl− shared the largest part of the total water soluble ions (61%) and PM2.5 concentration (36%). Among the secondary inorganic aerosol components in PM2.5, SO42− was the most abundant component followed by NO3− and Cl−. Ion balance and molar equivalent ratios indicated that the sufficient amount of NH4+ was available to neutralize SO42−, NO3− and Cl− in the winter season followed by summer and monsoon seasons. The formation of NH4NO3 was higher in winter due to low temperature and high humid conditions that drives the reaction between NH3 and HNO3 in forward direction.