Role of atmospheric ammonia in the formation of inorganic secondary particulate matter: A study at Kanpur, India

Levels of fine Particulate Matter (PMfine), SO2 and NOx are interlinked through atmospheric reactions to a large extent. NOx, NH3, SO2, temperature and humidity are the important atmospheric constituents/conditions governing formation of fine particulate sulfates and nitrates. To understand the formation of inorganic secondary particles (nitrates and sulfates) in the atmosphere, a study was undertaken in Kanpur, India. Specifically, the study was designed to measure the atmospheric levels of NH4+; Ca2+, Mg2+, Na+, K+, NO3-, SO42-, CI-; NH3 (gas), HNO3 (gas), NO2 and PM10 (PM2.5/PM10 ratio = 0.74) covering winter and summer seasons and day and night samplings to capture the diurnal variations. Results showed NO3-, SO42- NH4+, K+ are found to be significantly high in winter season compared to the summer season. In winter, the molar ratio of NH4+ to SO42- was found to be greater than 2: 1. This higher molar ratio suggests that in addition to ( NH4)(2)SO4, NH4NO3 will be formed because of excess quantity of NH4+ present. In summer, the molar ratio was less than 2:1 indicating deficit of NH4+ to produce NH4NO3. The nitrogen conversion ratio ( NO2 to NO3) was found to be nearly 50% in the study area that suggested quick conversion of NO2 into nitric acid. As an overall conclusion, this study finds that NH3 plays a vital role in the formation of fine inorganic secondary particles particularly so in winter months and there is a need to identify and assess sources of ammonia emissions in India.