High-frequency Evolution of Urban Atmospheric Ammonia and Ammonium and Its Gas-to-Particle Conversion Mechanism in Nanjing City

Abstract

In this study, hourly mass concentrations of atmospheric gases (mainly NH3) and secondary inorganic aerosols (mainly NH4+, NO3-, and SO42-) in Nanjing City were continuously measured during the fall of 2018 by an online gas and aerosol chemical component monitor. The dataset was used to investigate the variation characteristics of ambient NH3 and NH4+ during polluted and non-polluted periods, and to explore the potential chemical mechanism during gas-to-particle conversion between NH3 and NH4+. The results show that throughout the sampling period, the mean values (±1σ) of the mass concentrations of NH3 and NH4+ were (15.3±6.7) μg·m-3 and (11.3±7.8) μg·m-3, respectively, and that their diurnal profiles were distinct between pollution and non-pollution periods. Analysis of the potential contribution sources indicated that local contributions exceeded long-range transport as the dominant source of measured NH3 and NH4+, suggesting that urban areas can be hotspots of NH3 emissions. Further in-depth analysis revealed that the process of gas-to-particle conversion was the main driving force with respect to controlling diurnal variations in NH3 and NH4+. Specifically, pollution episodes were characterized by low temperature (7.5-12.5℃) and high humidity (50%-90%) meteorological conditions. These conditions tended to accelerate the reaction rate of gas-to-particle conversion and facilitate the formation of aerosol ammonium, leading to pronounced (NH4)2SO4 and NH4NO3 increases during pollution events. These findings clarify the sources of NH3 in the urban atmosphere and its potential contribution to the formation of particulate matter.