Atmospheric ammonia and its impacts on regional air quality over the megacity of Shanghai, China

Abstract

Atmospheric ammonia (NH 3) has great environmental implications due to its important role in ecosystem and global nitrogen cycle, as well as contribution to secondary particle formation. Here, we report long-term continuous measurements of NH 3 at different locations (i.e. urban, industrial and rural) in Shanghai, China, which provide an unprecedented portrait of temporal and spatial characteristics of atmospheric NH 3 in and around this megacity. In addition to point emission sources, air masses originated from or that have passed over ammonia rich areas, e.g. rural and industrial sites, increase the observed NH 3 concentrations inside the urban area of Shanghai. Remarkable high-frequency NH 3 variations were measured at the industrial site, indicating instantaneous nearby industrial emission peaks. Additionally, we observed strong positive exponential correlations between NH 4 + /(NH 4 + +NH 3) and sulfate-nitrate-ammonium (SNA) aerosols, PM 2.5 mass concentrations, implying a considerable contribution of gas-to-particle conversion of ammonia to SNA aerosol formation. Lower temperature and higher humidity conditions were found to favor the conversion of gaseous ammonia to particle ammonium, particularly in autumn. Although NH 3 is currently not included in China s emission control policies of air pollution precursors, our results highlight the urgency and importance of monitoring gaseous ammonia and improving its emission inventory in and around Shanghai.