Changes in ammonia and its effects on PM chemical property in three winter seasons in Beijing, China.

NH, SO, NO and the inorganic ions of PM in winter 2009, 2014 and 2016 were examined to investigate the change in NH and aerosol chemistry in Beijing, China. NH concentrations showed an increase by 59% on average, in contrast to the decrease of SO by 63% from winter 2009 to 2016. The mean mass ratio of NH/NH was 0.83 ± 0.12 in 2016, which is higher than those obtained in 2009 and 2014, implying more NH remaining as free NH in 2016 winter. Our findings suggest that vehicles exhaust emissions are an important NH source in urban central atmosphere of Beijing in winter. Despite the observed NO presenting declining trends from 2014 to 2016, nitrate concentrations even exhibited a significant increasing trend, which may be largely attributable to high NH levels. An in-depth analysis of measured NH and aerosol species in a heavy pollution episode in December 2016, combined with the acidity predicted by ISORROPIA II model demonstrated abundant NH most of the time in air, where NH is not only a precursor for NH but also effect the neutralization of SO and NO in PM. With high RH and low photochemical activity, elevated NO concentration was attributed to an enhanced heterogeneous conversion of NO to HNO to form NHNO in pollution transport stage. The decrease in NO from high level and the increase in NH, with peaks of SO occurring were observed in pollution cumulative stage. The aqueous-phase oxidation of SO by NO to sulfate might play an important role with high pH values. Our results suggested that the simultaneous control of NH emissions in conjunction with SO and NO emissions would be more effective in reducing particulate matter PM formation.