Role of ammonia in secondary inorganic aerosols formation at an ammonia-rich city in winter in north China: A comparative study among industry, urban, and rural sites

Ammonia is essential for the generation of secondary inorganic aerosols (SIA) in particulate matter, which affects severely the air quality in north China. In this study, PM2.5 sampling was conducted as well as gaseous pollutant concentration and meteorological parameters were measured from November 2017 to January 2018. PM2.5 concentration was highest in the industrial site (94.8 ± 41.7 μg m-3), followed by urban (40.9 ± 24.1 μg m-3) and rural (35.6 ± 20.3 μg m-3) sites. The mass ratio of NO3-/SO42- exhibited clear site variations, with the highest average value of 1.2 was found at the urban site, likely due to the dense traffic volume resulting in higher emissions of NO2, and the lowest value of 0.9 at the industry site. The presence of Excess-NHx (E-NHx), raising the pH 24 by 1.4, 1.3, and 1.4 units in industry, urban, and rural sites, respectively, might be vital for raising the aerosol pH. Correlation coefficients of Nitrogen oxidation rate (NOR, NOR = [NO3-]/[NO3-] + [NO2]) vs. Photochemical oxidants (Ox, NO2 +O3 in our study) and NOR vs. aerosol water content (AWC) at three sites were implied that both homogeneous and heterogeneous reactions occurred for nitrate formation in industry site, while heterogeneous reactions were dominant in urban and rural sites. Oxidation rates were most sensitive to the variation of E-NHx concentration at rural site, followed by the urban and industry sites, which was shown by the fact that the increase in E-NHx concentration by 1.0 μg m-3 increased the SIA concentration by 1.21, 1.02, and 0.37 μg m-3 at rural, urban, and industry sites, respectively. With the increase in NHx emissions at present, the role of NHx in SIA formation at ammonia-rich atmosphere requires more attention, especially in the less-noticed rural areas.