The isotopic patterns and source apportionment of nitrate and ammonium in atmospheric aerosol

Nitrate (NO₃^-) and ammonium (NH₄⁺) are the major components in inorganic aerosol. However, their sources and formation processes remain unclear. This study conducted a year-round field measurement of TSP, PM_2.5 and PM_1.0 in five different sites in the Beijing-Tianjin-Hebei (BTH) region to determine the concentrations of water-soluble inorganic ions (WSIIs) and the isotopic compositions of inorganic nitrogen (δ¹⁵N-NH₄⁺, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-). The results showed the highest concentration of WSIIs in winter and lowest in summer. δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁵N-NH₄⁺ were in the range of -6.1-18.2, 52.2-103.8, and -28.7-36.2‰, respectively. The seasonal variations of δ¹⁵N-NO₃^- and δ¹⁵N-NH₄⁺ were an indication of relative contributions of the main sources and effects of meteorological conditions. The source apportionment identified fossil fuel combustion (38.2-50.6%), agricultural emissions (18-24.7%), biomass burning (16.3-22.7%), and road dust/soil (8.7-23.4%) were the main sources of inorganic aerosols. The local sources and regional migration contributed to the level of inorganic aerosol pollution. In winter, the aerosol in the BTH region was affected by the air mass from the northwest. While in spring and summer, the air mass was mainly from the South China. The low temperature and high relative humidity favored to the formation of inorganic nitrogen aerosol, and solar radiation affected the formation processes of inorganic aerosols by changing the oxidation pathway of NO₃^- and accelerating the volatilization and dissociation of ammonium nitrate (NH₄NO₃). This study discovered the main source contributions of inorganic nitrogen aerosol using N and O isotopes composition, and the obtained information has a great importance in understanding the effects of meteorological conditions on formation and the contribution of regional transport.