Dominant contribution of combustion-related ammonium during haze pollution in Beijing

Aerosol ammonium (NH₄⁺), mainly produced from the reactions of ammonia (NH₃) with acids in the atmosphere, has significant impacts on air pollution, radiative forcing, and human health. Understanding the source and formation mechanism of NH₄⁺ can provide scientific insights into air quality improvements. However, the sources of NH₃ in urban areas are not well understood, and few studies focus on NH₃/NH₄⁺ at different heights within the atmospheric boundary layer, which hinders a comprehensive understanding of aerosol NH₄⁺. In this study, we perform both field observation and modeling studies (the Community Multiscale Air Quality, CMAQ) to investigate regional NH₃ emission sources and vertically resolved NH₄⁺ formation mechanisms during the winter in Beijing. Both stable nitrogen isotope analyses and CMAQ model suggest that combustion-related NH₃ emissions, including fossil fuel sources, NH₃ slip, and biomass burning, are important sources of aerosol NH₄⁺ with more than 60% contribution occurring on heavily polluted days. In contrast, volatilization-related NH₃ sources (livestock breeding, N-fertilizer application, and human waste) are dominant on clean days. Combustion-related NH₃ is mostly local from Beijing, and biomass burning is likely an important NH₃ source (∼15%–20%) that was previously overlooked. More effective control strategies such as the two-product (e.g., reducing both SO₂ and NH₃) control policy should be considered to improve air quality.