Unveiling the vertical dynamics of atmospheric ammonium in Asia megacity: A GRASP-based investigation of spatiotemporal patterns and source drivers

Abstract

Atmospheric ammonium (NH₄⁺), a key component formed through complex chemical reactions and particulate transformations involving nitrogen oxides (NOₓ) and ammonia (NH₃), plays a critical role in climate dynamics, ecological balance, and human health. Understanding the vertical distribution of NH₄⁺ is essential for comprehensively characterizing regional aerosol patterns. However, the lack of systematic and spatiotemporally resolved studies has hindered the development of accurate NH₄⁺ distribution models and predictive capabilities. In this study, we present an innovative algorithm based on the GRASP model to retrieve hourly NH₄⁺ mass concentration data from 2021 to 2022, enabling a detailed investigation of NH₄⁺ vertical distribution characteristics in Asia super megacity-Beijing. Our findings reveal that transportation emissions are a primary source of NH₄⁺ in Beijing, while elevated pollution sources and long-range transport also significantly influence local NH₄⁺ distribution. The vertical profiles exhibit strong seasonal consistency over the two-year study period, with higher concentrations typically observed in upper atmospheric layers compared to near-surface layers, and distinct peaks at 1 km and 300 m above ground level. NH₄⁺ concentrations are highest in spring and lowest in winter. In spring, the influence of northern dust leads to elevated NH₄⁺ levels, particularly on moderately polluted days, with more pronounced vertical gradients. During autumn, a significant peak in NH₄⁺ concentration at 1 km altitude is attributed to long-range transport from southern urban centers. The contribution of NH₄⁺ to PM₂.₅ remains relatively stable over the two years, with a slight increase during polluted hours due to enhanced hygroscopicity of NH₄⁺ particles driven by higher atmospheric humidity. Strong correlations among chemical components across various altitudes indicate a high degree of homogeneity. Additionally, ammonium nitrate (NH₄NO₃) is identified as the predominant form of NH₄⁺ in Beijing during the study period. These insights advance our understanding of NH₄⁺ dynamics and its role in urban air quality, providing a foundation for improved aerosol modeling and pollution mitigation strategies.