Formation of Nitrate in the Residual Layer of Beijing: Pathways Evaluation and Contributions to the Ground Level

High concentrations of particulate matter severely degrade air quality and pose significant threats to public health. Nitrate-driven pollution has long been recognized as a primary contributor to haze formation at the ground surface in Beijing. Here, comprehensive observations of N₂O₅ and related species were conducted using a unique 528 m platform in Beijing. Elevated N₂O₅ concentrations (1.3 ppb, 1 h average) were recorded. The average nighttime nitrate production rate from N₂O₅ heterogeneous hydrolysis was 5.8 μg·m^-3·h^-1, while the partitioning of HNO₃ at night significantly contributes to nitrate with an average production rate of 7.6 μg·m^-3·h^-1. As a result, chemical processes dominated by N₂O₅-NO₃ contribute approximately 65% of the total nitrate source. Using a large eddy simulation model, peak nitrate transfers of 14.1 μg//m³ from the residual layer to the surface were found, with a contribution of up to approximately 43% during the nitrate morning peak compared to simultaneous surface measurements. Entrainment effects even persisted into the afternoon, highlighting the vital contribution from thermal shear turbulence mixing. Sensitivity tests further suggest that reducing NO_<i>x</i> emissions during boundary-layer mixing could effectively mitigate nitrate formation, thereby improving the ground-level air quality.