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 N 2 O 5 and related species were conducted using a unique 528 m platform in Beijing. Elevated N 2 O 5 concentrations (1.3 ppb, 1 h average) were recorded. The average nighttime nitrate production rate from N 2 O 5 heterogeneous hydrolysis was 5.8 μg·m –3 ·h –1, while the partitioning of HNO 3 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 N 2 O 5 –NO 3 contribute approximately 65% of the total nitrate source. Using a large eddy simulation model, peak nitrate transfers of 14.1 μg//m 3 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 x emissions during boundary-layer mixing could effectively mitigate nitrate formation, thereby improving the ground-level air quality.