Vertically Resolved Formation Mechanisms of Fine Particulate Nitrate in Asian Megacities: Synergistic Lidar-Aircraft Observations and Process-Based Analysis

Abstract. The vertical distribution of particulate nitrate is crucial for understanding its formation mechanisms and developing urban haze reduction strategies. This study uses advanced technologies to collect continuous vertical data on nitrate concentrations in Beijing for 2021, providing a seasonal analysis of their distribution and influencing factors. Spring exhibits the highest nitrate concentration below 2 km (8.29 ± 3.14 μg/m³), followed by winter (7.34 ± 2.78 μg/m³), autumn (6.65 ± 2.11 μg/m³), and summer (2.23 ± 0.82 μg/m³). Below 300 m, nitrate formation in spring and summer is primarily driven by thermodynamic factors; relative humidity (RH) correlates at 0.64, while temperature (T) correlates at -0.76. In winter, both atmospheric oxidizing capacity (AOC, r = 0.52) and thermodynamic factors (r = 0.68) significantly influence nitrate formation. Between 0.8 km and 2 km, dynamic drivers such as turbulent kinetic energy (TKE, r = -0.41) and vertical wind speed (r = -0.43) dominate in spring and autumn. In contrast, photochemical factors, including AOC (r = 0.58) and ozone (r = 0.60), influence winter and summer. High nitrate levels are observed at the boundary layer top, peaking at 118.11 μg/m³ in late autumn, closely linked to photochemical processes and dynamic drivers. In winter, nitrate concentrations exhibit distinct diurnal variations, peaking at 13:00, 18:00, and 22:00, with variations and peak concentrations increasing with altitude due to the accumulation of photochemical products and enhanced AOC at night. These findings provide actionable insights for urban air quality management.