Vertically resolved formation mechanisms of fine particulate nitrate in Asian megacities: Integrated lidar - Aircraft observations and process analysis

Abstract

The vertical distribution of particulate nitrate is crucial for understanding its formation mechanisms and developing urban haze reduction strategies. Advanced technologies were used in this study to collect continuous vertical data on nitrate concentrations in Beijing for 2021, providing a seasonal analysis of their distribution and influencing factors. Spring exhibited the highest nitrate concentration below 2 km (8.29 ± 3.14 μg m-3), followed by winter (7.34 ± 2.78 μg m-3), autumn (6.65 ± 2.11 μg m-3), and summer (2.23 ± 0.82 μg m-3). Below 300 m, thermodynamic factors dominated nitrate formation in spring and summer (RH: R Combining double low line 0.64; temperature: R Combining double low line -0.76), whereas winter formation was driven by both atmospheric oxidizing capacity (AOC, R Combining double low line 0.52) and thermodynamic factors (R Combining double low line 0.68). Between 0.8 and 2 km, dynamic drivers prevailed in spring and autumn (TKE: R Combining double low line -0.41; vertical wind speed: R Combining double low line -0.43), while photochemical factors dominated in winter and summer (AOC: R Combining double low line 0.58; O3: R Combining double low line 0.60). High nitrate levels were observed at the boundary layer top (0.7-1.2 km), peaking at 118.11 μg m-3 in late autumn, closely linked to photochemical processes and dynamic drivers. In winter, nitrate concentrations exhibited distinct diurnal variations, peaking at 13:00, 18:00, and 22:00 LST, with variations and peak concentrations increasing with altitude due to the accumulation of photochemical products and enhanced AOC at night. These findings support targeted emission controls by reducing photochemical precursor emissions at the boundary layer top and strengthening NOx reductions at major sources.