| 01 Jul 2026
Vertical Structure and Seasonal Evolution of Atmospheric Oxidizing Capacity across Urban and Rural Regions: Observational Constraints from OH Radical Production Pathways
Abstract. Atmospheric oxidizing capacity (AOC) drives the formation of secondary pollutants, yet conventional surface observations fail to resolve its pronounced vertical heterogeneity, often leading to incomplete interpretations of regional pollution chemistry. Using ground-based hyperspectral vertical remote sensing observations collected between March and August 2023 at representative urban (AHU) and rural (CF) sites in the Yangtze-Huai River Basin, we quantified the vertical contributions of HONO, HCHO, and O3 photolysis to OH production. AOC showed a strong positive correlation with aerosol loading (R = 0.88–0.93), indicating that enhanced atmospheric oxidation promotes secondary aerosol formation. In urban air masses, the AOC regime exhibited distinct vertical stratification. Rapid oxidation below 1 km was primarily driven by HCHO and HONO, whereas O3 photolysis became the dominant OH source above 2.8 km, accounting for more than 74 % of total OH production. Urban OH production transitioned from near surface HONO dominance in spring (P(OH)HONO=4.43×10-4 ppb·s-1) to HCHO dominance in summer (P(OH)HCHO=5.22×10-4 ppb·s-1). A pronounced elevated HONO enhancement layer emerged near 2.4 km during summer, driven by intensified heterogeneous conversion, with a peak contribution of 30.6 % and a conversion rate C(HONO) of 0.053 h-1. By contrast, near surface OH production at the rural site remained consistently dominated by biogenic HCHO in both spring and summer (P(OH)HCHO=1.82×10-3 ppb·s-1). These findings challenge the conventional assumption that heterogeneous chemistry is confined to the near surface atmosphere. They further provide critical vertical constraints for three-dimensional atmospheric chemistry models and offer a mechanistic explanation for the limited effectiveness of surface-based NOx mitigation strategies under vertically decoupled upper-atmospheric photochemistry.
