OH, HO2 and RO2 Radical Chemistry Across North China Plain Informed by Multi-seasonal Observations and Experimental Budgets

RO_x radicals (including OH, HO₂ and RO₂) are central to atmospheric chemistry, governing the removal of trace gases and the formation of secondary pollutants such as ozone and secondary organic aerosols. Despite decades of research since OH radical was recognized as the core atmospheric oxidant, the chemical production and destruction processes of RO_x radicals under varied NO_x levels remain insufficiently constrained, limiting a mechanistic comprehension of atmospheric oxidation capacity. To address this, the Ensembled eXperiment of Atmospheric oxidation Capacity in the Troposphere (EXACT) campaign was conducted over one year across urban, regional, and background sites in the North China Plain. OH, HO₂ and RO₂ were measured online using an updated Peking University Laser Induced Fluorescence (PKU-LIF) system during one representative month per season (autumn, winter, spring, summer).

Preliminary results indicate that the daily maximum OH concentrations reached up to 1.5×10⁷ molecules cm^-3 in summer, which is five times higher than in winter (~3×10⁶ molecules cm^-3). HO₂ and RO₂ concentrations typically peaked at (2-3)×10⁹ molecules cm^-3 in summer across different sites, approximately an order of magnitude higher than (1-2)×10⁸ molecules cm^-3 in winter, with spring and autumn exhibiting intermediate levels. Total OH reactivity (k_OH) showed distinct spatiotemporal patterns, with daily peak values ranging from 10 s^-1 at the background site to over 30 s^-1 at the regional site, reflecting the complex mixture of anthropogenic and biogenic VOCs. The seasonal and diurnal variability of RO_x concentrations highlights distinct patterns influenced by local environmental conditions and photochemical activity. Based on the comprehensive observational datasets, we perform a detailed experimental budget analysis for individual radicals and their sum (RO_x). The research quantifies the contributions of critical radical sources and sinks, and identifies the dominant chemical pathways driving RO_x levels under varying NO_x conditions. Our findings advance the mechanistic understanding of radical chemistry and provide observational constraints that refine current chemical mechanisms for simulating atmospheric oxidation capacity and secondary pollution formation.