Overview of the EXACT Campaign and Establishing an Oxidation Network in China

Atmospheric radicals critically govern tropospheric oxidation processes, yet chemical models systematically underestimate OH and HO2 under high- and low-NOₓ conditions, limiting accurate prediction of haze and ozone in polluted regions like China. To address these gaps, the Ensembled eXperiment of Atmospheric oxidation Capacity in the Troposphere (EXACT) was conducted across the North China Plain, deploying advanced instrumentation at urban (Beijing), rural (Luancheng), and remote (Shangdianzi) sites over four seasons from autumn 2024 to summer 2025.Preliminary results reveal strong seasonal variability: peak OH concentrations reached ~2×107 cm-3 in spring and summer but dropped to ~2×106 cm-3 in winter. Significant nighttime HO2 and RO2 were observed in rural and remote areas, indicating active dark chemistry. OH-j(O1D) correlations strengthened from winter (R2≈0.5) to summer (R2≈0.8), reflecting increasing photochemical dominance. HONO photolysis dominated ROx production in winter, while O3 and carbonyl photolysis became more important in warmer seasons. Chlorine chemistry also contributed significantly to ROₓ, with distinct diurnal ClNO2 patterns suggesting multiple source mechanisms.Compared to earlier campaigns, EXACT shows elevated OH turnover rates since 2020, offering a partial explanation for slowed PM2.5 decline and rising ozone. Building on these findings, the EXACT-Plus campaign will expand into central China’s Gan-E-Xiang region, where complex terrain, high humidity, and unique chlorine sources, such as kilns, fireworks, and biomass burning, may drive unexplored oxidation pathways. This work advances understanding of radical-driven pollution and supports improved model development and air quality management.