OVOCs drive radical cycling and ozone formation in background air

Surface ozone pollution is a critical global environmental challenge driven by the complex, nonlinear photochemical cycling of RO _x radicals (OH + HO₂ + RO₂). Oxygenated volatile organic compounds (OVOCs) are central to these cycles as both radical sources and sinks, yet their quantitative impact on regional radical budgets remains poorly understood due to historical limitations in ambient measurements. This knowledge gap hinders the accurate prediction of persistent ozone exceedances. Here we show that constraining atmospheric models with a broad suite of 23 OVOCs-specifically reactive dicarbonyls-is essential for the accurate simulation of radical chemistry in southern China's background air through comprehensive field observations and photochemical modeling. We find that models constrained with only the three most common OVOCs (formaldehyde, acetaldehyde, and acetone) overestimate hydroxyl radical concentrations by 50 %-100 %, whereas comprehensive constraints align simulations with observations. This discrepancy is caused by complex offsetting errors, including the severe overestimation of isoprene-derived intermediates and the significant underestimation of secondary biacetyl production. Our results reveal that photolysis of the measured OVOCs contributes 49 %-61 % of total RO _x production, with species such as methylglyoxal and biacetyl playing unexpectedly dominant roles in driving ozone formation. These findings highlight critical deficiencies in current chemical mechanisms and demonstrate that high-resolution monitoring of reactive OVOC intermediates is vital for developing effective emission control strategies to mitigate persistent regional ozone pollution.