Ozone-driven non-linear controls on nighttime ClNO₂ formation: Unintended oxidant increases under NOx control

Nitryl chloride (ClNO₂) is an important but underrecognized driver of oxidant formation, linking nocturnal reactive chlorine chemistry to next-day ozone (O₃) production by enhancing daytime oxidation capacity through the photolytic releases of NO₂ and chlorine radicals. Despite its importance, its key controls remain difficult to isolate in ambient observations where chemical precursors and meteorological factors co-vary, limiting policy-relevant diagnosis of nocturnal chemistry. Here, we applied an explainable machine learning framework using seven routinely available predictors (O₃, NO₂, CO, PM_2.5, temperature, relative humidity, and solar radiation) to quantify their contributions of wintertime ClNO₂ formation in urban South Korea. The model reproduced ClNO₂ (R² = 0.96) and identified nocturnal O₃ as the dominant control, exceeding the influence of NO₂. Coupled O₃-NOx effects produced non-linear transitions among NO₂-, O₃-, and NO-limited regimes for ClNO₂ formation. Under observed wintertime long-term trends at this site-nocturnal O₃ rising (+0.62 ppbv yr^-1) and NO₂ declining (-0.53 ppbv yr^-1)- ClNO₂ is projected to increase by 1-2% annually, suggesting a potential positive feedback with next-day O₃ formation. These results suggest that, when ClNO₂ production is O₃-limited with the rise in nocturnal O₃, NOx controls targeting NO₂ may not yield the intended O₃ response due to the enhanced ClNO₂ and its photolysis products, thereby strengthening daytime oxidation. O₃-control strategies should therefore explicitly account for nocturnal chemistry and chlorine activation alongside daytime photochemistry to avoid unintended increases in oxidant exposure under NOx emission reductions.