Revisiting the critical role of stabilized Criegee intermediates (sCIs) in sulfuric acid formation: coupling mechanistic updates with interpretable machine learning

Abstract. Sulfuric acid (H2SO4) is a key driver of atmospheric new particle formation and subsequent growth, playing a critical role in the formation of sulfate aerosols. While stabilized Criegee intermediates (sCIs) are recognized to be one of the free radicals oxidated sulfur dioxide (SO2), alongside the dominant hydroxyl radical (OH), their role in the formation of H2SO4 remains poorly understood due to uncertainties in current chemical mechanisms. Here, we quantify the impact of updated sCIs chemistry within the MCM v3.3.1 mechanism using an XGBoost-SHAP model, revealing that the updated mechanism significantly amplifies the contribution of precursor species to the sCIs oxidation rate by a factor of 1.97–10.75. To identify scenarios where sCIs effectively compete with OH, sensitivity analysis highlights ozone (O3) and alkenes as the primary synergistic drivers promoting the fractional contribution of sCIs to H2SO4 (μsCIs%). Furthermore, nitrogen oxides (NOx) exert a distinct diurnal regulatory effect: lower NOx levels enhance μsCIs% during the day by limiting OH propagation, whereas high NOx promotes μsCIs% at night by accelerating OH termination. To assess ambient atmosphere implications, we used a Random Forest model to identify a period where gas-phase pathways dominated sulfate formation. Constrained AtChem simulations demonstrate the updated mechanism elevates sCIs contributions to H2SO4 from 1.11 % to 7.13 % by day and 2.95 % to 15.72 % by night. These findings underscore the significance of sCIs for H2SO4 production, especially in urban environments with high O3 from imbalanced VOC/NOx reductions, and under nighttime conditions with low photolysis-dependent OH.