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 (μ_sCIs%). Furthermore, nitrogen oxides (NO_x) exert a distinct diurnal regulatory effect: lower NO_x levels enhance μ_sCIs% during the day by limiting OH propagation, whereas high NO_x 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/NO_x reductions, and under nighttime conditions with low photolysis-dependent OH.