Regional Transport Constrains the Microphysical Response of Black Carbon to Short-Term Emission Controls.

Black carbon (BC) has strong influences on air quality and climate, yet its short-term microphysical response to abrupt emission reductions remains poorly constrained. Here, using single-particle observations and machine learning, we investigate how temporary emission controls during the Asian Games affected BC properties. BC mass doubled after controls were lifted, indicating a rapid rebound of emissions. In contrast, BC microphysical properties, i.e., BC mixing state, optical properties, hygroscopicity, and cloud condensation nuclei (CCN) activity, responded rapidly to emission changes. Random Forest-SHAP analysis identifies air mass origin and temporal emission indicators as key contributors, with regional transport exerting effects comparable to, or exceeding, those of local emission reductions. Meteorological normalization attributes ∼ 40% of the BC mass decrease during the control period to emission reductions, while changes in BC absorption and CCN activity are strongly modulated by regional transport. Counterfactual sensitivity experiments further demonstrate that shifts in air mass origin alone can substantially modify BC microphysical properties under identical emission conditions. These results demonstrate that short-term emission controls can effectively suppress BC microphysical properties, but their efficacy is largely constrained by regional transport, providing insight into adjustments of BC direct and indirect effects during such periods.