Unveiling the paradox of reduced surface solar radiation in February 2020 during COVID-19 lockdown: aerosol-cloud-dynamics interactions in Beijing

Abstract This study reveals the physical mechanism behind anomalous surface incident solar radiation ( R s ) attenuation in Beijing during the COVID-19 lockdown (February 2020). Sunshine Duration observations reveal the paradox of dimming event under strictest lockdown policies in February 2020 over Beijing. Modern-era retrospective analysis for research and applications, Version 2 (MERRA-2) reanalysis shows R s decreased abruptly by 12.89 W·m −2 in February 2020 compared to 2015–2019 (pre-pandemic), which is confirmed by Himiwari-8 satellite retrieved R s . Through multi-parameter attribution analysis, Shapley additive explanations indicated that an anomalous increase in aerosol optical depth contributed 31.700% to R s attenuation, primarily driven by chemical formation of secondary organic aerosols that offset benefits from reduced primary emissions. Simultaneously, modified cloud physical properties collectively accounted for 38.02% to R s attenuation. Specifically, doubled low cloud cover accounted for 23.89% while increased low cloud optical thickness contributed 13.52%. Further compounding these effects, weakened boundary layer dynamics exerted synergistic impacts: boundary layer height (BLH) compression (168.93 m) contributed 14.92% and reduced wind speed (0.63 m·s −1 ) contributed 9.51%, jointly promoting near-surface aerosol accumulation through suppressed pollutant diffusion. Additionally, elevated surface temperature (+1.36 °C) exacerbated R s attenuation by facilitating aerosol nucleation and cloud condensation nucleus formation. Collectively, these findings challenge the assumption that emission reduction invariably improves R s , ultimately confirming nonlinear climatic effects from coupled aerosol-cloud- R s interactions under abrupt anthropogenic disturbances.