Unraveling urban nighttime aerosol characteristics and meteorological factors on AOD-PM relationships in East Asia

Abstract

This study comprehensively investigates nighttime aerosol characteristics across East Asia, addressing a critical gap in current research, as most existing studies focus exclusively on daytime conditions. It systematically examines variations in column-integrated optical properties, vertically resolved extinction profiles, and aerosol-type distributions in six representative regions of East Asia, including Mongolia (MG), Beijing (BJ), Korea (KR), Japan (JP), Hong Kong (HK), and Taiwan (TW). Significant regional differences were identified, with northern regions (MG, BJ, KR, and JP) displaying consistent day-night aerosol optical depth (AOD) and Ångström exponent values. In contrast, southern regions (HK and TW) exhibit pronounced discrepancies. All regions exhibit smoother nighttime extinction profiles than during the daytime. However, northern regions showed sharper vertical decay, while southern regions exhibited secondary peaks attributed to transported smoke aerosols. Additionally, CALIPSO revealed peaks of vertical aerosol distribution at nighttime compared to daytime. Such abnormal phenomenon was then proved to be the more sensitivity of nighttime Lidar signal to upper thin aerosol layers. After removing this bias, the vertical distribution of aerosol types at nighttime is found to be generally consistent with that during the daytime. Meteorological analysis in BJ suggests consistency in AOD-PM relationships between day and night, highlighting temperature as the dominant factor driving seasonal variations in AOD–PM2.5 correlations, followed by relative humidity. The findings provide valuable insights into nighttime aerosol characteristics, supporting future research on diurnal aerosol radiative forcing.