What caused record-breaking aerosol loading over the South China Sea in April 2023

Abstract

Abstract. In April 2023, record-breaking aerosol optical depth (AOD) levels were observed over the South China Sea (SCS). This study investigates the sources, transport pathways, and large-scale dynamical conditions associated with this extreme aerosol event. Observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) revealed the highest April AOD in the 2003–2023 satellite record, with values increasing by approximately 150 % relative to the 2003–2022 climatological mean and exceeding 4σ above normal conditions. Enhanced carbon monoxide (CO) concentrations were simultaneously detected in the free to mid-troposphere (700–500 hPa) by Measurements of Pollution in the Troposphere (MOPITT) and Atmospheric Infrared Sounder (AIRS), indicating substantial long-range transport of combustion-related pollution. MODIS fire count and burned-area datasets further indicated intensified biomass-burning (BB) activity over northern Peninsular Southeast Asia (PSEA) during the same period. Backward trajectory simulations using the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model suggested that a large fraction of air masses arriving over the SCS originated from northern PSEA, supporting the interpretation that BB emissions from this region contributed substantially to the elevated aerosol loading over the SCS. Dynamical analyses revealed a persistent anti-cyclonic anomaly over northern PSEA at 500 hPa, accompanied by an eastward-shifted Bay of Bengal anticyclone at 700 hPa and a western North Pacific cyclonic anomaly at both levels. These circulation anomalies likely altered the prevailing regional flow by shifting climatological southerlies toward northerly anomalies, thereby favoring aerosol transport and accumulation over the SCS. Overall, the results suggest that the extreme aerosol event was associated with the combined influence of intensified BB emissions and anomalous atmospheric circulation, highlighting the important role of coupled emissions and atmospheric dynamics in shaping regional aerosol extremes.