Mapping daily PM_2.5 at 500 m resolution over Beijing with improved hazy day performance

The application of satellite-derived aerosol optical depth (AOD) to infer surface PM_2.5 has significantly increased the spatial coverage and resolutions (1-10 km) of ground-level PM_2.5 mapping as required for accurate exposure estimation. The remaining challenge is to further increase the mapping resolution to the sub-km level with improved algorithms to minimize misrepresentation of severe haze as clouds. In this study, we provide the first daily PM_2.5 estimation over Beijing at a 500 m resolution using AOD from the Simplified Aerosol Retrieval Algorithm (SARA) and linear mixed effects model. A novel cloud screen method is developed which significantly improves data availability during hazy days. The cross-validation R² for PM_2.5 estimations is 0.82 with the cloud-screened SARA AOD. Based on the satellite-predicted high-resolution PM_2.5 map, all-day population-weighted PM_2.5 is estimated to be 81.4 μg m^-3 over Beijing (2.3 times higher than China's NAAQS of 35 μg m^-3). Compared to the standard MODIS Dark Target 3 km product which presents a significant percentage of missing data, the 500 m resolution PM_2.5 mapping derived from SARA AOD reveals distinct pollution patterns and population exposure conditions during severe hazy days, thereby providing valuable information for pollution control and epidemiological studies.