Comparison of Satellite- based PM2.5 Estimation from Aerosol Optical Depth and Top-of-atmosphere Reflectance

Aerosol optical depth (AOD) and top-of-atmosphere (TOA) reflectance are two useful sources of satellite data for estimating surface PM2.5 concentrations. Comparison of PM2.5 estimates between these two approaches remains to be explored. In this study, satellite observations of TOA reflectance and AOD from the Advanced Himawari Imager (AHI) onboard the Himawari-8 geostationary satellite in 2016 over Yangtze River Delta (YRD) and meteorological data are used to estimate hourly PM2.5 based on four different machine learning algorithms (i.e., random forest, extreme gradient boosting, gradient boosting regression, and support vector regression). For both reflectance-based and AOD-based approaches, our cross validated results show that random forest algorithm achieves the best performance, with a coefficient of determination (R-2) of 0.75 and root-mean-square error (RMSE) of 18.71 mu g m(-3) for the former and R-2 = 0.65 and RMSE = 15.69 mu g m(-3) for the later. Additionally, we find a large discrepancy in PM2.5 estimates between reflectance-based and AOD-based approaches in terms of annual mean and their spatial distribution, which is mainly due to the sampling difference, especially over northern YRD in winter. Overall, reflectance-based approach can provide robust PM2.5 estimates for both annual mean values and probability density function of hourly PM2.5. Our results further show that almost all population lives in non-attainment areas in YRD using annual mean PM2.5 from reflectance-based approach. This study suggests that reflectance-based approach is a valuable way for providing robust PM2.5 estimates and further for constraining health impact assessments.

Automated summary

This study used satellite data and meteorological data combined with machine learning algorithms to estimate PM2.5 concentrations in the Yangtze River Delta region, finding random forest algorithm performed the best in reflectance and AOD methods. Additionally, there were significant differences in PM2.5 estimates between reflectance and AOD methods in terms of annual mean values and spatial distribution, possibly due to sampling differences, especially in the northern region during winter.