1 Fusing Observational , Satellite Remote Sensing and 2 Air Quality Model Simulated Data to Estimate 3 Spatiotemporal Variations of PM 2 . 5 Exposure in 4 China 5

Estimating ground surface PM2.5 with fine spatiotemporal resolution is a critical technique 17 for exposure assessments in epidemiological studies of its health risks. Previous studies have 18 utilized monitoring, satellite remote sensing or air quality modeling data to evaluate the 19 spatiotemporal variations of PM2.5 concentrations, but such studies rarely combined these data 20 simultaneously. Through assembling techniques, including linear mixed effect regressions with a 21 spatial-varying coefficient, a maximum likelihood estimator and the spatiotemporal Kriging 22 together, we develop a three-stage model to fuse PM2.5 monitoring data, satellite-derived aerosol 23 optical depth (AOD) and community multi-scale air quality (CMAQ) simulations together and 24 apply it to estimate daily PM2.5 at a spatial resolution of 0.1 ̊ over China. Performance of the three25 stage model is evaluated using a cross-validation (CV) method step by step. CV results show that 26 the finally fused estimator of PM2.5 is in good agreement with the observational data (RMSE = 23.0 27 μg/m and R2 = 0.72) and outperforms either AOD-derived PM2.5 (R2 = 0.62) or CMAQ simulations 28 (R2 = 0.51). According to step-specific CVs, in data fusion, AOD-derived PM2.5 plays a key role to 29 reduce mean bias, whereas CMAQ provides spatiotemporally complete predictions, which avoids 30 sampling bias caused by non-random incompleteness in satellite-derived AOD. Our fused products 31 are more capable than either CMAQ simulations or AOD-based estimates in characterizing the 32 polluting procedure during haze episodes and thus can support both chronic and acute exposure 33 assessments of ambient PM2.5. Based on the products, averaged concentration of annual exposure to 34 PM2.5 was 55.7 μg/m3, while averaged count of polluted days (PM2.5 > 75 μg/m3) was 81, across China 35 during 2014. Fused estimates will be publicly available for future health-related studies. 36