Estimating exposures to PM2.5 within urban areas requires surface PM2.5 concentrations at high temporal and spatial resolutions. We developed a mixed effects model to derive daily estimations of surface PM2.5 levels in Beijing, using the 3 km resolution satellite aerosol optical depth (AOD) calibrated daily by the newly available high-density surface measurements. The mixed effects model accounts for daily variations of AOD-PM2.5 relationships and shows good performance in model predictions (R(2) of 0.81-0.83) and cross-validations (R(2) of 0.75-0.79). Satellite derived population-weighted mean PM2.5 for Beijing was 51.2 μg/m(3) over the study period (Mar 2013 to Apr 2014), 46% higher than China's annual-mean PM2.5 standard of 35 μg/m(3). We estimated that more than 19.2 million people (98% of Beijing's population) are exposed to harmful level of long-term PM2.5 pollution. During 25% of the days with model data, the population-weighted mean PM2.5 exceeded China's daily PM2.5 standard of 75 μg/m(3). Predicted high-resolution daily PM2.5 maps are useful to identify pollution "hot spots" and estimate short- and long-term exposure. We further demonstrated that a good calibration of the satellite data requires a relatively large number of ground-level PM2.5 monitoring sites and more are still needed in Beijing.