Correlation coefficient between PM2.5 concentration predictions and observed PM2.5 concentration in μg/m3 stratified by the number of active monitoring sites within 50 miles of the prediction site for each day. The methods considered are: raw CMAQ output, ordinary least squares (“OLS”), inverse distance weighting (“IDW”), universal Kriging(“UK”), downscaler, random forests (“RF”), support vector regression (“SVR”) and Neural networks (“NN”). Methods use either CMAQ and/or other geographic covariates (“Covs”).