Correlation coefficient between PM2.5 concentration predictions and observed PM2.5 concentration in μg/m3 stratified by PM2.5 concentration level. Groupings are based on: whether PM2.5 is less than 6 μg/m3, 6–12 μg/m3 and greater than or equal to 12 μg/m3 (12 μg/m3 is the EPA standard). 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”). The last line reports the mean and standard deviation, in parenthesis, for the observed PM2.5 concentration within each category (low, medium or high).