Study region with training and independent test sample locations [four routine AQS monitoring locations, a (468 Manzanita Av., Chico, CA), b (3377 Tahoe Blvd., South Lake Tahoe, CA), c (II-1270 Natividad Rd., Salinas, CA) and d (6078 Adobe Rd., Twentynine Palms, CA) selected for the long-term time series plots of the observed, predicted and simulated concentrations in Figure 7]