Seasonal and regional variations of primary and secondary organic aerosols over the continental United States: semi-empirical estimates and model evaluation.

Seasonal and regional variations of primary (OC(pri)) and secondary (OC(sec)) organic carbon aerosols across the continental United States for the year 2001 were examined by a semi-empirical technique using observed OC and elemental carbon (EC) data from 142 routine monitoring sites in mostly rural locations across the country, coupled with the primary OC/EC ratios, obtained from a chemical transport model (i.e., Community Multiscale Air Quality (CMAQ) model). This application yields the first non-mechanistic estimates of the spatial and temporal variations in OC(pri) and OC(sec) over an entire year on a continental scale. There is significant seasonal and regional variability in the relative contributions of OC(pri) and OC(sec) to OC. Over the continental United States, the median OC(sec) concentrations are 0.13, 0.36, 0.63, 0.44, and 0.42 mictrog C m(-3) in winter (DJF), spring (MAM), summer (JJA), fall (SON), and annual, respectively, making 21, 44, 51, 42, and 43% contributions to OC, respectively. OC(pri) exceeds OC(sec) in all seasons except summer. Regional analysis shows that the southeastern region has the highest concentration of OC(pri) (annual median = 1.35 microg C m(-3)), whereas the central region has the highest concentration of OC(sec) (annual median = 0.76 microg C m(-3)). The mechanistic OC(sec) estimates from the CMAQ model were compared against the independently derived semi-empirical OC(sec) estimates. The results indicate that the mechanistic model reproduced the monthly medians of the semi-empirical OC(sec) estimates well over the northeast, southeast, midwest, and central regions in all months except the summer months (June, July, and August), during which the modeled regional monthly medians were consistently lower than the semi-empirical estimates. This indicates that the CMAQ model is missing OC(sec) formation pathways that are important in the summer.