Primary and secondary organic aerosols over the United States: estimates on the basis of observed organic carbon (OC) and elemental carbon (EC), and air quality modeled primary OC/EC ratios

The temporal and spatial distributions of primary and secondary organic carbon aerosols (OC) over the continental US from 15 June–31 August 1999, were estimated by using observational OC and elemental carbon (EC) data from Interagency Monitoring of Protected Visual Environments (IMPROVE) and Southeastern Aerosol Research and Characterization project (SEARCH) networks, coupled with the primary OC/EC ratios, (OC/EC)pri, obtained from an emission/transport-model (i.e., US EPA Models-3/Community Multiscale Air Quality (CMAQ) model). It was found that the mean primary OC concentrations over the Northeast, Southeast, Central, West and West Pacific regions were 0.39±0.09 (mean ± standard deviation), 1.02±0.55, 0.47±0.34, 0.51±0.24, and 0.96±0.68 μg C m−3, respectively, while the mean secondary OC concentrations were 1.27±0.15, 1.52±0.59, 0.90±0.51, 0.51±0.29, and 0.94±0.52 μg C m−3, respectively. The contribution of secondary OC to the measured OC ranged from 48±16% over the West to 77±3% over the Northeast. The mean values of modeled (OC/EC)pri ratios ranged from 1.16±0.13 over the Northeast to 3.49±1.22 over the West Pacific. The results at the SEARCH sites indicate that the daily mean values of modeled (OC/EC)pri ratios ranged from 0.84 to 2.99 at Yorkville and the contributions of secondary OC to OC ranged from 0% to 66% at North Birmingham. Our results indicate significant temporal and geographic variability in the relative contributions of primary and secondary OC and that the use of a constant value to represent the (OC/EC)pri ratio at a location is not appropriate over the time scales studied here.