Predicted secondary organic aerosol concentrations from the oxidation of isoprene in the eastern United States.

Isoprene, the most abundant non-methane hydrocarbon emitted into the troposphere, has generally not been considered a major source of SOA due to the relatively high volatility of its oxidation products. In this study, the SOA formed from the oxidation of isoprene is predicted using a three-dimensional chemical transport model, PMCAMx, across the eastern U.S. for July, October, January, and April 2001-2002. The variability of the measured SOA yields in the available smog chamber studies is captured by combining the base case scenario with upper and lower bound estimates of the measurements. For the base case simulation, the predicted annual average isoprene SOA concentration in the southeast is 0.09 microg m(-3) (bounds 0.04-0.23 microg m(-3)). Isoprene is predicted to produce 70% less SOA across the entire domain for spring and fall than during the summer and negligible amounts of SOA during the winter. During the summer, the average concentrations in the northeast are predicted to be 0.11 microg m(-3) (bounds 0.04-0.31 microg m(-3)) and in the southeast 0.19 microg m(-3) (bounds 0.11 -0.58 microg m(-3)). PMCAMx predictions are compared to available measurements of some isoprene SOA components in North Carolina and New York State. These modeling results suggest that on an annual basis isoprene oxidation is a small but non-negligible organic aerosol source in the eastern U.S. Its contribution is relatively more important during the summer and in the southeast U.S.