Modeling the atmospheric concentrations of individual gas-phase and particle-phase organic compounds

An Eulerian photochemical airshed model is adapted to track the concentrations of individual vapor-phase, semivolatile, and particle-phase compounds over the carbon number range from C-1 to C-34 The model incorporates primary emissions of organic gases and particles from sources based on recent source tests. These emissions are processed through a photochemical airshed model whose chemical mechanism has been expanded to explicitly follow the reaction or formation of 125 individual vapor-phase organic compounds plus 11 lumped vapor-phase compound groups. Primary organic compounds in the particle phase can be disaggregated at will from a lumped primary organic compound mass category; in the present model application, 31 individual primary particulate organic compounds are tracked as they are transported from sources to receptor air monitoring sites. The model is applied to study air quality relationships for organics in California's South Coast Air Basin that surrounds Los Angeles during the severe photochemical smog episode that occurred on September 8-9,1993. The ambient concentrations of all normal alkanes and most aromatic hydrocarbons are predicted within the correct order of magnitude over 6 orders of magnitude concentration change from most abundant gas phase to least abundant particulate species studied. A formal evaluation of model performance shows that, with the exception of a Few outliers, the concentrations of over 100 organic compounds studied were reproduced with an average absolute bias of +/-47% and with roughly equal numbers of compounds underpredicted (58) versus overpredicted (46). The time series of observed aromatic hydrocarbons concentrations are reproduced closely, production of methylglyoxal from aromatic precursors is tracked, and the predicted olefinic hydrocarbon concentrations decline dramatically in concentration due to chemical reaction and dilution during downwind transport as is observed in the ambient monitoring database. This ability to simultaneously account for the concentrations of individual gas-phase and particulate organic compounds lays a foundation for future calculations of secondary organic aerosol formation and gas/particle repartitioning in the atmosphere.