Modeling the atmospheric concentrations of individual volatile organic compounds

An Eulerian photochemical air quality model is developed in which the transport and chemical reactions of 53 individual organic species are tracked explicitly. The model is applied to the Los Angeles, California area for the period 27–28 August 1987. An extensive set of speciated ambient concentration data for hydrocarbons and carbonyls is available for this period, which provides the basis for comparison between predicted and observed concentrations of individual organic compounds. Hot stabilized exhaust emissions of volatile organic compounds from motor vehicles were increased to three times the baseline (EMFAC 7E) values, consistent with measurements made in the Van Nuys tunnel during 1987. Results of the model performance analysis show that the ambient concentrations of most individual organic species are predicted to within a normalized bias of ± 50%, with better performance in many cases. Most of the aromatic hydrocarbons for which ambient concentration data are available are predicted with a bias of ± 20% or less. Acetylene and ethene concentration predictions are higher than ambient concentrations, consistent with previous findings that the southern California atmosphere contains more unburned gasoline and less of the combustion-derived species such as acetylene and ethene than are found in the present emission inventory. A comparison of model predictions and observed isoprene concentrations indicates that at most sites where measurements were made, the model is predicting the concentrations of this biogenic compound correctly. However, at Claremont, both ozone and isoprene concentrations are seriously underpredicted by the model. This suggests that isoprene emissions near Claremont may be higher than previously thought. Given the high reactivity of isoprene, further exploration of its possible role in creating the peak ozone levels near Claremont is warranted