Comparison of Emissions and Ambient Concentrations of Carbon Monoxide, Nitrogen Oxides and Nonmethane Organic Gases in California's South Coast Air Basin

In the present study, a "top-down" validation of the reactive organic gas and carbon monoxide emission inventories for California's South Coast Air Basin was performed by comparing speciation profiles for nonmethane organic gases (NMOG) and ratios of CO/NO_ {rm x} and NMOG/NO_ {rm x} derived from early-morning (0700 to 0800) ambient measurements taken during the 1987 Southern California Air Quality Study with the corresponding ratios and speciation profiles derived from day-specific, hourly, gridded emission inventories. Twenty separate comparisons were examined for each pollutant ratio, each comparison representing a different combination of season, emission category, and spatial and temporal averaging of emissions. For the most appropriate comparisons, the ambient CO/NO_{rm x} and NMOG/NO_{rm x} ratios are about 1.5 and 2 to 2.5 times higher, respectively, than the corresponding inventory ratios. On the assumption that inventories of NO_{rm x } emissions are reasonably correct, these results indicate that on-road motor vehicle CO and NMOG emissions are significantly underestimated. Comparisons of long -term trends in ambient 0600-0900 CO/NO_ {rm x} and NMHC/NO_ {rm x} ratios with corresponding trends in emission inventory ratios show similar differences, indicating that a discrepancy between ambient and emission inventory pollutant ratios has existed for some time. It is postulated that actual ROG emissions in the SoCAB were 700 to 1000 tons/day higher than the ARB's emission inventory estimates for 1987. The underestimation of emissions has important implications for the validation and application of air quality simulation models, establishing baseline inventories to monitor the five percent annual emission reductions mandated by the California Clean Air Act, and quantifying the effectiveness of current and future emission control measures.