Comparison of Emission Inventory and Ambient Concentration Ratios of CO, NMOG, and NO_x in California's South Coast Air Basin

Abstract

In the present study, we performed a "top-down" validation of the reactive organic gas and carbon monoxide emission inventories for California's South Coast Air Basin by comparing speciation profiles for nonmethane organic gases (NMOG) and ratios of CO/NO_x and NMOG/NO_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. We considered twenty separate comparisons for each ratio, each representing a different combination of season, emission category, and spatial and temporal averaging of emissions. We determined that the most appropriate comparison in summer was ambient pollutant ratios with ratios derived from morning on-road motor vehicle emission inventories, and in the fall, ambient ratios with ratios derived from overnight on-road motor vehicle emission inventories with some contribution from overnight stationary-source NO_x emission inventories. From these comparisons, the ambient CO/NO_x and NMOG/NO_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_x emissions are reasonably correct, these results indicate that on-road motor vehicle CO and NMOG emissions are significantly underestimated. Our comparisons of measured CO, NMOG, and NO_x concentrations and CO/NO_x and NMOG/NO_x ratios with air quality model predictions obtained by the California Air Resources Board show similar differences.