Receptor model and emissions inventory source appontionments of nonmethane organic gases in Californiaʹs San Joaquin valley and San Francisco bay area

The chemical mass balance (CMB) receptor model was applied to the nonmethane organic gas (NMOG) database acquired during the San Joaquin Valley Air Quality Study (SJVAQS)/Atmospheric Utility Signatures—Predictions and Experiment (AUSPEX) Regional Model Adaptation Project (SARMAP). During SARMAP, the major contributors to NMOG were vehicle exhaust, liquid gasoline, gasoline vapor, oil production, acetone and unidentified organic compounds. Oil production was the major contributor to NMOG in the southern SJV during the morning hours, ranging from about one-third to one-half of the total NMOG. Contributions of oil production were lower during the afternoon due to increased ventilation, and larger contributions from secondary organic compounds. In the afternoon, the combined fraction of acetone and unidentified or unexplained (difference between calculated and measured mass) NMOG, which is mostly of secondary origin, accounted for about half of the total NMOG at receptor sites. Only the Yosemite and Giant Forest sites showed significant contributions from biogenic emissions. The fact that CMB did not detect significant contributions from biogenic sources in samples collected from sites in the SJV where estimated biogenic emission rates exceed those of either Yosemite or Giant Forest, suggests that biogenic emissions are overestimated in the SARMAP inventory. Source contribution estimates for total motor vehicle emissions averaged 75 and 70% of the total measured NMOG in urban areas during the 0800–1000 and 1200–1400 sampling periods, respectively, compared to the average daily emission inventory contribution of 44%. These results support recent studies which indicate that motor vehicle emissions have been seriously underestimated.