VALIDATION OF THE CHEMICAL MASS-BALANCE RECEPTOR MODEL APPLIED TO HYDROCARBON SOURCE APPORTIONMENT IN THE SOUTHERN CALIFORNIA AIR-QUALITY STUDY

The non-methane hydrocarbon (NMHC) data base acquired during the South ern California Air Quality Study (SCAQS) was used to assess the perfor mance of the chemical mass balance (CMB) receptor model following the CMB applications and validation protocol. As a prelude to the actual C MB effective variance runs, initial source contribution estimates were made to determine the optimal combination of source profiles and fitt ing species. Several different source composition profiles were select ed for major source types to determine the effect of alternative profi les on the source contribution estimates and on overall model performa nce. The ambient NMHC data were also examined by less complex tracer a nd bivariate regression methods to gain additional insights about prob able source contributions, spatial and temporal patterns of emission s ources, and photochemical reactions of various hydrocarbon species. NM HC was apportioned to motor vehicle exhaust, liquid fuel, gasoline vap or, gas leaks, architectural and industrial coatings, and biogenic emi ssions. Attribution of source contributions among the motor vehicle so urce categories is highly sensitive to the abundance of ethyne and lig ht olefins to NMHC in the exhaust composition profile, which varies wi th emission control technology and vehicle maintenance and operation.