Atmospheric transport of visibility degrading pollutants into the California mojave desert

Three atmospheric tracer experiments using SF₆ and fine aerosol measurements were conducted in California to determine the relative impact of pollutant sources in the San Fernando Valley area of Los Angeles and the southern San Joaquin Valley on visibility in a portion of the Mojave Desert. Dilution ratios calculated for SF₆ and various gaseous and aerosol chemical species were used to indicate atmospheric transformation processes between source and receptor. The evolution of the aerosol sulfur mass distribution resulting from transport and transformation was measured. The SF₆ data were compared with predictions based upon the Gaussian dispersion model. SF₆ released in the San Fernando Valley was found to impact the southern Mojave Desert including the towns of Palmdale and Adelanto. SF₆ released at Oildale in the southern San Joaquin Valley was found to directly impact the northern Mojave Desert including the towns of Mojave and China Lake. Some SF₆ released in the San Joaquin Valley was also detected in the southern Mojave Desert. SF₆ was found to be diluted by factors of only 2-3 during passage over the mountains separating the source and receptor areas. The SF₆ dispersion during the San Fernando Valley experiment could be modeled with the Gaussian plume model if the experimentally determined mixing height, transport speed and plume trajectory were used, and the stability class was chosen to give good agreement with the data. A simple Gaussian plume model could not provide an adequate description of the San Joaquin Valley test because of the complexities of the wind reversal that occurred during the transition from morning to afternoon flow conditions during this test. Dilution ratios for conserved fine aerosols between Oildale and China Lake were found to be about 3. in excellent agreement with the SF₆ data. The timing of the diurnal degradation in visibility at China Lake was found to coincide with the arrival of SF₆, thus indicating that the southern San Joaquin Valley was the source of visibility degrading aerosol. The aerosol sulfur mass distribution was found to be essentially conserved between Oildale and China Lake and substantially different from Los Angeles-Palm Springs sulfur mass distributions. Our data indicate that pollutants from both the southern San Joaquin Valley and the Los Angeles Basin can impact the Mojave Desert during their transport eastward by the predominantly westerly summertime winds; consequently, both "plumes" should be taken into account when considering the impact of southern California pollutants upon the southwestern United States.