Regional haze case studies in the southwestern U.S.—II. Source contributions

Abstract

The contributions of major source types to air quality and visibility in the desert southwestern U. S. near Page, AZ are determined for five distinctly different periods in June, July and December, 1979. The analysis is based on temporal and spatial patterns of parameters observed during 4 weeks of sampling carried out as part of Project VISTTA (1979) and other studies. These patterns are related to known source characteristics and computed air trajectories. An episode of low visibility accompanied by very high concentrations of carbon and potassium is traced to smoke from unusually large wildfires in southern Arizona. The unambiguous chemical and physical signal observed during this period serves to calibrate the emission transport simulation and back trajectory calculations. Three incursions of southern California pollutants throughout the southwest are identified—one in each of the months in which sampling was conducted. A low visibility summer incursion, during which sulfate concentrations increased by about 2μg m −3, followed several days of stagnation in southern California with high sulfate and ozone concentrations. A high visibility summer incursion, during which ozone concentrations increased but sulfate concentrations did not, followed an episode of high ozone and low sulfate concentrations in southern California. Both incursions were accompanied by a pronounced shift toward larger sulfate particle sizes. The bulk of the secondary pollutants observed during both incursions may have been formed in the strongly oxidizing southern California atmosphere.