Surface ozone at the Devils Postpile National Monument receptor site during low and high wildland fire years

Abstract

Surface ozone (O3) was measured at the Devils Postpile National Monument (DEPO), eastern Sierra Nevada Mountains, California, during the 2007 (low-fire) and 2008 (high-fire) summer seasons. While mean and median values of O3 concentrations for the 2007 and 2008 summer seasons were similar, maximum O3 concentrations in June and August 2008 were higher than in any month of the 2007 summer season. This increase of maximum concentrations in the high-fire year is attributed to emissions of O3 precursors from wildland fires upwind of DEPO in addition to transport of polluted air from the California Central Valley (CCV) and the San Francisco Bay Area (SFBA). Analysis of backward trajectories for high O3 episodes in June 2007 and 2008 showed the lowest O3 pollution at DEPO when air masses originated over the Pacific Ocean (PO) and passed from West to East over PO and CCV at high altitudes. The highest O3 levels occurred when air masses originated in the vicinity of SFBA, swept through CCV in the NW–SE direction before reaching DEPO at low altitudes. Diurnal O3 concentrations were characterized by a sharp increase early morning and maximum values in late afternoon, followed by gradual evening decreases with very low pre-dawn minima, a phenomenon explained by local generation of O3 combined with the long range transport of polluted air masses from CCV, boundary layer destruction and surface deposition. Once in 2007, and 3 times in 2008, the 8-h average concentration exceeded 75 ppb, counting towards exceedance of the present primary Federal O3 standard (4th highest 8 h concentration <75 ppb over 3 years). The California 8-h average standard (<70 ppb) was exceeded 5 times in 2007 and 6 times in 2008, and these instances counted towards exceedance of the newly proposed primary Federal primary standard (4th highest 8-h concentration <70 ppb over 3 years). In addition, in 2008, the California 1-h average standard (<90 ppb) was exceeded 3 times. The proposed Federal secondary O3 standard of 13 ppm h was exceeded reaching 16 ppm h in 2007 and 15 ppm h in 2008 that indicates potentially phytotoxic effects on local flora.