Surface ozone in the White Mountains of California

Surface ozone concentrations are presented for four high-elevation sites along a north-south transect along the spine of the White Mountains and a fifth site located at lower elevation approximately 15 km to the west on the floor of the Owens Valley. The ozone data, which were collected from mid-June through mid-October of 2009, include results from two sites, White Mountain Summit (4342 m elevation) and Barcroft Station (3783 m), that are believed to be higher in elevation than any previously investigated sampling locations in North America. Average daily ozone values from the five sampling sites display similar day-to-day and week-to-week temporal fluctuations, which suggest that the sites are experiencing the same regional-scale background patterns in air quality and meteorology. Ozone concentrations increase with increasing elevation, consistent with findings from prior studies in Europe and North America. A linear elevation gradient of +0.0042 ppb m(-1) is obtained for July 15-August 15, but analogous gradients for August 15-September 15 and September 15-October 15 show reduced linearity and possibly the onset of a plateau in ozone concentrations for elevations above 2000 m. Average diurnal cycle magnitudes decrease with increasing elevation, falling from similar to 25-35 ppb for the Owens Valley site to similar to 3-7 ppb at three of the four high-elevation sites. Diurnal cycle magnitudes decrease (or remain roughly constant) at the non-Summit sites during the progression from mid-July to mid-October, but the magnitude of the diurnal cycle at the Summit increases from similar to 3 ppb to similar to 7 ppb over this same time frame. This latter result is inconsistent with results from previous investigations at other alpine sites, and may indicate the presence of local, topography-influenced mixing dynamics that are unique to the White Mountains. High hourly ozone concentrations at White Mountain Summit are found to correlate with 72-hour HYSPLIT back-trajectories that reflect enhanced levels of ozone transport from polluted regions (such as the Central Valley of California) or meteorological conditions that are favorable for ozone production. Low ozone concentrations at the Summit are found to correlate with HYSPLIT back-trajectories that reflect reduced levels of ozone transport from polluted areas or meteorological conditions that are unfavorable for ozone production. (C) 2011 Elsevier Ltd. All rights reserved.