High-Altitude Ozone Concentrations in Yosemite National Park, Sierra Nevada

Degrading air quality has become a major concern for many national parks in North America. A new report released by the EPA has listed Yosemite National Park as one of eight national parks that now fail the federal smog standard. This has major implications for park management. This paper discusses preliminary results from a pilot study that was conducted in Yosemite National Park in August 2003 to better understand the structure of thermally-driven circulations and ozone transport at high elevations in the park and in the Sierra Nevada.

Preliminary analyses show a distinct diurnal pattern of surface ozone concentration at Tuolumne Meadows (2612 m MSL)that correlates well with the breakup of the nocturnal stable layer and the onset of upvalley winds. At Tioga Pass (3029 m MSL), the diurnal variation of ozone is less and the wind regime there is more complex than at Tuolumne Meadows. Another interesting feature observed during the experiment was a nocturnal down-mixing event. This occurred at Tuolumne Meadows after the initial buildup of the nocturnal stable layer. Sodar profiles showed southerly winds aloft that mixed downward to the surface, destroying the surface inversion. At this same time a strong spike in ozone concentrations occurred indicating that either an elevated layer of higher ozone concentrations was mixed vertically downwards to the surface or higher concentrations were advected horizontally by the southerly winds.

High-resolution simulations were made using the Penn State MM5 mesoscale forecast model with the innermost domain centered over Tuolumne Meadows in Yosemite National Park and a horizontal grid size of 1 km. Model simulations suggest that one mechanism that may be responsible for higher observed surface ozone concentrations is the role of downward vertical motion caused by the development of mountain waves. Similar events were shown in simulations conducted by Hacker et al (2001) for Asian dust events in the Coast Mountains of British Columbia. Further simulations are planned using a coupled MM5/CMAQ modeling system to shed light on the dynamical and chemical mechanisms responsible for the observed high ozone concentrations in Yosemite National Park and in the Sierra Nevada.