Surface Ozone in the Lake Tahoe Basin

Abstract:

Surface ozone (O₃) concentrations were measured in and around the Lake Tahoe Basin using both active monitors (12 sites in 2010) and passive samplers (31 sites 2002; 34 sites in 2010). The 2010 data from active monitors indicate average summertime diurnal maxima of approximately 50-55 ppb. Minimal site-to-site variability is observed within the Basin during the well-mixed hours of 10:00 to 17:00 PST, but large differences between different sites are observed in the late evening and pre-dawn hours. The observed trends correlate most strongly with elevation, topography, and surface vegetation. High elevation sites with steeply sloped topography and drier ground cover experience elevated O₃ concentrations throughout the night because they maintain good access to downward mixing of ozone-rich air from aloft with minimal losses due to dry deposition. Low elevation sites with flat topography and wetter surface vegetation experience low O₃ concentrations in the pre-dawn hours because of greatly reduced downward mixing coupled with enhanced O₃ removal via efficient dry deposition. Very high average O₃ concentrations (overall seasonal average = 64 ppb) were measured with passive samplers in the middle of the Lake in 2010. This latter finding may reflect high emissions of O₃ precursors from vehicular traffic around the Lake, emissions from motorboats, and/or elevated rates of photochemical processes due to high solar radiation and stagnant air masses over the Lake. Tahoe Basin sites with good nocturnal exposure to ozone-rich air from aloft experience average O₃ concentrations that are frequently higher than concurrent averages from the polluted upwind comparison sites of Sacramento, Folsom, and Placerville.