Surface ozone in the Lake Tahoe Basin
Surface ozone (O3) concentrations were measured in and around the Lake Tahoe Basin using both active monitors (2010) and passive samplers (2002, 2010). The 2010 data from active monitors indicate average summertime diurnal maxima of approximately 50–55 ppb. Some 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 O3-rich air from aloft with smaller losses due to dry deposition. Low elevation sites with flat topography and more dense 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. Additionally, very high average O3 concentrations were measured with passive samplers in the middle of the Lake in 2010. This latter result likely reflects diminished dry deposition to the surface of the Lake. High elevation Tahoe Basin sites with exposure to nocturnal O3-rich air from aloft experience daily maxima of 8-h average O3 concentrations that are frequently higher than concurrent maxima from the polluted upwind comparison sites of Sacramento, Folsom, and Placerville. Wind rose analyses of archived NAM 12 km meteorological data for the summer of 2010 suggest that some of the sampling sites situated near the shoreline may have experienced onshore "lake breezes" during daytime hours and/or off-shore "land breezes" during the night. Backtrajectory analysis with the HYSPLIT model suggests that much of the ozone measured at Lake Tahoe results from the transport of "polluted background" air into the Basin from upwind pollution source regions. Calculation of ozone exposure indices indicates that the two most polluted sites sampled by active monitors in 2010 — the highest Genoa Peak site, located on the eastern side of the Lake at an elevation of 2734 m above sea level, and Angora Lookout, located to the south–southwest (SSW) of the Lake at an elevation of 2218 m above sea level — likely experienced some phytotoxic impacts, while the other Tahoe Basin locations received lower ozone exposures.