Measurements of Ozone Precursors in the Lake Tahoe Basin, USA

摘要

The results of this and the previous study indicate that the majority of O_3 precursors (VOC and NOx) are emitted in the urbanized areas of the Central Valley and possibly the San Francisco Bay Area. The concentrations of ozone precursors, VOC and NOx at all four sites are very low, therefore, substantial in-situ photochemical production of ozone is unlikely at most sites. The only site which showed slightly higher NOx and VOC concentrations is TRPA, which is located in South Lake Tahoe near the Kingsbury Grade Highway. South Lake Tahoe is the biggest city in the Tahoe Basin and experiences higher local traffic. Thus photochemical production of ozone may be non-negligible in this area. The Tahoe sites display great site-to-site variability in O_3 concentrations during the evening and pre-dawn hours of 1900 to 0700 PST, but consistent maxima of 50 to 60 ppb from 1000 to 1700 PST. The observation that all of the Tahoe sites tend to rapidly reach the same middle day maximum between the hours of 0600 and 1000 PST suggests that this rise in surface ozone is due primarily to the vigorous vertical mixing that occurs in the hours immediately after sunrise, when ozone-depleted air near the surface is warmed by solar heating of the surface, while cooler, ozone-rich air from aloft is simultaneously mixed downwards. These well-mixed conditions persist throughout the Tahoe Basin until approximately 1700 PST, when there is marked decrease in solar insolation. During the evening hours, those sites that have good exposure to ozone-rich air from aloft, which is being transported in via regional-scale and long-range transport, continue to exhibit high ozone concentrations. Sites that are more conducive to the formation of nocturnal temperature inversions see a decrease in surface level ozone, primarily via dry deposition. These observations likely indicate that much of the ozone measured at Lake Tahoe sites results from the transport of polluted background air into the Basin from upwind pollution source regions (e.g., Sacramento, San Joaquin Valley and San Francisco Bay Area).