CHAPTER THREE AIR QUALITY Steven

Introduction Lake Tahoe resides in a high elevation basin separated from the Sacramento Valley by the dominant Sierra Nevada divide along the Crystal Range. Lower ridges of the Carson Range to the east separate the lake from the Great Basin. These physical attributes define atmospheric processes in the Tahoe basin as much as define hydrological processes. The presence of the cold lake at the bottom of the Tahoe basin determines an atmospheric regime that, in the absence of strong synoptic weather systems, develops very strong, shallow subsidence and radiation inversions at all times throughout the year. In addition, the rapid radiation cooling at night generates gentle but predictable downslope winds each night, moving from the ridgetops down over the developed areas at the edge of the lake and out over the lake itself. Local pollutants within this basin are trapped by inversions, which occur almost nightly in the summer and between storms during the winter, greatly limiting the volume of air into which they can be mixed. This condition then allows pollutants to build up to elevated concentrations. Downslope winds each night move local pollutants from developed areas around the periphery of the lake out over the lake, increasing the opportunity for these pollutants to deposit into the lake itself. This meteorological regime of weak or calm winds and a strong inversion is the most common atmospheric pattern at all times of the year (Cahill et al. 1989, 1997). The location of Lake Tahoe directly to the east of the crest of the Sierra Nevada Mountains creates the second most common meteorological regime, that of transport from the Sacramento Valley into the Lake Tahoe basin by mountain upslope winds. This pattern develops when the western slopes of the Sierra Nevada are heated, causing the