Observations of NO2, total peroxy nitrates, total alkyl nitrates, and HNO3 along with a variety of correlative measurements over the full annual cycle (March 2003- February 2004) were collected at Big Hill CA (38.842 °N, 120.408 °W, 1860 m). The elevation and location of the Big Hill site, approximately 30 km west of Lake Tahoe, were chosen so that observations made there could be analyzed to quantify the influence of urban areas, such as Sacramento and San Francisco, which are upwind of Lake Tahoe during westerly flow. Examining the seasonal and diurnal behaviour of reactive nitrogen at the site shows that in the winter:
· total reactive nitrogen is lower, net flow at the surface is downhill and the urban plume rarely reaches the western rim of the Basin.
· individual episodes of high NO2 and inorganic nitrates associated with small-scale burning events along the western slope may generate HNO3 that can reach Tahoe
Combining our data with corresponding measurements at UC Blodgett Forest, we have developed a highly constrained model of the processes that govern reactive nitrogen distribution during the summer months in the region. Based on our analyses of the observations made, we can draw the following conclusions:
· During summer months, the Sacramento plume is the dominant source of reactive nitrogen in the region of the western slope of the Sierra Nevada, but this plume rarely reaches as far east as the Tahoe Basin
· HNO3 deposition is sufficiently fast that very little remains in the plume by the time it reaches high elevation sites near the western rim of the Lake Tahoe Basin
· At Big Hill, similar concentrations of HNO3 are found in airmasses coming from Sacramento (west) and Tahoe (east), demonstrating that urban areas to the west of Lake Tahoe cannot be identified as a net source of nitric acid to the Tahoe Basin
· Organic nitrates are significantly elevated in the plume compared to background conditions but their contribution to nitrogen deposition remains poorly understood
Yearlong measurements from Big Hill, just west of the Lake Tahoe Basin, show that the chemical processing, deposition, and dilution of urban emissions result in a negligible direct contribution of this upwind source to dry deposition of nitrogen oxides to Lake Tahoe.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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