Observations of the diurnal and seasonal trends in nitrogen oxides in the western Sierra Nevada
Observations of speciated nitrogen oxides, namely NO sub(2), total peroxy nitrates (Σ PNs), total alkyl nitrates (Σ ANs), and HNO sub(3) by thermal dissociation laser induced fluorescence (TD-LIF), and supporting chemical and meteorological measurements at Big Hill (1860 m), a high elevation site in California's Sierra Nevada Mountains, are described. From May through October, terrain-driven winds in the region routinely bring air from Sacramento, 100 km southwest of the site, upslope over oak and pine forests to Big Hill during the day, while at night, the site often samples clean, dry air characteristic of the free troposphere. Winter differs mainly in that the meteorology does not favour the buildup of Sacramento's pollution over the Sierra Nevada range, and the urban-influenced air that is seen has been less affected by biogenic VOC emissions, resulting in longer lifetime for NO sub(2) and a predominance of the inorganic forms of nitrogen oxides. Summertime observations at Big Hill can be compared with those from Granite Bay, a Sacramento suburb, and from the University of California's Blodgett Forest Research Station to examine the evolution of nitrogen oxides and ozone within the urban plume. Nitrogen oxide radicals (NO and NO sub(2)), which dominate total nitrogen oxides (NO sub(y)) at Granite Bay, are rapidly converted into HNO sub(3), Σ PNs, and Σ ANs, such that these compounds contribute 29, 30, and 21% respectively to the NO sub(y) budget in the plume at Big Hill. Nevertheless, the decreasing concentrations of NO sub(2) as the plume is advected to Big Hill lead to decreases in the production rate of HNO sub(3) and ozone. The data also demonstrate the role that temperature plays in sequestering NO sub(2) into peroxy nitrates, effectively decreasing the rate of ozone production. The important contribution of Σ ANs to NO sub(y) in the region suggests that they should be considered with regards to export of NO sub(y) from the boundary layer. Nocturnal observations of airmasses characteristic of the free troposphere showed lower NO sub(y) concentrations, which were dominated by HNO sub(3) with a relatively small contribution from the organic nitrates.