Observations of the diurnal and seasonal trends in nitrogen oxides in the western Sierra Nevada

Observations of speciated nitrogen oxides, namely NO2, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), and HNO3 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 NO2 and a predominance of the inorganic forms of nitrogen oxides.