Nitrogenous air pollutants and ozone exposure in the central Sierra Nevada and White Mountains of California – Distribution and evaluation of ecological risks
Ammonia (NH3), nitric oxide (NO), nitrogen dioxide (NO2), nitric acid (HNO3), and ozone (O3) were measured in summers of 2012 and 2013 with passive samplers. Nine monitoring sites were on W-E transect (511 to 3490 m) across central Sierra Nevada Mountains (SNM), and five sites on elevational gradient (1237 to 4346 m) in White Mountains (WM) of California. Levels of pollutants were similar in 2012 and 2013 in all sites. NH3, NO2, and HNO3 were highest near highly polluted Central Valley of California (CVC): maximum summer season means 7.8 μg m−3, 3.0 ppb, and 3.0 μg m−3, respectively. Regional background for NH3, NO2, and HNO3 in SNM occurred >20 km from CVC and >1500 m with seasonal averages: 2.1–4.8 μg m−3; 0.8–1.7 ppb; 1.0–1.8 μg m−3, respectively, during two seasons. Levels of NH3, NO2, and HNO3 in WM remote locations were similar: 1.2–3.3 μg m−3, 0.6–1.1 ppb, and 1.0–1.3 μg m−3, respectively. Seasonal mean O3 (38–60 ppb) in SNM did not change with distance from CVC nor elevation. In WM, O3 and NO mixing ratios were 41–61 ppb and 2.3–4.1 ppb, respectively, increasing with elevation. Even the lowest NH3 concentrations determined in this study were higher than NH3 continental background. This fact, as well as high values of Nreduced/Noxidized near CVC of 1.9 in 2012 and 2.0 in 2013, decreasing with distance to 0.7 in 2012 and 0.8 in 2013, show importance of NH3 emissions from CVC as a contributor to N deposition and ecological impacts in SNM. The phytotoxic O3 indices, AOT40 and W126, for selected sites on SNM and WM transects, showed high potential for negative O3 impacts on vegetation, including forest trees.