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. CAPSULE: Elevated NH3, NO2, and HNO3 on the western slopes of the Sierra Nevada Mountains (SNM) near the Central Valley of California (CVC) decreased with distance from CVC and elevation to regional background levels also recorded at high elevation sites of the White Mountains (WM).

The Nevada Rural Ozone Initiative (NVROI): Insights to understanding air pollution in complex terrain.

The Nevada Rural Ozone Initiative (NVROI) was established to better understand O3 concentrations in the Western United States (US). The major working hypothesis for development of the sampling network was that the sources of O3 to Nevada are regional and global. Within the framework of this overarching hypothesis, we specifically address two conceptual meteorological hypotheses: (1) The high elevation, complex terrain, and deep convective mixing that characterize Nevada, make this state ideally located to intercept polluted parcels of air transported into the US from the free troposphere; and (2) site specific terrain features will influence O3 concentrations observed at surface sites. Here, the impact of complex terrain and site location on observations are discussed. Data collected in Nevada at 6 sites (1385 to 2082 m above sea level (asl)) are compared with that collected at high elevation sites in Yosemite National Park and the White Mountains, California. Average daily maximum 1-hour concentrations of O3 during the first year of the NVROI ranged from 58 to 69 ppbv (spring), 53 to 62 ppbv (summer), 44 to 49 ppbv (fall), and 37 to 45 ppbv (winter). These were similar to those measured at 3 sites in Yosemite National Park (2022 to 3031 m asl), and at 4 sites in the White Mountains (1237 to 4342 m asl) (58 to 67 ppbv (summer) and 47 to 58 ppbv (fall)). Results show, that in complex terrain, collection of data should occur at high and low elevation sites to capture surface impacts, and site location with respect to topography should be considered. Additionally, concentrations measured are above the threshold reported for causing a reduction in growth and visible injury for plants (40 ppbv), and sustained exposure at high elevation locations in the Western USA may be detrimental for ecosystems.

Variability and sources of surface ozone at rural sites in Nevada, USA: Results from two years of the Nevada Rural Ozone Initiative.

Ozone (O3) has been measured at Great Basin National Park (GBNP) since September 1993. GBNP is located in a remote, rural area of eastern Nevada. Data indicate that GBNP will not comply with a more stringent National Ambient Air Quality Standard (NAAQS) for O3, which is based upon the 3-year average of the annual 4th highest Maximum Daily 8-h Average (MDA8) concentration. Trend analyses for GBNP data collected from 1993 to 2013 indicate that MDA8 O3 increased significantly for November to February, and May. The greatest increase was for May at 0.38, 0.35, and 0.46 ppb yr(-1) for the 95th, 50th, and 5th percentiles of MDA8 O3 values, respectively. With the exception of GBNP, continuous O3 monitoring in Nevada has been limited to the greater metropolitan areas. Due to the limited spatial detail of O3 measurements in rural Nevada, a network of rural monitoring sites was established beginning in July 2011. For a period ranging from July 2011 to June 2013, maximum MDA8 O3 at 6 sites occurred in the spring and summer, and ranged from 68 to 80ppb. Our analyses indicate that GBNP, in particular, is ideally positioned to intercept air containing elevated O3 derived from regional and global sources. For the 2 year period considered here, MDA8 O3 at GBNP was an average of 3.1 to 12.6 ppb higher than at other rural Nevada sites. Measured MDA8 O3 at GBNP exceeded the current regulatory threshold of 75 ppb on 7 occasions. Analyses of synoptic conditions, model tracers, and air mass back-trajectories on these days indicate that stratospheric intrusions, interstate pollution transport, wildfires, and Asian pollution contributed to elevated O3 observed at GBNP. We suggest that regional and global sources of ozone may pose challenges to achieving a more stringent O3 NAAQS in rural Nevada.