Distribution of ozone, N air pollutants and N deposition and their potential ecological effects in the Lake Tahoe Basin, California
Ground-level ozone (O3) and atmospheric nitrogen (N) deposition are important factors in understanding threats to human and ecosystem health in the Lake Tahoe Basin, including diminishing clarity of the lake and increasing potential for catastrophic wildland fires. While some information on O3 distribution in the basin exists, very little is known about spatial and temporal distribution of O3 precursors (such as nitrogen oxides, NOx, and volatile organic compounds, VOCs) or diurnal changes of O3 concentrations. There is also a lack of information on distribution of ammonia (NH3) and nitric acid (HNO3), which are the main drivers of N dry deposition. Such knowledge is crucial for science-based development of air pollution control and land management strategies. In a 2010/2011 study we used passive samplers for monitoring concentrations of NOx, NH3, HNO3, O3 and VOCs at 34 sites distributed inside and outside of the basin. On a subset of 10 collocated sites real time O3 concentrations were measured with UV absorption 2B Technologies instruments and bulk and throughfall N deposition were determined with ion exchange resin (IER) collectors. We used ARC GIS Geostatistical Analyst to generate maps of air pollutant distribution. We also calculated indices of primary (health) and secondary (ecological) O3exposures, atmospheric N deposition and exceedances of critical loads for nutritional N.
High summertime concentrations of O3, NH3 and HNO3 on the western slope of the Sierra Nevada Mountains were caused by emissions from the California Central Valley (CCV). Elevated NO and NO2 levels and NO/NO2 ratios near the lake as well as elevated O3 and HNO3 levels above the lake and at high-elevation eastern basin locations indicate the influence of local emission sources and long-range transport of pollutants aloft from CCV. Two-week long average O3 concentrations for various sites ranged from 26.8 to 72.1 ppb, while the hourly maximum values reached 88.4 ppb. Two of the high elevation sites within the basin exceeded the Federal and state primary (human health) O3 standards. Nitrogen deposition as throughfall ranged from 1.3 to 5.6 kg N ha-1 yr-1 and at several sites where N deposition exceeded 3.1 kg N ha-1 yr-1 changes in lichen communities can be expected. Other potential ecological effects of O3 and N deposition and their interactions with climate variability will also be discussed.