Concentrations, deposition and ecological effects of reactive nitrogen in California ecosystems

Negative impacts of air pollution on California ecosystems are mainly caused by elevated levels of ozone (O3) and nitrogen (N) deposition. While O3 air pollution in California has improved significantly since the 1970s, ambient concentrations of reactive N species and their deposition have remained elevated in many ecologically important areas. Nitrogen deposition to California ecosystems is dominated by dry deposition of ammonia (NH3), nitrogen dioxide (NO2) and nitric acid (HNO3) vapor. These compounds have been measured in various areas of California using passive samplers working on the principle of diffusion of gases into various types of collecting media. The point data from passive samplers have been converted to surfaces (maps) of pollutant concentration by using the ArcGIS Geostatistical Analyst software (ESRI, Redlands, CA). Maps of distribution of individual N air pollutants and total reactive N have been developed for selected areas in California including the Sierra Nevada, the San Bernardino Mountains and the Mojave Desert. Our studies confirmed that transport of NH3 and HNO3 into complex terrain and subsequent N deposition are restricted in space due to the high deposition velocity of these pollutants. Consequently steep landscape gradients of N deposition occur in California mountain ranges with the highest potential for negative effects near the emission source areas in contrast to relatively unaffected more distant areas. Examples for such gradients occur for NH3 and HNO3 across the Sierra Nevada (caused by urban air pollution generated in the San Francisco Bay Area) and the California Central Valley (both urban and agricultural air pollution). Strong West to East gradients of NH3, NO, NO2 and HNO3 in southern California are caused by the Los Angeles urban pollution and emissions from motor vehicles along the major traffic routes with much lower importance of agricultural sources. For example, in the San Bernardino Mountains east of Los Angeles N deposition decreases from 70 to 7 kg/ha/yr over a 50 km west-to-east gradient. Air quality data from passive samplers have been used for development of an inferential method allowing for fine-scale calculation of N deposition and estimates of critical loads exceedance for nutritional N. Nitrogen deposition has also been measured in the Sierra Nevada and in southern California using ‘passive' ion exchange resin throughfall collectors. Data from all these relatively fine scale monitoring studies of atmospheric concentrations and deposition, in conjunction with simulation modeling of N deposition, all indicate very high exposures to N pollutants in more exposed sites, but variable deposition across the landscape. National monitoring networks such as the National Atmospheric Deposition Program (NADP) and dry deposition networks either do not account for dry deposition or do not adequately measure dry deposition. Also national deposition network sites do not occur at fine enough scale to provide ecologically relevant data and do not characterize N deposition in California. In such national maps, California is usually misrepresented as a state with relatively low N deposition. Prominent examples of the most sensitive indicators of harmful effects of elevated N deposition to California ecosystems include: shifts in epiphytic lichen communities and nitrogen enrichment of lichen tissue in mixed conifer forests, oak woodlands and chaparral ecosystems; enhanced biomass accumulation of exotic grasses to levels capable of causing major biodiversity changes and sustaining wildland fires in desert scrub, pinyon-juniper, and coastal sage scrub ecosystems. Increased nitrate (NO3-) concentrations in streamwater and groundwater from montane catchments is another effect of N deposition, but the land area thus affected in California is much less than that for the biodiversity effects on lichens and native plant communities. It is estimated that 35% of the California land area is in exceedance of the critical loads of N deposition for these various impacts in seven major vegetation types in California.