Characteristics and Ecological Impacts of Atmospheric Deposition in Urban and Urban-Affected Regions

Mark Fenn1, Andrzej Bytnerowicz2, Susan Schilling3, Michael Bell4 and James Sickman5

Urban air pollution causes major effects on human and ecosystem health and the environment. On-road emissions are major drivers of air quality in urban regions. Technologies for reducing NOx emissions result in production of reduced forms of N. Industrial, urban and on-road emissions of NOx have declined dramatically while NH3 emissions have increased. Simulation models underestimate NH4+ deposition, and models and regional monitoring networks do not capture fine-scale spatial heterogeneity in dry deposition or fogwater deposition to receptor sites affected by urban emissions. The highest N deposition reported for the Western hemisphere (70 kg/ha/yr as throughfall) occurs in the San Bernardino National Forest within the eastern region of the Los Angeles Air Basin. Deposition can vary ten-fold over a 50-60 km gradient in forests downwind of Los Angeles. Contrary to previous assumptions, sulfur deposition in Los Angeles remains unusually high (30-40 kg/ha/yr), due to emissions from  shipping, heavy transport and industry. Wet deposition data are of limited utility in characterizing deposition in arid regions. Notwithstanding, long-term data at the NADP site near Los Angeles (CA42) indicate decreasing temporal trends in deposition of SO42+ and NO3- and slightly increasing trends in NH4+. At several such urban-dominated NADP sites across the U.S., wet deposition switched to NH4+ values that are consistently greater than NO3- molar values during the 2005 to 2010 period. Data on urban NH3 concentrations support this trend of increasing importance of reduced forms of N, primarily from on-road emissions of NH3. In this light, the Lake Tahoe Air Basin case study will also be discussed.

Nitrogen critical loads in ecosystems of highly-urbanized southern California have been determined for streamwater NO3-, epiphytic lichen community change, vegetation type change, tree health, soil acidification and mycorrhizal responses. Urban emissions have resulted in extensive areas of CL exceedance, although agricultural emissions are also important in some locations. Urban emissions of N from Mexico City have likewise caused eutrophication impacts to forested catchments in the air basin of this megacity. Ecosystems downwind of Mexico City and Los Angeles are also exposed to phytotoxic levels of ozone---thus multipollutant effects are important in evaluating ecosystem responses to urban air pollution. Urban air pollution affects ecosystem services within the urban environment and in wildland ecosystems located within the path of urban and transport corridor plumes. Impacted services include the provision of clean drinking water and aesthetic, residential and recreational use of forests and other vegetation types.


1USDA Forest Service, PSW Research Station,
2USDA Forest Service, PSW Research Station,
3USDA Forest Service, PSW Research Station,
4National Park Service, Air Resources Division,
5University of California, Riverside, Department of Environmental Sciences,