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 NO_x emissions result in production of reduced forms of N. Industrial, urban and on-road emissions of NO_x 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.