94Exposures to ozone (O3), nitrogenous (N), and sulfurous (S) pollutants may have serious effects on vulnerable ecosystems in California and elsewhere. Elevated O3 concentrations have been suggested as a factor contributing to the decline of sensitive forest tree species in the San Bernardino and Sierra Nevada Mountains. Long-term exposure to elevated deposition of N pollutants may lead to N saturation of mountain ecosystems and may affect their health and sustainability. Direct toxic effects of ambient concentrations of some N air pollutants (e.g., gaseous nitric acid or peroxyacetyl nitrate) are also possible. The methodology used by the authors for measuring concentrations and deposition of air pollutants in remote locations of California is reviewed. Special emphasis is put on methodologies which can be applied in remote locations without access to electric power. Information on concentrations of O 3 , gaseous and particulate N and S species, as well as nitrate, ammonium, and sulfate in wet precipitation for selected case studies and monitoring networks is presented. In general, summer season average 24-h concentrations of ozone were elevated on western slopes of the Sierra Nevada, approaching 75 ppb in the Mountain Home site. Wet deposition of N and S were the highest at elevations below 2100 m on the western slopes of the Sierra Nevada (approaching 2.83 kg N ha-1 yr -1 at Giant Forest) and similar to the values determined at Tanbark Flat in southern California. The sites above 2200 m in the eastern Sierra Nevada had the lowest values of N wet deposition values. Similar trends to wet N deposition were also determined for gaseous and particulate N pollutant1> as well as dry deposition to pines. Total levels of wet and dry N deposition in the Sierra Nevada sites are still well below saturation levels. In the Sierra Nevada, wet N deposition was about two- to threefold higher than wet deposition of sulfur. Results from the elevational gradient in the San Bernardino Mountains clearly indicated that deposition of NO3 -, SO4 2-, and NH4 + were proportional to the amount of rain precipitation. In the San Bernardino Mountains, the highest ozone concentrations were found at about 1500 m, with significantly reduced levels at higher elevations. This chapter also presents distribution of N pollutant concentrations and their deposition on a vertical gradient in the mixed conifer canopy, models for atmospheric deposition estimates, ecological 95consequences of elevated concentrations of ozone, N saturation in vulnerable eco-systems, and research recommendations.