Acidic deposition in California: Findings from a program of monitoring and effects research

California's 14-year, $25 million acidic deposition program has studied the causes and effects of acidic air pollutants. In contrast to the eastern United States where sulfur-derived (S-derived) by-products from coal combustion dominate precipitation chemistry, nitrogen-derived (N-derived) acids predominate in wet and dry deposition in California. Adverse effects on the human lung have not been observed after short-term exposures to acidity, but extended exposures to ambient acidity may pose a chronic risk. No irreversible, adverse effects on surface waters in the Sierra Nevada mountain range or to the state's forests have been found due to extant acidic inputs. The longer-term outlook for forests is less certain because the impacts observed elsewhere occurred after decades of S and N deposition, but at lower ambient ozone levels. Ozone is the major air pollutant stressor for forests, but atmospheric N has the potential to cause adverse changes in soil nutrient cycling. Impacts on man-made materials in southern California (e.g., galvanized steel) were found to be minor. While California does not have an ambient air quality standard for acidic air pollutants, emissions of precursors have declined since the 1960's due to changes in industrial practices, improvements in technology, and adoption of control measures for ozone. Lowering emissions from motor vehicles will be emphasized to prevent future increases in N deposition.