Air pollution distribution patterns in the San Bernardino Mountains of southern California: a 40-year perspective

Since the mid-1950s, native pines in the San Bernardino Mountains (SBM) in southern California have shown symptoms of decline. Initial studies in 1963 showed that ozone (O-3) generated in the upwind Los Angeles Basin was responsible for the injury and decline of sensitive trees. Ambient O-3 decreased significantly by the mid-1990s, resulting in decreased O-3 injury and improved tree growth. Increased growth of trees may also be attributed to elevated atmospheric nitrogen (N) deposition. Since most of the N deposition to mixed conifer forest stands in the SBM results from dry deposition of nitric acid vapor (HNO3) and ammonia (NH3), characterization of spatial and temporal distribution of these two pollutants has become essential. Although maximum daytime O-3 concentrations over last 40 years have significantly decreased (similar to 3-fold), seasonal means have been reduced much less (similar to 1.5-fold), with 2-week long means occasionally exceeding 100 ppb in the western part of the range. In the same area, significantly elevated concentrations of HNO3 and NH3, up to 17.5 and 18.5 mg/m(3) as 2-week averages, respectively, have been determined. Elevated levels of O-3 and increased N deposition together with long-term drought predispose the SBM forests to massive bark beetle attacks making them susceptible to catastrophic fires.