Ozone distribution and phytotoxic potential in mixed conifer forests of the San Bernardino Mountains, southern California

In the San Bernardino Mountains of southern California, ozone (O3) concentrations have been elevated since the 1950s with peaks reaching 600 ppb and summer seasonal averages >100 ppb in the 1970s. During that period increased mortality of ponderosa and Jeffrey pines occurred. Between the late 1970s and late1990s, O3 concentrations decreased with peaks ∼180 ppb and ∼60 ppb seasonal averages. However, since the late 1990s concentrations have not changed. Monitoring during summers of 2002-2006 showed that O3 concentrations (2-week averages) for individual years were much higher in western sites (58-69 ppb) than eastern sites (44-50 ppb). Potential O3 phytotoxicity measured as various exposure indices was very high, reaching SUM00 - 173.5 ppm h, SUM60 - 112.7 ppm h, W126 - 98.3 ppm h, and AOT40 - 75 ppm h, representing the highest values reported for mountain areas in North America and Europe.