Surface ozone in Joshua Tree National Park

Surface ozone concentrations are high throughout Joshua Tree National Park (JOTR), as demonstrated by direct monitoring at multiple sites within JOTR. Multiple years of data from continuous ozone monitors and passive samplers indicate that the highest ozone concentrations occur in the northwest corner of JOTR, and decrease as one moves to the south and east. Analysis of long-term (1994–2010) hourly data from the Black Rock monitoring station indicates that average summertime ozone concentrations have remained essentially constant, with a concurrent decrease in the number, and magnitude, of high ozone events. Calculation of ozone exposure indices suggests that ozone concentrations may be high enough to adversely affect local flora, especially in the northwestern region of the park. Back-trajectory analysis using the HYSPLIT model suggests that most of the ozone measured in JOTR is transported into the park from a high source area (Los Angeles and surrounding suburbs) to the west.