Surface ozone in Joshua Tree National Park

- Joshua Tree National Park (JOTR) experiences very high ozone concentrations.
- Ozone concentrations in JOTR are highest in the late spring and early summer.
- There are frequent exceedances of the 8-h NAAQS for ozone (75 ppb).
- O3 is highest in the northwest region of JOTR and decreases to the south and east.
- The main source for JOTR O3 is transport from the Los Angeles metropolitan area.

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.