COMPARISON OF OZONE EXPOSURE CHARACTERISTICS IN FORESTED REGIONS NEAR MEXICO-CITY AND LOS-ANGELES

This comparison of forest exposure to ozone in the vicinity of Mexico City and Los Angeles provides preliminary evidence of the seasonal differences in ozone concentrations. Summer concentrations near Mexico City are not as high as those near Los Angeles because most of the precipitation and associated cloudiness occurs near Mexico City during the months of June through September. Winter concentrations remain nearly as high as summer concentrations at Mexico City, because in winter skies are clearer and incident sunlight remains high. Latitudinal influences on solar zenith angle and the higher altitude of the Mexico City region both contribute to a higher actinic flux than in the Los Angeles region. The primary difference in forest exposure is that there is very little respite from adverse ozone concentrations during the entire year in the Mexico City region. Also, the rainy summer season would likely diminish water stress and result in greater ozone uptake at the Desierto de los Leones compared to dry summer conditions in California. The closer proximity of the Desierto de los Leones monitoring site to the urban area also contributes to high winter exposures. There is some respite from exposure during the winter in the San Bernardino mountain region; however, summer concentrations are higher than near Mexico City. The greater transport distance from the Los Angeles source region also contributes to lower winter exposures.