California Environmental Protection Agency

Air Resources Board

 No. 96-3

August 1996

 RESEARCH NOTES

Brief Reports to the Scientific and Technical Community

 Research Division, John R. Holmes, Ph.D., Chief

P.O. Box 2815, Sacramento CA 98512

Background:

Methods:

Results:

Significance and Application:

Related Projects:

In the 1980s, field surveys were conducted in Sequoia and Kings Canyon National Parks to determine whether ponderosa and Jeffrey pine trees were being injured by ambient air pollutants. The results of the surveys revealed that large numbers of trees exhibited the characteristic type of yellow spotting caused by ozone (chlorotic mottle). However, the absence of quality-assured ozone concentration data prevented researchers from establishing a definitive causal relationship between ambient air quality and tree injury. To address this need, the Sierra Cooperative Ozone Impact Assessment Study (SCOIAS) was initiated in 1990 to measure ambient concentrations of ozone and several meteorological parameters in mid-elevation conifer forests in the Sierra Nevada. Results from 1990-1991 were reported in Research Note 94-2; the results from 1992-1994 are presented here.

To characterize air quality and meteorological conditions in conifer forests, six aerometric monitoring stations were established in five national forests on the western slope of the Sierra Nevada. The sites were equipped with an ozone analyzer and weather station (to measure wind speed and direction, air temperature, relative humidity, and solar radiation). Table 1 lists the sites, their elevations, and the national forests in which they are located.

Table 1

During the period 1992-1994, data were collected from May through October, when ambient ozone concentrations are highest. From June through September, daytime 12-hr average ozone concentrations varied slightly (± 5 percent) from year to year at each site, ranging from a low of 61 ppb at Sly Park (a northern site) to 82 ppb at Mountain Home (the southernmost site). Maximum 1-hr ozone concentrations were at or above 60 ppb nearly every day, 80 ppb on about 50 percent of days, and 100 ppb for a few days each month. Three sites (White Cloud, Five-Mile, and Jerseydale) exhibited flat ozone profiles, where daily maximum and minimum concentrations differed by only 10-15 ppb (Figure 1). The other sites (Mountain Home, Shaver Lake, and Sly Park) exhibited diurnal profiles with distinct maxima and minima that differed by 25-70 ppb.

Figure 1

The results of the SCOIAS for the period 1992-1994 demonstrate that ozone concentrations reached levels that could cause damage to tree species such as ponderosa and Jeffrey pine. Concurrent surveys of ozone injury were performed by the USDA Forest Service on trees growing in the vicinity of the monitoring stations. These surveys provided data to link the occurrence of tree injury to ozone concentrations in ambient air for the period 1992-1994. Ozone concentrations and tree injury were also monitored during SCOIAS by the National Park Service in Lassen Volcanic, Sequoia, Kings Canyon, and Yosemite national parks. For purposes of comparison, data from the San Bernardino mountains in southern California were collected by the USDA Forest Service.

As more people relocate to rural areas in the Sierra Nevada, ambient levels of ozone are likely to rise as a result of increased emissions of ozone precursors from motor vehicles and other anthropogenic activities in these areas. Present-day ozone levels are harmful to selected pine species; these tree species are likely to sustain even greater amounts of injury if ozone levels increase in future years.

Following the completion of Contract No. 92-346, the ARB's Monitoring and Laboratory Division assumed responsibility for aerometric monitoring at the White Cloud, Five-Mile, Jerseydale, and Shaver Lake sites. By continuing to monitor ozone levels at these sites, any significant changes in local ozone concentrations from 1991-1994 levels can be identified. Under a memorandum of understanding with the USDA Forest Service, surveys of tree injury in future years will be performed every third year.