Summer smog: not just an urban problem.

Summer smog: Not just an urban problem

Last summer, New York City's air exceeded the federal 1-hour smog-ozone standard of 120 parts per billion on 42 occasions, or an average of about once every two days. Washington's summer air violated the standard an average of once every three days. Frequent ozone excesses also hit most other large U.S. cities. And this year, the National Weather Services's latest forecast envisions above-normal temperatures -- the type that can cook up high levels of ozone, the primary irritant in smog--from July to October for one-third of the nation.

If that makes you long for a country stroll to breathe clean, fresh air, think again. A new study shows that unhealthy ozone levels plaue rural air, too--at least in the eastern United States.

Though people have traditionally viewed ozone as an urban problem, Jennifer A. Logan, an atmospheric chemist at Harvard University, focused her analysis on data collected from 18 rural stations recording 24-hour ozone readings in 1978 and 1979. She sought to characterize not only the high-ozone trends but also the atmospheric processes that may have fostered them.

In the East, she found that high-ozone periods lasting up to three days tended to occur simultaneously or within a day or two of each other at sites hundreds of kilometers apart. Most such episodes appeared linked to the passage of large, slow-moving and persistent high-pressure weather systems, Logan reports in the June 20 JOURNAL OF GEOPHYSICAL RESEARCH.

These high-pressure systems tend to maintain a low-altitude boundary area -- a well-mixed region of air extending about 2 kilometers from Earth's surface. The boundary area, explains Logan, "acts like a low lid," preventing pollutant dilution by restricting low-altitude air from mixing with the upper atmosphere. This gives hydrocarbons and nitrogen oxides entrained within it an especially good chance to interact with each other in ways that generate ozone, she says.

Logan's data show that peak ozone concentrations in rural areas rivaled or exceeded those reported in many urban areas. For instance, ozone peaks at the eastern sites topped 80 parts per billion (ppb) on 26 percent of the days between May and August in 1979 and on 39 percent of those days in 1978. Moreover, brief peaks commonly exceeded 120 ppb. This is well above the 40-tl 60-ppb ozone level that can cause significant crop-yield losses. While all stations recorded similar median values--30 to 40 ppb--from April to September of both years, the three western stations (in Arizona, Montana and Oregon) almost never exceeded 80 ppb. Logan suspects this difference stems primarily from the West's lower levels of hydrocarbons and nitrogen oxides.

Her data correlating acute ozone episodes with high-pressure areas hold open the prospect that weather forecasters might one day give farmers a couple days' warning before plant-choking ozone conditions arrive. Farmers would need such warnings to use chemical protectants now being developed to save plants from ozone damage, notes Joseph Miller at the Agriculture Department's air-quality program in Raleigh, N.C. His preliminary data indicate that the leading candidate protectant, best known as EDU, "was detrimental to plant growth" under all but extreme ozone concentrations.

Air-quality consultant Allen S. Lefohn of Helena, Mont., thinks Logan's most important contributions may be two observations that call into question the credibility of previous agricultural data. Logan observed that the 7-hour ozone-exposure period used in most crop studies -- 9 a.m. to 4 p.m. -- differs from the period of highest rural ozone levels, which ranges from midafternoon to near midnight. Moreover, she found that the practice of assessing ozone in terms of a 7- or even 12-hour seasonal average masks the brief peaks that may occur every few days and that could pose a far greater threat to crops.