Environmental▼News PERSPECTIVE with low VOC emissions,” he says. Emissions in the Southeast have Scientists are beginning to blame volume (ppbv), of the U.S. national jumped by 14% per decade, and local forests and pollutants blown in mean concentrations of ozone in four states in the Deep South have from overseas for the fact that conthe summer, which range from 40 seen emissions increase by 25% per centrations of ground-level ozone— to 80 ppbv, says Arlene Fiore, an atdecade because of tree plantations, and its accompanying smog—have mospheric chemist at Princeton which use high-VOC-emitting tree not declined during the past 10 University and coauthor of the species. “Sweetgum, one of the years in the United States, despite research. highest-emitting species for VOCs, cuts in ozone precursors. Thus, naFiore suggests that U.S. regulatory is increasingly appearing as an untional smog-fighting regulations measures to curb ozone, such as the invited understory tree in pine may actually be doing a better job NOx state implementation plan call, plantations,” says Purves. Meanthan they have been given credit which targets summer ozone in the while, in the natural forests, loggers for, but measurably reselectively remove the ducing ozone levels low-emitting species, may require internawhich leaves communitional efforts and reties dominated by highthinking forestry emitting species. The management. combined effect of When the U.S. EPA sweetgum growth and hands over its report on selective logging is acthe science behind the tually larger than the country’s ozone stanemissions from the dards to outside reviewplantation species, he ers this spring, one of notes. the issues for discussion “EPA says there is will be determining the nothing we can do level of background about natural VOCs, but Scientists increasingly suspect that cross-border pollution and forest concentrations of they are important conemissions are the reason that U.S. ozone levels have not dropped. ozone, says Joe Pinto, tributors to ozone and an atmospheric chemist their levels are changing with EPA. Northeast, and maximum achievable quickly due to our land-use deciBecause ozone causes and exaccontrol technologies for processes sions,” Purves says. He suggests that erbates respiratory diseases and like painting operations, may be researchers should calculate the relahurts agricultural production, the more successful than they appear. tive costs and benefits of making But the contribution of these regulacuts in NOx, anthropogenic VOC reUnited States and many other tory programs is masked by the leases, and natural VOC emissions countries have implemented conlong-range transport of ozone and before making new policies. “It is trols over its precursors—nitrogen its precursors, which contributes to possible that intentionally controloxides (NOx), which primarily come from fossil fuel burning, and volatile background levels of ozone. ling VOC emissions from forests organic compounds (VOCs) from Another neglected source of would be cost-effective, but this does gasoline and solvents. But in the ozone is the production of VOCs not mean cutting down forests—this past decade, the U.S. average nafrom forests, says Drew Purves, an would be expensive and unnecestional ozone concentrations have ecologist at Princeton University. sary, since in many areas old-growth not changed, despite a 12% decline Until now, modelers have assumed forest emits almost no VOCs,” he in NOx and a 25% cut in VOC emisthat the levels of natural VOCs from says. However, changes to current sions, according to EPA. This flat vegetation remained relatively conlogging and plantation forestry pracozone trend, which is discussed in stant and roughly equal to humantices could reduce VOC emissions an EPA report, Air Quality Criteria caused emissions of VOCs. “But significantly, he adds. for Ozone and Related Photochemiforest survey data show that natural EPA underestimated the imporcal Oxidants, has puzzled scientists. VOC emissions are increasing at 6% tance of natural VOCs and longOne clue comes from global per decade in the United States,” he range transport of ozone in the past, modeling studies that show that says. Fiore says. “Reductions in emissions ozone and its precursors are being The trend is not uniform across of NOx, possibly achieved through a transported from Asia and Europe the country, Purves adds. “In some hemispheric treaty, should decrease to North America (Environ. Sci. areas, such as New England and surface ozone concentrations since Technol. 2003, 37, 4535–4542). Wisconsin, natural VOC releases these ozone precursors contribute According to the model, these forhave fallen as forests created by substantially to global background eign pollutants constitute nearly farm abandonment have matured, ozone concentrations,” she con10%, about 4–7 parts per billion by becoming dominated by species cludes. —JANET PELLEY PHOTODISC
Neglected sources of ozone
86A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / MARCH 1, 2004
