Halocarbon Emissions Change With The Seasons

Although a more than 20-year-old international treaty ended production of most ozone-depleting halocarbons, the chemicals still find their way into the atmosphere. Now a new study reveals that the abundance of one harmful halocarbon may jump during the summer—at least in one location—which could throw off national emission estimates (Environ. Sci. Technol. DOI 10.1021/es1005362).

Since 1987, representatives from 196 nations have ratified the Montreal Protocol and have vowed to phase out the production of halocarbons that destroy the ozone layer. The banned compounds, which include methylchloroform, were in widespread use as industrial and household solvents, refrigerants, and aerosol propellants. As more nations have signed the treaty, banned halocarbon production has plummeted. But low-level emissions continue from lingering sources such as pre-ban refrigerators tossed into dumps.

Environmental engineer Drew Gentner and his colleagues at the University of California, Berkeley monitored these halocarbon emissions as part of their ongoing studies on seasonal changes in Southern California's air quality. During the fall and summer in Riverside, Calif., the researchers used gas chromatography to measure concentrations of eight anthropogenic halocarbons in air samples collected at ground level.

The researchers observed ongoing emissions of methylchloroform that were comparable to national emission estimates. But more importantly, the halocarbon fluctuated with the season: Its emissions were 4.5 times greater in the summer than in the fall. Although the researchers don't yet know the variability's exact source, previous studies have suggested that gases from landfills could be responsible. When temperatures rise in the summer, Gentner says, halocarbon gases diffuse from buried garbage such as old aerosol hairspray cans more readily than in the fall.

Seasonal variability could skew current estimates of annual halocarbon emissions, depending on when officials take the measurements, the researchers suggest. But, Gentner cautions that the researchers need further studies to extend these conclusions nationwide. Atmospheric chemist Elliot Atlas of the University of Miami concurs: "It's not easy to extrapolate these findings without more detailed knowledge of the emission sources and the factors that might influence emission strength." Still, chemist Stephen Montzka of the National Oceanographic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo. says that the study does serve as a reminder that "harmful substances can linger for a long time after production essentially stops."