Aggressive efforts to improve urban air quality could be undermined by rising levels of methane, a compound more closely linked to global warming than air pollution. Using a global model of tropospheric chemistry, researchers at Harvard University, Argonne National Laboratory, and the U.S. EPA determined that higher methane levels could increase ozone background levels worldwide, lead to a greater frequency of days with high ozone levels in the summer, and produce a longer “season” of ozone pollution days.

Anthropogenic methane comes from many sources, including leaking pipelines and cattle herds. Because it has a lifetime of 8 to 9 years, effective methane reductions would require worldwide cooperation. Indeed, the researchers warn that efforts in Europe and North America to improve local air quality could be undermined by rising methane emissions by developing nations.

The study used the GOES-CHEM model, which follows ozone, NO_x, CO, and volatile organic compound (VOC) chemistry, to predict changes in the troposphere. The researchers simulated ozone levels for daily afternoons during the months June through August, which typically are the most polluted days.

It is already known that methane is a major source of the worldwide tropospheric ozone background, and this study supports that finding. However, the surprise is that a 50% reduction in anthropogenic methane in their scenario is as effective as a 50% drop in anthropogenic NO_x concentrations at lowering summer afternoon ozone levels over the United States. CO and VOCs in the presence of NO_x are known to generate ozone in urban settings and have historically been the targets of air quality regulations.

Moreover, by taking a threshold value of 70 parts per billion by volume (ppbv) ozone as a pollution event, the researchers found that reducing methane by 50% decreased the number of high ozone days by 45%. By way of comparison, the ozone standard in many European countries is 55–65 ppbv, and 80 ppbv (8-hour average) in the United States. Finally, using a relatively pessimistic scenario for increasing methane, the researchers find that by 2030, the high ozone pollution season in the United States could begin as early as April. (Geophys. Res. Lett. 2002, 29, 10.1029/2002GL015601)