Carbon monoxide in the US Mid-Atlantic troposphere: Evidence for a decreasing trend

Nearly continuous measurements of carbon monoxide (CO) were made at Shenand oah National Park-Big Meadows in rural Virginia, a site considered represen tative of regional air quality, from December 1994 to November 1997. Simila r observations were also made at this location from October 1988 to October 1989. These observations combine to indicate a decreasing trend in CO conc entration over the U.S, mid-Atlantic region of about 5.0 ppbv yr(-1), with greater than 95% confidence that the slope is significantly different from zero. The decrease suggests U.S. reductions in anthropogenic CO emissions h ave been effective in reducing pollutant levels. The observed trend is cons istent with the U.S. EPA reported trend in emissions and the decrease in No rthern Hemisphere tropospheric background CO mixing ratios observed by othe r researchers.