Anthropogenic sources of halocarbons, sulfur hexafluoride, carbon monoxide, and methane in the southeastern United States


  • P. S. Bakwin,

  • D. F. Hurst,

  • P. P. Tans,

  • J. W. Elkins


Mixing ratios of several radiatively active and ozone depleting trace gases were measured at 1-hour intervals from December 1994 to August 1996 at 496 m above the ground on a communications tower in eastern North Carolina. The time series show variability on a range of timescales, most apparently diurnal cycles, pollution episodes with synoptic time scale, seasonal cycles, and long-term trends. We focus on the pollution episodes and analyze the time series to reveal relationships between species in polluted air. These relationships are used to determine relative sources of the constituents on a regional scale. Using tetrachloroethene (C2Cl4) as a reference compound and assuming that the tower site is representative of the whole continent with respect to emissions ratios, we estimate source strengths of the other species for North America. For methyl chloroform (CH3CCl3) our emissions estimate is in excellent accord with an estimate from industrial production figures, giving confidence in our methods and assumptions. For carbon monoxide (CO) our estimate also agrees very well with an independent estimate of North American sources based on national inventories. Sources of chlorofluorocarbons 11, 12, and 113 are much lower than expected from global emissions inventories, a finding that may indicate that the ratio of North American to global emissions of these compounds has dropped dramatically since the mid-1980s, as was indeed the case for CH3CCl3, or that emissions of these compounds have decreased more rapidly in recent years than industry estimates, or most likely both. We also estimate the North American source of sulfur hexafluoride (SF6).