Behavior of trace gas mixing ratios on a very tall tower in North Carolina

We present a 15-month record of mixing ratios of CO, CH4, N2O, and eight halogenated gases (CCl3F, CCl2F2, CCl2FCClF2, CH3CCl3, CCl4, CHCl3, C2Cl4, and SF6) at a rural site in eastern North Carolina. The data result from hourly gas chromatographic analyses of air sampled at three heights on a 610-m-tall telecommunications tower during November 1994 through January 1996. At night, most of these gases were more abundant near the ground (51 m) than aloft (496 m) because of the buildup of local and regional surface emissions in the shallow nocturnal stable layer. The abundance and variability of trace gases at this continental site were generally higher than those at similar latitude remote locations. Mixing ratios of most gases were well correlated in polluted air masses occasionally advected to the tower. Frequent, strong enhancements in CHCl3 at the lower sampling level(s) indicate a local point source(s) of this gas that is not associated with combustion. Temporal trends of regional background mixing ratios at this continental site are, for the most part, in good agreement with recent trends of remote background mixing ratios in the northern hemisphere.