In situ chloroform measurements at Advanced Global Atmospheric Gases Experiment atmospheric research stations from 1994 to 1998

Measurements of atmospheric chloroform (CHCl3) by in situ gas chromatograph y using electron capture detection are reported from the Advanced Global At mospheric Gases Experiment (AGAGE) network of atmospheric research stations . They are some of the most comprehensive in situ, high-frequency measureme nts to be reported for CHCl3 and provide valuable information not only on c lean "baseline" mixing ratios but also on local and regional sources. Emiss ions from these sources cause substantial periodic increases in CHCl3 conce ntrations above their baseline levels, which can be used to identify source strengths. This is particularly the case for measurements made at Mace Hea d, Ireland. Furthermore, these local sources of CHCl3 emissions are signifi cant in relation to current estimates of global emissions and illustrate th at the understanding of competing sources and sinks of CHCl3 is still fragm entary. These observations also show that CHCl3 has a very pronounced seaso nal cycle with a summer minimum and winter maximum presumably resulting fro m enhanced destruction by OH in the summer. The amplitude of the cycle is d ependent on sampling location. Over the 57 months of in situ measurements a global average baseline concentration of 8.9 +/-0.1 ppt was determined wit h no appreciable trend in the baseline detected.