Acetone in the free troposphere and lower stratosphere: Aircraft-based CIMS and GC measurements over the North Atlantic and a first comparison

Acetone was measured in the free troposphere and lower stratosphere over th e North Atlantic by aircraft-based CIMS (Chemical Ionization Mass Spectrome try) and GC (Cas Chromatography) instruments in fall 1997 within the framew ork of the POLINAT-2/SONEX campaigns. The measurements included 28 flights covering altitudes from 2.4 to 12.5 km, and geographic latitudes from 20 to 69 degrees N. The data are the most extensive measurements of acetone in t he free troposphere and lower stratosphere published to date. A first compa rison of the CIMS and the GC method is presented. The acetone data obtained by both instruments in the same air masses agree within 50% which is consi stent with the uncertainty limits quoted. An inspection of all flights reve als upper tropospheric acetone to vary very substantially between 200 and 2 200 pptv with a mean value of about 600 pptv. By contrast, in the lower str atosphere the acetone abundance is much less variable and is mostly around 100 to 200 pptv. Acetone rich air masses with volume mixing ratios exceedin g 1000 pptv and reaching up to 2200 pptv were surprisingly frequently encou ntered in the upper troposphere. Probably these air masses had recent conta ct with the continental planetary boundary layer where they took up acetone and/or acetone precursor gases from ground-level sources.