In situ measurements of long-lived trace gases in the lower stratosphere by gas chromatography

Detailed information on the four-channel Airborne Chromatograph for Atmosph eric Trace Species (ACATS-IV), used to measure long-lived atmospheric trace gases, is presented. Since ACATS-IV was last described in the literature, the temporal resolution of some measurements was tripled during 1997-99, ch romatography was significantly changed, and data processing improved. ACATS -IV presently measures CCl3F [chlorofluorocarbon (CFC)-11], CCl2FCClF2 (CFC -113), CH3CCl3 (methyl chloroform), CCl4 (carbon tetrachloride), CH4 (metha ne), H-2 (hydrogen), and CHCl3 (chloroform) every 140 s, and N2O (nitrous o xide), CCl2F2 (CFC-12), CBrClF2 (halon-1211), and SF6 (sulfur hexafluoride) every 70 s. An in-depth description of the instrument operation, standardi zation, calibration, and data processing is provided, along with a discussi on of precision and uncertainties of ambient air measurements for several a irborne missions.