Three generations of ACATS instruments have been built at NOAA/ESRL/GMD since 1991. Each instrument was designed specifically to make autonomous measurements of chlorofluorocarbons and other long-lived atmospheric tracers on board the high altitude NASA ER-2 aircraft. The succession of instruments reflects the incorporation of additional channels to the gas chromatograph (GC), measurements of more atmospheric tracers, and faster measurement rates.
Below we describe the lineage of ACATS instruments and their utility in NASA upper atmosphere science missions since 1991.
ACATS-I, a one-channel gas chromatograph (GC) with electron capture detection (ECD), was designed and constructed in 1990-1991 for operation on the high-altitude NASA ER-2 aircraft during the October 1991 - March 1992 AASE-II mission. The GC resided inside the NOAA Aeronomy Laboratory Reactive Nitrogen Instrument (NOy) and its operation and data acquisition were handled by the host instrument computer. ACATS-I measured chlorofluorocarbon-11 (CFC-11, CCl3F) every 120 seconds during this mission, providing key information about ozone-depleting chlorine in the lower stratosphere.
The original instrument was upgraded to a two-channel GC-ECD for the April-May 1993 SPADE mission. The new ACATS-II incorporated measurements of chlorofluorocarbon-113 (CFC-113, CCl2FCClF2) on the original CFC-11 channel and added a second channel using a nitrous oxide (N2O)-doped ECD to measure methane (CH4). Both channels reported data at 120 second intervals. Instrument control and data acquisition were handled by the host NOy instrument.
A new instrument, ACATS-IV, was constructed in 1993 for the 1994 ASHOE/MAESA mission. ACATS-IV is a four-channel GC-ECD designed to operate external to and independent of the NOy instrument. The new instrument added measurements of 1,1,1-trichloroethane (methyl chloroform, CH3CCl3) and carbon tetrachloride (CCl4) to the CFC-11 and CFC-113 channel, measurements of hydrogen (H2) to the CH4 channel, and incorporated two new channels: one to measure N2O and sulfur hexafluoride (SF6), the other to measure halon-1211 (CBrClF2) and chlorofluorocarbon-12 (CFC-12, CCl2F2). Measurement intervals for the four channels ranged from 3 to 6 minutes.
ACATS-IV was part of the ER-2 payload for the 1995-1996 STRAT, 1997 POLARIS, and 2000 SOLVE missions. Improvements to the instrument during 1997-2000 were primarily advances in chromatography that increased the frequency of measurements. In general, chromatography columns were shortened, column inside diameters were decreased, sample loop volumes were reduced, and carrier gas flows were programmed to decrease retention times while maintaining peak separation. Stacked sample injections, where a new sample is injected before all peaks of interest from the previous sample have eluted, were also incorporated. Improved chromatography allowed the detection of chloroform (CHCl3) on the CFC-11 channel.
In March 2001 ACATS-IV was configured as a ground-based instrument for the TROICA-7 expedition along the trans-Siberian railway in Russia. Detection of carbon monoxide (CO) was added to the H2 and CH4 channel in April 2002.