| Instrument: | Lightweight Airborne Chromatograph Experiment (LACE) |
| Principal Investigator: | James W. Elkins |
| Organization: | Mail Stop R/E/CG1 Climate Monitoring and Diagnostics Laboratory (CMDL) National Oceanic and Atmospheric Administration 325 Broadway Boulder, CO 80303 |
| Co-Investigators: | Fred L. Moore, Eric A. Ray, Geoffrey S. Dutton, and Erik S. Kline |
| Organization: | Cooperative Institute for Research in Environmental Sciences University of Colorado Campus Box 216 Boulder, CO 80309-0216 |
Measurement Description: The Lightweight Airborne Chromatograph Experiment (LACE) is designed to measure a variety of organic chlorine and bromine, and other trace species in the upper troposphere and lower stratosphere on the JPL balloon gondola for SOLVE or NASA WB- 57 aircraft for ACCENT [Moore et al., 1999; Ray et al., 1999]. LACE is similar in design to the ACATS-IV instrument [Elkins et al., 1996] operated in the pressurized Q-bay on the NASA ER- 2 aircraft. The key difference is LACE is housed in a pressurized container and can be operated at sub-ambient pressure for higher altitude sampling (up to 32+ km) than the ER-2 version.
LACE also can be operated in an unpressurized section in many aircraft. The instrument comprises three separate gas chromatographic (GC) channels (see Figure 1), each incorporating an electron capture detector (ECD). Channels can be configured by selecting different GC columns to measure a variety of different species. LACE was configured before the last OMS flight (May 18, 1998) on channel 1 for CFC-11, halon-1211, and CFC-12 (70 seconds, 10% OV- 101 packed columns), on channel 2 for CFC-11, CFC-113, CHCl3, CH3CCl3, and CCl4 (140 seconds, 3% OV-101 packed columns), and on channel 3 for SF6 and N2O (70 seconds, tri- column of Porapak Q-PorapakQ-5 A molecular sieve). The sample inlet is located far outside the boundary layer of the balloon's gondola or the fuselage of the WB-57 aircraft to avoid local contamination. Samples of ambient air, zero gas, and a calibrated standard are selected by switching the Stream Selection Valve (SSV). Mixing ratios are then calculated by comparing the sample and standard responses. The recording of the frequency output for the ECDs and engineering data, as well as valve switching, are controlled by an 80486-based computer.
Improvements for SOLVE and ACCENT: We plan to replace the channel 2 columns with unibeads columns and N2O-doping to the make-up of the ECD similar to that used on ACATS- IV to measure CO and/or CH4 for the SOLVE OMS balloon deployments and for the WB-57 aircraft ACCENT experiment.
| Accuracy: | Less than ±2% absolute error plus precision. |
| Precision: | In percent of CMDL surface values N2O, CH4:1% CFC-11, CFC-12, CFC-113, CH3CCl3, CCl4, SF6: 1.5% Halon-1211: 2% CHCl3: 3% CO: <5% |
| Detection Limit: | Less than 1% of current tropospheric value for most gases, 5-10 ppb for CO |
| Sample Period: | 70 seconds, except 140 seconds for CO, CH4, CHCl3, CH3CCl3, and CCl4 |
Figure 1. (a) Schematic layout of LACE. Gas from two calibration tanks, a nitrogen carrier gas tank and a diaphragm pump are plumbed into the pressurized GC. Sample gas from the diaphragm pump is held at a constant pressure by venting the excess gas outside the pressurized shell through an absolute pressure Tavco valve. A four position Stream Selection Valve (SSV) is used to select sample flow for analysis. Each channel has a twelve port, two position Gas Sample Valve (GSV) to direct flow. Channel #1 shows this valve in the load/backflush position and channel #2 is shown in the inject position. Also shown are the oven-controlled columns and electron capture detectors (ECD). Active control of the input nitrogen carrier flows, and the exhaust pressures of the ECD's, the pre- column in backflush, and sample loops are obtained using custom light weight low power controllers.
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