Whole Air Sampler-- University of California, Irvine

 During TRACE-P the University of California-Irvine (UCI) research group will monitor more than 50 trace gas species including nonmethane hydrocarbons (NMHCs), halocarbons, alkyl nitrates, and DMS. The measurements will be made using a combination of whole air sampling and analysis by gas chromatography (GC) with flame ionization detection (FID), electron capture detection (ECD) and mass spectrometer detection (MSD).

The whole air samples will be collected in individual electropolished 2-L stainless steel canisters. During each TRACE-P flight, up to 168 whole air samples will be collected aboard the DC-8, with up to 144 aboard the P-3. Prior to each flight, the canisters will be conditioned and evacuated, and 10 Torr of water will be added into each canister to quench active surface sites. To collect a sample, outside air will be collected from beyond the laminar boundary layer of the aircraft via ¼” tubing. A wider diameter tubing may be used aboard the P-3 to allow faster sampling for application towards flux calculations. On both aircraft the sample air will be pressurized by a two-stage metal bellows pump and distributed to a gas-handling manifold via ¼” stainless steel tubing.

The whole air samples will be collected throughout each flight on both aircraft. A typical sampling rate is every 3-7 minutes during horizontal flight legs, and 1-3 minutes during vertical legs. During both horizontal and vertical flight legs, the sampling duration can be lengthened or shortened by adjusting a bellows valve located on the gas-handling manifold between the pump and the canisters. A typical sampling time on horizontal flight legs is 1 minute, which corresponds to a sampling distance of roughly 12 km. During standard vertical flight legs, the samples will be collected every 1500-2000 ft. A typical vertical sampling distance is roughly 1000 ft for the DC-8, and 450 ft for the P-3.

After each flight, the filled canisters will be couriered to our laboratory at UCI for analysis using two three-GC, five-column, five-detector analytical systems. Within 10 days of being collected, the air samples will be analyzed for C2-C10 NMHCs, halocarbons including the methyl halides, C1-C4 alkyl nitrates, and DMS. For each sample, 1520 ± 1 cm3 (STP) of canister air will be passed through a preconcentration loop that is filled with glass beads and immersed in liquid nitrogen. The loop then will be isolated before being warmed in a hot water bath to revolatilize the gases. The contents of the loop are then flushed into a helium carrier gas, and the sample flow is quantitatively split into 5 streams, with each stream directed to a different column-detector combination. In previous missions we have found that the split ratios are highly reproducible. During TRACE-P, the 5 column-detector combinations will be DB5ms/MSD; DB1/FID; PLOT-DB1/FID; Restek1701/ECD; and DB5-Restek1701/ECD. Because it takes a few days for the analytical systems to equilibrate, they will be operated continuously (24 hours a day) throughout the project in order to generate a self-consistent data set.

The range of accuracies for the gases we report is 2-20%. The precision of the measurements varies by compound and by mixing ratio. For example, the measurement precision for the NMHCs is 1% or 1.5 pptv (whichever is larger) for the alkanes and alkynes, and 3% or 3 pptv (whichever is larger) for the alkenes. The precision for CFC-12 at 550 pptv is ±3 pptv, while that for methyl iodide at 0.02 pptv is ±0.01 pptv. The limit of detection (LOD) is 3 pptv for the NMHCs, and 1 pptv for DMS. Although the LOD is different for each halocarbon, the halocarbons that we report are usually present at mixing ratios above their detection limits.