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.