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.
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