Whole
Air Sampling aboard the DC-8 and P-3B Aircraft During
TRACE-P
Donald
R. Blake and F. Sherwood Rowland
Department of Chemistry, University of California, Irvine 92697-2025
Canisters will be
evacuated in our UCI laboratory after which 10 torr of purified
water vapor will be added to each canister. Adding water helps
avoid the problem of wall reactions for some gases in some very
dry samples. Once the approximately 150 samples from each flight
are filled the cans will be ferried back to our lab by research
group members returning from the field. The average time between
canister filling and sample analysis is one week.
Each analytical apparatus
utilizes two gas chromatographs (GCs) and with 4 detectors and
one gas chromatograph mass spectrometer (GC-MS). Each whole air
sample is cryogenically trapped with liquid nitrogen, warmed, and
injected into a hydrogen flow stream. This stream is then split
into five, with each stream feeding a separate GC column. One DB-5MS,
one PLOT A12O3/KCl, and two DB-1 columns are used. The DB-5MS is
plumbed into an electron capture detector (ECD) and separates about
20 C1-C2 halocarbons. The DB-1/ECD combination detects essentially
the same halocarbons as the DB-5MS/ECD but the separation order
is different enough that gases co-eluting on one column are usually
resolved on the other. All ECDs are oxygen-doped which yields a
substantial increase in ECD response, especially for many of the
hydrohalocarbons. Several halocarbons and alkyl nitrates are quantified
on the GC-MS system. The
DB-1 FID combination separates C3-C10 NMHCs. The PLOT column, also
plumbed to an FID, is used for separating the C2-C5 NMHCs, some
of which are not resolved adequately by the DB-1 column.
The preparation of
standards for the halocarbons employs a pressure balancing method
using three different sections of a glass vacuum line. Pure gas
is introduced into the first section of the line and is ultimately
diluted to a mixing ratio that most closely matches the concentration
of the gas in the atmosphere. The range of these halocarbon standards
falls between 0.5-600 pptv.
Calibration of the
NMHC compounds has been achieved by employing Scott calibration
gases available in the 1-100 ppmv mixing ratio range. A NIST propane
standard with an absolute accuracy of 0.99±0.0l ppmv is also used
in the preparation of standards. In order to simulate whole air
characteristics, a known amount of a particular calibration gas
is added to a known amount of a previously assayed stratospheric
sample. These high altitude samples contain less than 50 pptv of
ethane and are usually devoid of any other hydrocarbons (<2
pptv), except methane. After
adding the Scott standards containing the various gases, the augmented
air sample concentrations are calculated. In this manner, standards
have been prepared over a wide range of concentrations which are
then used as primary standards for calibrating the 7500 liter pressurized
whole air working standards collected at either Niwot Ridge, CO,
Scripps Institute of
Oceanography, La
Jolla, CA, or White Mountain Research Station, Bishop, CA. During
analysis one aliquot from the working standard is assayed every
fifth analysis, thereby quickly indicating any malfunction of the
analytical apparatus. To determine any drift in the working standard
several other whole air pressurized Aculife and SpectraSeal treated
Luxfer cylinders and other stainless steel containers are assayed
at rates ranging from once per day to once per week. Alkyl nitrate
standards will be provided by E. Atlas and F. Flocke of NCAR.
The stainless steel
canisters that will be used in this study were fabricated at UCI
and prepared for use by a technique developed over the 18 years
since the first canisters were made here. The canisters have been
used on several previous NASA and NSF airborne missions during
which very clean tropospheric or stratospheric air was collected.
Samples from those projects have been assayed within a few days
after collection and then again several weeks later and the concentrations
of the reported gases exhibited no statistically significant differences
between the two data sets.
TRACE-P Measurement Parameters
Compound
|
Limit
of Detection
|
Precision
|
Accuracy
|
Ethane
|
3
pptv
|
1%
or 2 pptv
|
5%
|
Ethene
|
3
pptv
|
3
pptv
|
10%
|
Ethyne
|
2
pptv
|
1%
or 2 pptv
|
5%
|
Propane
|
3
pptv
|
1%
or 2 pptv
|
5%
|
Propene
|
2
pptv
|
3
pptv
|
10%
|
Propadiene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
n-Butane
|
2
pptv
|
1%
or 2 pptv
|
5%
|
i-Butane
|
2
pptv
|
1%
or 2 pptv
|
5%
|
1-Butene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
cis-2-Butene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
trans-2-Butene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
Isobutylene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
n-Pentane
|
2
pptv
|
1%
or 2 pptv
|
5%
|
Isoprene
|
2
pptv
|
3%
or 3 pptv
|
10%
|
i-Pentane
|
2
pptv
|
1%
or 2 pptv
|
5%
|
Benzene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
Toluene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
o-Xylene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
m-Xylene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
p-Xylene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
Ethylbenzene
|
3
pptv
|
2%
or 2 pptv
|
5%
|
CFC-12
|
1.0
pptv
|
0.7%
|
2%
|
CFC-11
|
3
pptv
|
0.8%
|
2%
|
CFC-113
|
3
pptv
|
1.4%
|
2%
|
CFC-114
|
1
pptv
|
1.5%
|
5%
|
HCFC-22
|
10
pptv
|
1.5%
|
5%
|
H-1301
|
0.1
pptv
|
2%
|
5%
|
H-1211
|
0.1
pptv
|
2%
|
5%
|
H-2402
|
0.1
pptv
|
5%
|
5%
|
CH3CCl3
|
3
pptv
|
1%
|
5%
|
CCl4
|
3
pptv
|
1%
|
5%
|
C2Cl4
|
0.05
pptv
|
1-5%
|
10%
|
CH2Cl2
|
2
pptv
|
5%
|
10%
|
CHBrCl2
|
0.05
pptv
|
5-10%
|
10-20%
|
CHClBr2
|
0.005
pptv
|
5-10%
|
10-20%
|
CHBr3
|
0.005
pptv
|
5-10%
|
10-20%
|
CH3Cl
|
100
|
5-10%
|
5%
|
CHCl3
|
2
pptv
|
5-10%
|
5%
|
CHBr3
|
0.1
pptv
|
5-10%
|
10
%
|
CH3Br
|
1
pptv
|
4%
|
5%
|
CH2Br2
|
0.4
pptv
|
5-10%
|
10-20
%
|
CH3I
|
0.05
pptv
|
5-10%
|
10
%
|
Methyl
Nitrate
|
0.05
pptv
|
5-10%
|
10-20%
|
Ethyl
Nitrate
|
0.05
pptv
|
5-10%
|
10-20%
|
1-Propyl
Nitrate
|
0.05
pptv
|
5-10%
|
10-20%
|
2-Propyl
Nitrate
|
0.05
pptv
|
5-10%
|
10-20%
|
2-Butyl
Nitrate
|
0.05
pptv
|
5-10%
|
10-20%
|
Canister arrays like
the one shown in the photo each consist of 24 2-L evacuated canisters. Before
each flight, seven arrays will be loaded onto the DC-and 6 arrays
onto the P-3B. Outside air is collected from beyond the laminar
boundary layer of the aircraft and is pumped via 1/4" stainless
steel tubing through a two-stage metal bellows pump and a gas handling
manifold. To fill an individual canister the operator first builds
up pressure in the manifold, opens the valve to the canister, then
waits for the pressure inside the canister to reach 40 psi before
closing the valve.
One sample will be
collected approximately every 3-4 minutes during horizontal flight,
and one sample every 1.5 to 2 minutes during vertical profile spirals.
Each canister takes about 2.5 minutes to fill at an altitude of
12 km but only about 40s in the boundary layer.
|