5.5. STEALTH PROJECT: AUTOMATED FOURCHANNEL
FIELD GAS CHROMATOGRAPHS
The STEALTH GC was installed at ALT, HFM, ITN, and LEF with different custom configurations for each client. The four-channel STEALTH GC will replace the old HP5920 GCs at the Radiatively Important Trace Species (RITS) stations (BRW, NWR, MLO, SMO, and SPO) and is currently being constructed and laboratory tested.
The data acquisition software and operating system of the STEALTH GC computer is being upgraded. NOAH is currently cooperating with personnel at Harvard University in the development of new data acquisition software for the PC-based UNIX operating system, QNX. QNX is a multitasking and multi-user operating system that will facilitate data acquisition, data retrieval, data archival, and real-time display. The airborne GC, ACATS-IV, is currently being configured to test the QNX data acquisition software. In 1996, NOAH scientists will implement the software on an ACATS-IV deployment and on the new STEALTH station GCs.
The STEALTH GC is an ECD/GC system based on technology developed on ACATS-IV
and LACE. The first channel encompasses a Shimadzu mini-2E ECD and a Porapak
Q packed column (Figure 5.28). This channel allows for the measurement of N2O
and SF6. Channel two of the instrument uses a Valco ECD along with
a Unibeads 2S packed column. This configuration is capable of measuring N2O,
CFC-12, H-1211, CFC-11, and CFC-113. The third channel also uses a Valco ECD
and an OV-101 packed column and is used to measure CFC-11, CFC-113, CHCl3,
CH3CCl3, CCl4, and C2Cl4
(perchloro-ethylene, PCE). All of the aforementioned channels have been proven
in other similar instruments that have been constructed and deployed to various
Fig. 5.28. Chromatograms of all four channels of the STEALTH GC that will
replace the old RITS HP5890 GCs.
Channel four, however, has just recently been developed and uses quite a different
setup than the other three channels. This channel also uses a Valco ECD, but
incorporates a GS Q capillary column rather than a packed column. This channel
also uses a Neslab cryocooler which allows for the preconcentration trapping
of the three trace gasses, HCFC-22, CH3Cl, and CH3Br being
measured. With the current configuration of this channel, one is capable of
measurements of better than 1% for HCFC-22, 0.5% for CH3Cl, and 2%
for CH3Br. Figure 5.28d is a chromatogram of the newly developed
5.5.2. TOWER GC AT WITN IN COOPERATION WITH CCG
The GC and instruments that monitor CO2 and 222Rn are housed in a building adjacent to a tall tower (WITN) in rural North Carolina. Diaphragm pumps located in the building continuously draw air from 51, 123, and 496 m above ground through 1 cm i.d. Dekabon tubing affixed to the tower. Detailed descriptions of sample handling and drying, and initial results of CO2 measurements at WITN were published [Bakwin et al., 1995]. GC analyses of air from each sampling level and of two calibrated whole-air standards are performed hourly. Standards are stored at high pressure in "Aculife"-treated aluminum cylinders. Chromatographic and housekeeping data are logged by a 486SX PC and archived on 1.2 Gb optical disks that are sent to the NOAH laboratory each week for analysis. Instruments and gas supplies are maintained by a technician who visits the site weekly .
Monthly statistics of mixing ratios for several halocompounds and N2O
at the WITN tower are presented in Figure 5.29. Statistics for each sampling
height (51, 123, and 496 m) are denoted by 1, 2, and 3, respectively, along
the bottom of each plot . For each month, the mean (circle) and standard deviation
(distance between circle and asterisk) of the mixing ratios of each species
generally decrease with increasing sample height. Variability in trace gas mixing
ratios within the continental boundary layer is determined by sources, sinks,
boundary layer dynamics, and horizontal transport. Since each species plotted
in Figure 5.29 has solely ground-based sources, it is expected that mixing ratios
and variability should be greatest near the ground. This effect is inflated
at night by the accumulation of emissions from local, ground-based sources in
the shallow nocturnal stable layer.
Fig. 5.29. Monthly statistics of CFC-11, CFC-12, CFC-113, methyl chloroform, carbon tetrachloride, nitrous oxide, and sulfur hexafluoride mixing ratios at the WITN tower. Crosses represent medians (horizontal bars) and interquartile range (vertical bars). Circles and asterisks are means and means ±1 standard deviation, respectively. The numbers across the bottom of each plot are the sampling level (1, 2, and 3 refer to 51, 123, and 496 m, respectively).
Figure 5.30 gives statistics for the 51-496 m mixing ratio gradients, binned
by hour of day, of N2O, CH3CCl3, and SF6
for November 1995. Significant vertical gradients of N2O and CH3CCl3
were observed at night, indicating these compounds were emitted by local, ground-based
sources. In contrast, the insignificant accumulation of SF6 in the
nocturnal stable layer suggests an absence of local, ground-based sources. During
the late morning and afternoon, convection rapidly mixes air from the ground
to >500 m, and vertical gradients approach zero.
Fig. 5.30. Statistics of 51 m - 496 m mixing ratio gradients for N2O,
CH3CCl3, and SF6, binned by hour, for November
1995. Crosses indicate means (horizontal bars) ± the 95% confidence interval
(vertical bars). Circles represent medians, and asterisks indicate upper and
lower quartiles. The left panel gives statistics of 51 m - 496 m gradients for
the entire month.
In studying regional emissions of trace gases, it is critical that the influences
of local sources are minimized. At WITN, the boundary layer height during the
night is typically <500 m. Hence, mixing ratio variability at 496 m during
the nighttime is primarily driven by horizontal transport of polluted air to
the site, and mixing ratios of long-lived species should reflect regional-scale
emissions. Figure 5.31 shows the correlation of CH3CCl3
and C2Cl4 mixing ratios at 496 m between 2200-0900 EST
during November 1995. An orthogonal distance regression through the data yields
a slope of 0.62, which can be taken as the regional emission ratio of these
two compounds. Using accurate (±5%) estimates of North American emissions
of C2Cl4 [McCulloch and Midgely, 1996], CH3CCl3
emissions can be calculated. Using this methodology, emissions of halocompounds,
especially those whose production and emissions are controlled by the Montreal
Protocol, are monitored.
Fig, 5.31. Correlation between CH3CCl3 and C2Cl4
mixing ratios at 496 m between 2200 and 0900 (EST) during November 1995. The
slope of an orthogonal distance regression (0.62) is taken as the regional emission
ratio of these two compounds.