2.7. AUTOMATED AIRCRAFT SAMPLING
The aircraft sampling project has been in continuous operation at the Carr,
Colorado [40.9°N, 104.8°W] site since November 1992. Until April 1995,
profiles of 20 samples each were taken on a biweekly basis. After April, profiles
were scheduled on a weekly basis to investigate improved signal to noise from
the increased data rate and to gain experience with the logistics of more frequent
flights. Approximately 30 flights were made in 1995 alone. An additional change
was the use of a Cessna Model T210 aircraft beginning in June 1995 that increased
the profile height to 7.9 km typical (9.1 km maximum). An example of a smoothed-curve
fit to a profile altitude interval from the last 3 years of data is shown in
Figure 2.24. Although the plot shows a good seasonal cycle and trend, the irregularities
in the fit and outliers in the data are a reminder of the difficulty in measuring
continental air masses and the complexities of individual profiles.
Fig. 2.24. Carbon dioxide in samples obtained between 3-4 km above
Also in 1995, collaboration began with the Aviaecocentre group in Moscow,
Russia, to fly the CCG automated sampling system on a Russian Air Force twin-turboprop
AN-24. The project acquired six profiles between June and the end of the year
at altitudes from 0.3 to 6.1 km at a location about 100 km southeast of Moscow
(54.9°N, 35.5°E). Meteorology and air mass back-trajectory data were
also recorded. A typical profile from the Moscow site is shown in Figure 2.25.
The Russian collaborators are working to develop a climatology for the site
that can be correlated with the long-term carbon data. It is expected that this
work will continue at least through August 1996.
Fig. 2.25. Vertical profiles of trace species over Yuchonov, Russia,
near Moscow, on November 14, 1995.
All of the sampling was performed with the automated sampling package developed by the Carbon Cycle Group. This package continues to evolve with experience and the testing of various components. A newer version of the package uses a more compact valve-actuator system so that each flask can be equipped with two valves and flushed more thoroughly and reliably than evacuated flasks. An internal Global Positioning System receiver was also included in the package for fully automatic operation.
As part of this research and development effort, year-long sample storage
tests were performed on our "standard" 2.5-L glass flasks equipped
with Teflon o-rings, to measure the trend of trace gas variability. Over a 12-month
period, the tests show a 0.4 ppm loss of CO,
a 0.5 ppb gain of CH
(not significant), a 7.2 ppb gain of CO, a 5.7 ppb gain of H2 (not significant),
and a 1.1 ppb loss of NO.
These trends imply that the trace gas mixing ratio drift within the sample flask
is easily less than the analysis limits if the samples are stored for a month
or less, which is our normal operating procedure.