Long-term greenhouse gas measurements from aircraft

Abstract

Abstract. In March 2009 the NOAA/ESRL/GMD Carbon Cycle and Greenhouse Gases Group collaborated with the US Coast Guard (USCG) to establish the Alaska Coast Guard (ACG) sampling site, a unique addition to NOAA's atmospheric monitoring network. This collaboration takes advantage of USCG bi-weekly Arctic Domain Awareness (ADA) flights, conducted with Hercules C-130 aircraft from March to November each year. Flights typically last 8 h and cover a large area, traveling from Kodiak up to Barrow, Alaska, with altitude profiles near the coast and in the interior. NOAA instrumentation on each flight includes a flask sampling system, a continuous cavity ring-down spectroscopy (CRDS) carbon dioxide (CO2)/methane (CH4)/carbon monoxide (CO)/water vapor (H2O) analyzer, a continuous ozone analyzer, and an ambient temperature and humidity sensor. Air samples collected in flight are analyzed at NOAA/ESRL for the major greenhouse gases and a variety of halocarbons and hydrocarbons that influence climate, stratospheric ozone, and air quality. We describe the overall system for making accurate greenhouse gas measurements using a CRDS analyzer on an aircraft with minimal operator interaction and present an assessment of analyzer performance over a three-year period. Overall analytical uncertainty of CRDS measurements in 2011 is estimated to be 0.15 ppm, 1.4 ppb, and 5 ppb for CO2, CH4, and CO, respectively, considering short-term precision, calibration uncertainties, and water vapor correction uncertainty. The stability of the CRDS analyzer over a seven-month deployment period is better than 0.15 ppm, 2 ppb, and 4 ppb for CO2, CH4, and CO, respectively, based on differences of on-board reference tank measurements from a laboratory calibration performed prior to deployment. This stability is not affected by variation in pressure or temperature during flight. We conclude that the uncertainty reported for our measurements would not be significantly affected if the measurements were made without in-flight calibrations, provided ground calibrations and testing were performed regularly. Comparisons between in situ CRDS measurements and flask measurements are consistent with expected measurement uncertainties for CH4 and CO, but differences are larger than expected for CO2. Biases and standard deviations of comparisons with flask samples suggest that atmospheric variability, flask-to-flask variability, and possible flask sampling biases may be driving the observed flask versus in situ CO2 differences rather than the CRDS measurements.

Evaluation of a new JMA aircraft flask sampling system and laboratory trace gas analysis system

“…These mean differences are not viewed as significant, compared with the WMOrecommended limits (e.g., 0.1 ppm for CO 2 , 2 ppb for CH 4 and 2 ppb for CO) (WMO, 2011), although the SDs are larger. The larger deviations might have been caused by several factors such as atmospheric variability, flask-to-flask variability, and possible flask sampling biases in the comparison between flask and in situ measurements, as pointed out by Karion et al (2013). These results strongly suggest that CO 2 , CH 4 and CO mole fractions in the sampled air are not significantly changed at least by passing it through the air-conditioning system of the C-130H aircraft.…”

“…These advantages of the laser-based instruments are suitable for the long-term operational monitoring at JMA, although the higher initial cost compared with nondispersive infrared (NDIR) analyzers or GC techniques is disadvantageous (e.g., Stephens et al, 2011). The laser-based instruments have been field tested and are available for in situ continuous measurements at ground-based stations and aircraft observations (e.g., Chen et al, 2010;Winderlich et al, 2010;WMO, 2011;Richardson et al, 2012;Zellweger et al, 2012;Karion et al, 2013), but their applications for measuring discrete atmospheric samples were limited except for the analysis of the AirCore tubing sample (Karion et al, 2010). We report on the trace gas measurement system using the laserbased instruments that went into our flask sample analysis.…”

“…The C-130H aircraft would provide a unique opportunity for a regular observation of atmospheric trace gases in the middle troposphere because of the aircraft's cruising altitude of about 6 km. However, we faced several logistical issues using the military aircraft, such as lack of available electric power and limited space for our instruments, eliminating any possibility of setting up and operating an automated air sampling system (e.g., Machida et al, 2008;Karion et al, 2013). Thus, sampling is performed manually.…”

Accurate measurements of carbon monoxide in humid air using the cavity ring-down spectroscopy (CRDS) technique

“…The CRDS analyzer does not need frequent calibration Karion et al, 2013;Richardson et al, 2012) or a drying system, and could be an accurate and low-maintenance instrument useful for monitoring the atmospheric mole fractions of CO.…”

Atmospheric CO2, CH4, and CO with CRDS technique at the Izaña Global GAW station: instrumental tests, developments and first measurement results

“…The raw variables associated to each "species" value are: a) Species=1, CO2, peak_14 (p14), and CO; b) Species=2, CH4, CO2_dry; c) Species=3, H2O, h2o_reported (hr), CO2_dry, and CH4_dry; and d) Species=4, CO, b_h2o_pct (bh), and peak84_raw (p84); where CO2 and CH4 are raw values not corrected from H2O dilution nor pressure broadening, whereas CO2_dry and CH4_dry have been 5 (factory) corrected from those effects, p14 is the raw value associated to the main CO2 peak, CO is already (factory) corrected from the CO2 and H2O influences, H2O is the calibrated value obtained from hr, which is the reported H2O raw value associated to the main H2O peak (in the CH4-peak laser wavelength range), bh is the H2O raw value associated to the secondary H2O peak (in the CO-peak laser wavelength range), and p84 is the raw value associated to the CO peak. 3.1 Inlet pressure sensitivity correction for raw CO2 15 As pointed out by Karion et al (2013), the Picarro G2401 CRDS has a critical orifice (indeed a proportional valve kept always at the same aperture or closed) at the inlet of the cavity, and a proportional valve at the outlet of the cavity and upstream the vacuum pump. Since the cavity pressure is kept at 140 Torr (by controlling the opening of the Outlet Valve), the flow in the critical orifice is supersonic, and therefore, ideally only depends on the CRDS upstream quantities (mainly on the inlet pressure; see Appendix A for more details) and not on the cavity quantities.…”