A multi-year record of airborne CO 2 observations in the US Southern Great Plains
We report on 10 yr of airborne measurements of atmospheric CO 2 mole fraction from continuous and flask systems, collected between 2002 and 2012 over the Atmospheric Radiation Measurement Program Climate Research Facility in the US Southern Great Plains (SGP). These observations were designed to quantify trends and variability in atmospheric mole fraction of CO 2 and other greenhouse gases with the precision and accuracy needed to evaluate ground-based and satellite-based column CO 2 estimates, test forward and inverse models, and help with the interpretation of ground-based CO 2 mole-fraction measurements. During flights, we measured CO 2 and meteorological data continuously and collected flasks for a rich suite of additional gases: CO 2, CO, CH 4, N 2O, 13CO 2, carbonyl sulfide (COS), and trace hydrocarbon species. These measurements were collected approximately twice per week by small aircraft (Cessna 172 initially, then Cessna 206) on a series of horizontal legs ranging in altitude from 460 m to 5500 m a.m.s.l. Since the beginning of the program, more than 400 continuous CO 2 vertical profiles have been collected (2007–2012), along with about 330 profiles from NOAA/ESRL 12-flask (2006–2012) and 284 from NOAA/ESRL 2-flask (2002–2006) packages for carbon cycle gases and isotopes. Averaged over the entire record, there were no systematic differences between the continuous and flask CO 2 observations when they were sampling the same air, i.e., over the one-minute flask-sampling time. Using multiple technologies (a flask sampler and two continuous analyzers), we documented a mean difference of < 0.2 ppm between instruments. However, flask data were not equivalent in all regards; horizontal variability in CO 2 mole fraction within the 5–10 min legs sometimes resulted in significant differences between flask and continuous measurement values for those legs, and the information contained in fine-scale variability about atmospheric transport was not captured by flask-based observations. The CO 2 mole fraction trend at 3000 m a.m.s.l. was 1.91 ppm yr −1 between 2008 and 2010, very close to the concurrent trend at Mauna Loa of 1.95 ppm yr −1. The seasonal amplitude of CO 2 mole fraction in the free troposphere (FT) was half that in the planetary boundary layer (PBL) (~ 15 ppm vs. ~ 30 ppm) and twice that at Mauna Loa (approximately 8 ppm). The CO 2 horizontal variability was up to 10 ppm in the PBL and less than 1 ppm at the top of the vertical profiles in the FT.