Radiatively active gases: ACME airborne observations in the U.S. Southern Great Plains

Description

The vertical profile of atmospheric greenhouse gas mixing ratios is used in large-scale inversions of surface fluxes, as tracers of atmospheric transport, in characterization of climate-carbon cycle interactions, and in validation of satellites. The ARM Airborne Carbon Experiment (ARM-ACME) collects profiles semi-weekly from a small aircraft (Cessna 206) on a series of horizontal legs ranging from 17,500 feet down to 1,500 feet above sea level. Since November 2007, more than 300 continuous CO2 vertical profiles have been collected, along with NOAA/ESRL 12-flask (carbon cycle gases and isotopes) packages for validation and fingerprinting the source of CO2 anomalies. Currently these are the only semi-weekly atmospheric profiles collected in the United States, and the data are used in inversions, satellite validation, and other studies. We report here on three years of these airborne measurements. The continuous CO2 observations are measured using an Atmospheric Observing System, Inc. instrument. The analyzer has non-imaging optics, an NDIR core, and negligible sensitivity to motion of platform. Accuracy, including bias, is approximately 0.1 ppm of CO2 at 1 Hz. Comparison between the continuous and flask CO2 measurements indicates no systematic differences. In addition to the long-term trend in CO2 mixing ratio of about 2 ppm per year in the free troposphere, the seasonal cycle in mixing ratio spans more than 20 ppm within the boundary layer and 10 ppm in the free troposphere within a single year. We have discovered that the CO2 mixing ratio varies over the vertical profile by as much as 6 ppm on a single day, and the direction of the concentration gradient across the boundary layer reverses between spring and fall. This research is supported by the ARM AAF and TES programs.