||We propose to continue our long-term airborne study of atmospheric composition and carbon cycling in the Southern Great Plains (SGP), with scientific objectives that are central to carbon-cycle and radiative-forcing goals of the US Climate Change Science Program. The goals of this measurement program are to improve understanding of: (a) land-atmosphere carbon exchanges of the ACRF region; (b) how CO2 and associated water and energy fluxes influence radiative forcing, convective processes, and CO2 concentrations; and (c) how greenhouse gases are transported on continental scales. To meet these goals, we propose airborne measurements to support the following scientific objectives: 1) Quantify trends and variability in atmospheric concentrations of CO2 and other greenhouse gases in North America, e.g., for NACP continental synthesis; 2) Contribute to the Earth System Model test-bed; 3)Apply carbonyl sulfide as a tracer of Gross Primary Productivity; 4) Improve understanding and modeling of PBL and FT exchange dynamics; 5) Contribute to characterization of cloud dynamics and precipitation processes; 6) Characterize fossil fuel emissions in the ACRF; and 7) Reduce uncertainties in GCM parameterization of radiative transfer. We will maintain and enhance the ongoing airborne carbon cycle measurements using ACRF aircraft (Cessna 206) in the ARM-SGP. We request 300 flight hours per year, in the following missions: (1) vertical profiles from the surface to mid-troposphere (0-5 km); and (2) horizontal transects for spatial patterns in clear sky and cloudy day campaigns. During flights, we will measure CO2 and meteorological data continuously and collect flasks for a rich suite of additional gases: CO, CH4, N2O, 13CO2, 14CO2, carbonyl sulfide (COS), and many trace hydrocarbon species. These flights are the only regular, continuous CO2 vertical profiles collected in the U.S.