1. CO 2 in the middle troposphere Chang-Yu Ting 1, Mao-Chang Liang 1, Xun Jiang 2, and Yuk L. Yung 3 ¤ Abstract Measurements of CO 2 in the middle troposphere are made globally by AIRS. Significant zonal variation is seen but could not be simulated by existing global chemical transport models such as MOZART2 and GEOS-CHEM. Since there is no sink of CO 2 in the atmosphere, the prominent zonal variation in the middle troposphere suggests that (1) there are significant spatial and temporal variations of CO 2 emissions at the surface (2) the transport/dynamics is more turbulent than what we thought. Here we examine the former by employing daily and 6-hourly CO 2 variation at the surface. We find that the model (MOZART2 driven by NCEP winds) constrained by the CO 2 at the surface can better reproduce the AIRS measurements in the middle troposphere. Ground and aircraft measurements are compared to model results. Implications for utilizing AIRS CO 2 measurements for constraining CO 2 sources and sinks at the surface are discussed. ¤ Data and Model ¤ Comparison of CO 2 Between Model and Measurements ►Figure 5: Aircraft observations between 8 km and 13 km (red dots) [Matsueda et al., 2002] and modeled CO 2 mixing ratios from the CTM model averaged at the layer between 9 km and 13 km (solid line) from 2001 to 2007. The panels are for 35.7  S, 24.3  S, 15.7  S, 4.3  S, 4.3  N, 15.7  N, 24.3  N, and 35.7  N, respectively.  CarbonTracker ♦Mole fractions of CO 2 are determined with an accuracy of 0.1 parts per million (ppm) from surface air samples collected globally and from tall towers and small aircraft in North America. ♦An estimate of net CO 2 exchange between the terrestrial biosphere and the atmosphere is derived from a set of 28,000 CO 2 mole fraction observations in the global atmosphere that are fed into a state-of-the-art data assimilation system for CO 2 called CarbonTracker.  Chemistry and Transport Model ♦MOZART-2 driven by the meteorological inputs every 6 hours form the NCEP Reanalysis 1 is used ♦Model time step is 20 minutes with the flux-form semi-Lagrangian method for advection. ♦The horizontal resolution is 2.8 ﾟ (latitude) × 2.8 ﾟ (longitude) with 28 vertical levels extending up to approximately 40 km altitude. ♦The model is constrained by daily and 6 hourly mole fractions of CO 2 at the surface. ¤ Distribution ♦Significant zonal variation of >3 ppmv is observed (upper panel). ♦The model constrained by daily CO 2 variation at the surface can better reproduce the AIRS measurements in the middle troposphere. ♦Figures 1-4 demonstrate an example of CO 2 variation at the surface. ►Figure 6 : AIRS retrieved CO 2 averaged over the month of July 2003 overlain by monthly averages of the National Center for Climate Prediction global reanalysis (NCEP2) 500hPa geopotential height for reference. (top). Modeled CO 2 mixing ratios averaged over the month of July 2003. (bottom) ▲Figure 3: Monthly average of the surface fluxes obtained by CarbonTracker for January, April, July and October of 2003. ▲Figure 4: Standard deviation of the surface fluxes obtained by CarbonTracker for January, April, July and October of 2003. ♦Shown on the right are the simulated CO 2 mixing ratios and aircraft observations made at 8 to 13 km. ¤ Reference ¤ Results 1 Research Center for Environmental Changes, Academia Sinica, Taipei,Taiwan 2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 Peters et al. (2007), An atmospheric perspective on North American carbon dioxide exchange: CarbonTracker, PNAS, 104(48), 18925-18930. Shia et al. (2006), CO2 in the upper troposphere: Influence of stratosphere troposphere exchange, Geophys. Res. Lett., 33, L14814, doi:10.1029/2006GL026141. Chahine et al. (2008), Satellite remote sounding of mid-tropospheric CO2, Geophys. Res. Lett., 35, L17807, doi:10.1029/2008GL035022. Matsueda et al. (2002), Aircraft observation of carbon dioxide at 8 – 13 km altitude over the western Pacific from 1993 to 1999, Tellus, Ser. B, 54(1), 1-21. Jiang et al., Simulation of Upper Tropospheric CO2 From Chemistry and Transport Models, Global Biogeochemical Cycles, in press. ▲Figure 1: Monthly average of mole fractions for January, April, July and October of 2003. The unit of mole fractions is ppm. ▲Figure 2: Standard deviation of the 6 hourly CO 2 mole fractions normalized by the monthly mean values for January, April, July and October of 2003. MODEL