Angel J. Gomez-Pelaez

Observations and modelling of the global distribution and long-term trend of atmospheric 14CO2.

Global high-precision atmospheric Δ14CO2 records covering the last two decades are presented, and evaluated in terms of changing (radio)carbon sources and sinks, using the coarse-grid carbon cycle model GRACE. Dedicated simulations of global trends and interhemispheric differences with respect to atmospheric CO2 as well as δ13CO2 and Δ14CO2, are shown to be in good agreement with the available observations (1940–2008). While until the 1990s the decreasing trend of Δ14CO2 was governed by equilibration of the atmospheric bomb 14C perturbation with the oceans and terrestrial biosphere, the largest perturbation today are emissions of 14C-free fossil fuel CO2. This source presently depletes global atmospheric Δ14CO2 by 12–14‰ yr−1, which is partially compensated by 14CO2 release from the biosphere, industrial 14C emissions and natural 14C production. Fossil fuel emissions also drive the changing north–south gradient, showing lower Δ14C in the northern hemisphere only since 2002. The fossil fuel-induced north–south (and also troposphere–stratosphere) Δ14CO2 gradient today also drives the tropospheric Δ14CO2 seasonality through variations of air mass exchange between these atmospheric compartments. Neither the observed temporal trend nor the Δ14CO2 north–south gradient may constrain global fossil fuel CO2 emissions to better than 25%, due to large uncertainties in other components of the (radio)carbon cycle.

Global CO2 fluxes inferred from surface air-sample measurements and from TCCON retrievals of the CO2 total column

We present the first estimate of the global distribution of CO_2 surface fluxes from 14 stations of the Total Carbon Column Observing Network (TCCON). The evaluation of this inversion is based on 1) comparison with the fluxes from a classical inversion of surface air-sample-measurements, and 2) comparison of CO_2 mixing ratios calculated from the inverted fluxes with independent aircraft measurements made during the two years analyzed here, 2009 and 2010. The former test shows similar seasonal cycles in the northern hemisphere and consistent regional carbon budgets between inversions from the two datasets, even though the TCCON inversion appears to be less precise than the classical inversion. The latter test confirms that the TCCON inversion has improved the quality (i.e., reduced the uncertainty) of the surface fluxes compared to the assumed or prior fluxes. The consistency between the surface-air-sample-based and the TCCON-based inversions despite remaining flaws in transport models opens the possibility of increased accuracy and robustness of flux inversions based on the combination of both data sources and confirms the usefulness of space-borne monitoring of the CO_2 column.

Thermal Instability in a Cooling and Expanding Medium Including Self-Gravity and Conduction

A systematic study of the linear thermal stability of a medium subject to cooling, self-gravity and thermal conduction is carried out for the case when the unperturbed state is subject to global cooling and expansion. A general, recursive WKB solution for the perturbation problem is obtained which can be applied to a large variety of situations in which there is a separation of time-scales for the different physical processes. Solutions are explicitly given and discussed for the case when sound propagation and/or self-gravity are the fastest processes, with cooling, expansion and thermal conduction operating on slower time-scales. A brief discussion is also added for the solutions in the cases in which cooling or conduction operate on the fastest time-scale. The general WKB solution obtained in this paper permits solving the problem of the effect of thermal conduction and self-gravity on the thermal stability of a globally cooling and expanding medium. As a result of the analysis, the critical wavelength (often called Field length) above which cooling makes the perturbations unstable against the action of thermal conduction is generalized to the case of an unperturbed background with net cooling. As an astrophysical application, the generalized Field length is calculated for a hot (10^4 - 10^8 K), optically thin medium (as pertains, for instance, for the hot interstellar medium of SNRs or superbubbles) using a realistic cooling function and including a weak magnetic field. The stability domains are compared with the predictions made on the basis of models for which the background is in thermal equilibrium. The instability domain of the sound waves, in particular, is seen to be much larger in the case with net global cooling.

CO2 total column amounts at TCCON sites Izana (28.3 N, 16.5 W) and Karlsruhe (49.1 N, 8.5 E)

The Total Carbon Observing Network (TCCON) is a global network of ground-based Fourier Transform Spectrometers recording direct solar spectra in the near-infrared (NIR) spectral region. Accurate and precise columnaveraged abundances of different greenhouse gases (GHGs) are retrieved, which are used for carbon cycle research [1] and for satellite validation (e.g. SCIAMACHY, GOSAT, OCOII). Official TCCON data are generated using the GFIT code compared FTIR retrieval results with in-situ measurements developed at NASA/JPL [2]. In this study, we compare FTIR retrieval results with in-situ measurements as well as the GFIT code with the radiative transfer and retrieval algorithm PROFFIT [3].