Corrigendum to "First direct observation of the atmospheric CO<sub>2</sub> year-to-year increase from space" published in Atmos. Chem. Phys., 7, 4249–4256, 2007

Abstract

After the ACPD open review process and after our manuscript Buchwitz et al. (2007), entitled "First direct observation of the atmospheric CO 2 year-to-year increase from space", appeared in ACP, we have received some relevant private communications, which we would now like to address. In summary, we conclude that we would like to refer readers of our manuscript to information given in three other manuscripts not cited in Buchwitz et al. (2007), namely Chédin et al. (2002Chédin et al. ( , 2003 and Strow et al. (2006). We also conclude that the title of our paper now appears somewhat general and not sufficiently specific to atmospheric CO 2 columns. In our manuscript we report that the CO 2 annual increase has been observed indirectly already by Aumann et al. (2005) by analying AIRS brightness temperatures in the thermal infrared (TIR) spectral region. We have used the term indirectly because Aumann et al. (2005) is not focussing on CO 2 retrieval but on the sensitivity of AIRS temperature measurements to CO 2 variability, including CO 2 trends. We were not aware of results shown in other relevant manuscripts also discussing mid tropospheric CO 2 trends using satellite measurements in the TIR spectral region, namely Chédin et al. (2002Chédin et al. ( , 2003 using HIRS and Strow et al. (2006) using AIRS. In the following we would like to shortly discuss the results presented in these manuscripts focussing on aspects related to the interpretation of the title of our manuscript. As written in Chédin et al. (2002) the TIR radiances in the CO 2 absorption bands "mostly depend on the temperature of the atmosphere but also on the CO 2 concentration". In contrast, our results are based on absorption spectroscopy in the near-infrared (NIR) spectral region using reflected solar radiation, which primarily depends on CO 2 and only weakly depends on temperature. Because of this the NIR CO 2 retrievals may be characterized as more direct but we used direct in our title primarily because of the results shown in Au- (2002, 2003) are not focussing on temperature but on CO 2 retrieval and they show in both manuscripts that the annual CO 2 increase in the tropical middle troposphere can be retrieved from HIRS TIR radiances. Because of the strong temperature dependence of the HIRS TIR radiances atmospheric temperature variability has to be constrained. This is accomplished in Chédin et al. (2002) by using collocated radiosonde measurements and in Chédin et al. (2003) by using MSU microwave measurements. In addition, both papers focus on the tropical region because this enables to better decorrelate CO 2 from temperature variations. Strow et al. (2006) focusses on AIRS radiances and discusses differences to radiative transfer simulations based on ECMWF temperature profiles. Apart from the different temperature sensitivities there are several other important differences between the NIR and the TIR measurements. An important difference is is that the NIR measurements have nearly equal sensitivity to CO 2 concentration changes at all a...