Stratospheric CH4 and CO2 profiles derived from SCIAMACHY solar occultation measurements

Abstract

Stratospheric profiles of methane (CH4) and carbon dioxide (CO2) have been derived from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). The retrieval is performed using a method called onion peeling DOAS (ONPD), which combines an onion peeling approach with a weighting function DOAS (differential optical absorption spectroscopy) fit in the spectral region between 1559 and 1671ĝ€nm. By use of updated pointing information and optimisation of the data selection as well as of the retrieval approach, the altitude range for reasonable CH4 could be broadened from 20 to 40ĝ€km to about 17 to 45ĝ€km. Furthermore, the quality of the derived CO2 has been assessed such that now the first stratospheric profiles (17-45ĝ€km) of CO2 from SCIAMACHY are available. Comparisons with independent data sets yield an estimated accuracy of the new SCIAMACHY stratospheric profiles of about 5-10ĝ€% for CH4 and 2-3ĝ€% for CO2. The accuracy of the products is currently mainly restricted by the appearance of unexpected vertical oscillations in the derived profiles which need further investigation. Using the improved ONPD retrieval, CH4 and CO2 stratospheric data sets covering the whole SCIAMACHY time series (August 2002-April 2012) and the latitudinal range between about 50 and 70°ĝ€N have been derived. Based on these time series, CH4 and CO2 trends have been estimated. CH4 trends above about 20ĝ€km are not significantly different from zero and the trend at 17ĝ€km is about 3ĝ€ppbvĝ€yearĝ'1. The derived CO2 trends show a general decrease with altitude with values of about 1.9ĝ€ppmvĝ€yearĝ'1 at 21ĝ€km and about 1.3ĝ€ppmvĝ€yearĝ'1 at 39ĝ€km. These results are in reasonable agreement with total column trends for these gases. This shows that the new SCIAMACHY data sets can provide valuable information about the stratosphere.