Esa Bulletin February 2000 [antikvár]

MIPAS - An Envisat Instrument for Atmospheric Chemistry and Climate ResearchM. Endemann, P. Garé, J. Langen, H. Nett C.J. ReadingsESA Directorate for Applications Programmes, ESTEC, Noordwijk, The NetherlandsScientific rationaleThe past decade has seen increasing public concern about the Earth, its environment and mankind's Impact on It. Global threats such as climate warming, stratospheric ozone depletion, tropospheric pollution etc. have demonstrated the need to develop a much better understanding of the Earth system and the role of the atmosphere. In many areas knowledge is woefully lacking despite its relevance to the development of climate models and numerical forecasting.MIPAS, the Michelson Interferometer for Passive Atmospheric Sounding, is a high-resolution Fourier-transform spectrometer designed to measure concentration profiles of atmospheric constituents on a global scale. It will observe the atmospheric emissions from the Earth's horizon (limb) throughout the mid-infrared region, which will allow the simultaneous measurement of more than 20 atmospheric trace gases, including the oxides of nitrogen and several chlorofluorocarbons (CFCs). Of these, the five major species, as well as pressure and temperature profiles, will be routinely retrieved by the ESA ground segment. These MIPAS data will provide global data coverage, including in particular the polar regions, where the stratospheric chemistry is currently exhibiting some alarming changes.In considering the way forward, it is Important to realise the need to study not only the lower atmosphere (or troposphere), but also the other layers of the atmosphere, namely the stratosphere and the mesosphere. All layers are Important and to progress It is essential to study all three. Despite their importance, the processes (dynamic as well as chemical) controlling the levels of trace species and their distribution In the atmosphere are far from understood.The basic requirement Is for data to identify processes, to formulate models to represent these processes, to validate the models, and to monitor levels of key geophysical variables.Responding to this need for data, the ESA Envisat mission (Fig. 1) Includes three chemistry experiments, namely GOMOS (Global Ozone Monitoring by Occultation of Stars), SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Cartography) and MIPAS (Michelson Interferometer for Passive Atmospheric Sounding).GOMOS has already been described in a companion article In the ESA Bulletin (March 1999). This article focuses on MIPAS (Fig. 2), which Is a high-resolution Fourier-Transform spectrometer designed to observe the atmospheric emissions from the Earth's horizon (limb) throughout the mid-infrared region (685 -2410 cm-1, 14.6 - 4.15 micron). It will be able to make simultaneous measurements of more than 20 atmospheric trace gases, Including almost the complete family of nitrogen oxides (!.e.NOy) and several CFCs (chlorofluorocarbons).Mission objectivesStratospheric chemistry The primary aim of MIPAS Is to advance the understanding of the chemistry of the stratosphere, which is essentially the study of stratospheric ozone. Large decreases in stratospheric ozone have been recorded over the Antarctic for many years, but more recently significant reductions in ozone levels have also been observed at northern middle and high latitudes. The main cause of these reductions In ozone levels in the stratosphere is the anthropogenic emission of certain trace species into the atmosphere, in particular the CFCs.Following the Implementation of the Montreal Protocol (an International Treaty to eliminate the use of CFCs), the increase in the average concentrations of these compounds in the troposphere has now slowed down and, as a consequence, chlorine loading In the stratosphere is expected to start to decline9