Definition of Mission Objectives and Observational Requirements for an Atmospher ic Chemistry Explorer Mission Executive

Advisory Committee concerning the 1996 proposal for an Atmospheric Chemistry Explorer mission. The objectives were, firstly, to define mission objectives and observational requirements for a mission commencing in 2008 and, secondly, to assess systematically the observational capabilities with which to address these requirements. The study was undertaken by a consortium of thirteen European institutes, comprising a Modelling Group, with primary responsibility for the first objective, and an Observational Group, with primary responsibility for the second objective. The Modelling Group consisted of KNMI (coordinator , Netherlands), IPSL/CNRS and Istitut de Astrofisica de Andalucia (Spain). The study therefore involved a number of Europe's leading groups in atmospheric modelling and satellite remote-sensing, and is arguably the most comprehensive of its kind undertaken to date. To assess observational capabilities, the full range of passive remote-sensing techniques was analysed on a common basis through retrieval simulations. Both limb-and nadir-viewing geometries were examined for wavelengths ranging from uv to mm, and several options were assessed for each class of instrument 1. The study demonstrated that major scientific advances can be expected for an Explorer mission to follow Envisat and Eos Aura, by combining feasible technical advances for certain sensors with established capabilities of others. These scientific advances were found to be particularly significant for the troposphere and lower stratosphere, the regions of most importance to the scientific objectives of the mission. In addition to the assumption of launch in or after 2008 (ie beyond the expected lifetimes of Envisat and Eos Aura), a second assumption was that, during the Explorer mission, METOP-1,-2 or-3 could be depended upon to deliver: (1) high-quality data on tropospheric temperature, humidity, ozone and other trace gases and (2) high-resolution vis/ir images over a broad swath within which cloud, aerosol and surface parameters could be derived. The extent to which the Explorer could or should attempt to exploit these METOP resources was an important consideration for orbit geometry and payload options. 1 The focus was on passive spectrometric techniques to measure trace gases. Retrieval simulations were not performed for limb-nir, occultation or active techniques (ie lidar or radar), and nor were requirements for the following addressed quantitatively: aerosol, cloud, contrails and polar stratospheric clouds, temperature, wind, actinic flux, lightning, fire.