NEW CANDIDATE EARTH EXPLORER CORE MISSIONS THEIR IMPORTANCE FOR ATMOSPHERIC SCIENCE AND APPLICATION P. Ingmann and J. Langen European Space Research and Technology Centre (ESTEC) P.O. Box 299, NL-2200 AG Noordwijk, The Netherlands e-mail: paul.ingmann@esa.int Abstract The European Space Agency (ESA) 'Living Planet' programme is ESA's programme for observing the Earth. The programme represents a flexible and user-friendly approach to the whole concept of Earth observation from space. Within this programme various types of missions are considered including research and demonstration missions. In 2005 the Agency released a Call for Ideas for Earth Explorer Core missions. The scientific priorities for the call for proposals were the global water cycle, the global carbon cycle, atmospheric chemistry and the human element in the Earth system. The Agency received 24 proposals. A set of six mission candidates has been selected for further study including three atmospheric missions, namely TRAQ TRopospheric composition and Air Quality: the mission aims at monitoring of air quality, long-range transport of air pollutants and global climate change. A new synergistic sensor concept for process studies, particularly with respect to aerosol-cloud interactions PREMIER PRocess Exploration through Measurements of Infrared and millimetre-wave Emitted Radiation: central to this mission is the understanding of processes that link trace gases, radiation, chemistry and climate in the atmosphere concentrating on the upper troposphere/lower stratosphere (UTLS) region. Linking with MetOp/EPS data would also provide useful insights into processes occurring in the lower troposphere A-SCOPE Advanced Space Carbon and Climate Observation of Planet Earth: the target of this mission is the observation of total column CO 2 with a nadir-looking pulsed CO 2 differential Absorption Lidar (DIAL) for better understanding of the global carbon cycle and regional CO 2 fluxes, as well as for validation of emission inventories. In 2008 the decision will be made which out of the six candidate missions will be recommended for Phase A study. THE EARTH EXPLORER MISSIONS The Earth Explorer missions within ESA s Living Planet Programme seek to advance the understanding of the different Earth System processes and to demonstrate associated new observing techniques. Currently there are six missions under implementation: GOCE (Gravity Field and Steady-State Ocean Circulation Explorer), due for launch in early 2008, will provide high spatial resolution gravity gradient data that will improve global and regional models of the Earth's gravity field and geoid. SMOS (Soil Moisture and Ocean Salinity), due for launch in 2008, will provide global maps of soil moisture and ocean salinity to provide a better representation of land surfaces in global circulation models and to characterize the role of the ocean in the climate system.
ADM-Aeolus (Atmospheric Dynamics Mission), due for launch in 2009, will provide global wind-profile observations crucial to climate research and numerical weather prediction. CryoSat-2, due for launch in 2009, will determine the rate of change of variations in the thickness and mass of polar marine ice and continental ice-sheets in response to climate changes. CryoSat-2 replaces CryoSat, the first Explorer mission, which was lost at launch in 2005. Swarm, due for launch in 2010, is a constellation of three satellites with the objective of providing a survey of the geomagnetic field and its temporal evolution. The mission data will allow gaining new insights into the Earth system by improving our understanding of the Earth's interior and by allowing an analysis of the influence of the Sun on the Earth system. EarthCARE (Earth Clouds and Radiation Explorer), due for launch in 2013, is a joint European-Japanese mission that will measure cloud and aerosol properties to improve the understanding of their interactions with the Earth's radiative processes as well as climatechange predictions. Figure 1 provides an overview over the Earth Explorer missions under implementation. Figure 1: The Earth Explorer missions under implementation In March 2005, ESA released a new Call for Ideas to scientists from ESA Member States and Canada to solicit proposals for the next series of Earth Explorer Core missions. The scientific priorities defined in the Call focused on the global carbon and water cycles, atmospheric chemistry and climate as well as on the human element as a cross cutting issue. 24 proposals were received and evaluated against their relevance to the scientific themes of the Call as well as their potential for development within the programmatic constraints of the Core Explorer Missions. Six candidate missions were shortlisted for entering dedicated assessment studies. These missions are, in order of priority, BIOMASS, TRAQ, PREMIER, FLEX, A-SCOPE and CoreH 2 O. BIOMASS, FLEX and CoreH 2 O are dealing with a better understanding and quantification of land contribution to global carbon cycle, the observation of photosynthetic activity from space and the observation of snow water equivalent for hydrology,
climate, and NWP. These missions will not be addressed here in detail. However, TRAQ, PREMIER and A-SCOPE address the atmosphere by focusing on atmospheric composition. These will be the focus of this document. THE CANDIDATE EARTH EXPLORER ATMOSPHERIC MISSIONS TRAQ Overview and Mission Objectives The thin atmospheric layer is dramatically affected by the continuous increase in human population and activity. Changes in the tropospheric composition are a serious and growing problem in many regions of the world. The tropospheric composition affects air quality, both regionally and globally, as well as the climate via radiatively active trace gases and aerosols. The TRAQ mission addresses three major issues related to tropospheric composition and chemistry: How fast is air quality changing on a global and regional scale? Air quality has become a problem of at least hemispheric extent and interactions. Large numbers of deaths are caused by air pollution episodes in densely populated regions. Spaceborne measurements of the major primary and secondary pollutants are needed to provide quasi-global coverage, spatial representativity and links between the scattered ground-based data. Fast variations due to emission patterns, illumination and transport require measurements with high temporal resolution. What is the strength and distribution of the sources and sinks of trace gases and aerosols influencing air quality? Existing emission inventories are characterised by large uncertainties on all time scales. Many emissions are not periodic or predictable. Anthropogenic emissions mix with natural. Due to long-range transport, local sources overlap with imported pollutants. Chemical processing and deposition change the composition. Good geographical coverage and a comprehensive set of observed species are necessary to disentangle these processes. What is the role of tropospheric composition in global change? The role of aerosols is one of the largest uncertainties in climate forcing and feedback, in particular its coupling to cloud and precipitation. The budgets of CH 4, the most important non- CO 2 greenhouse gas, are not well understood. Tropospheric O 3, globally increased through primary air pollutants, is another strong forcing agent. TRAQ provides measurements of the relevant trace gases, such as O 3, NO 2, SO 2, CO, HCHO, CH 4 and H 2 O, using spectrometers in the UV, VIS, near IR and thermal IR; as well as detailed aerosol characterisation by a multi-directional polarisation imager. A cloud imager will be used to identify cloud-free scenes. The system is highly synergistic for improved vertical resolution of O 3 and CO, higher accuracy of CH 4, better treatment of partially cloudy scenes, and more detailed aerosol characterisation. Small pixel size provides for high horizontal resolution and good coverage in regions with frequent cloud contamination. Mission design will aim at achieving up to 5x daily coverage in highly polluted areas at mid-latitudes globally. Figure 1 shows an example of what would be looked at in terms of air quality using the day-to-day variations of NO 2 as an example: on a Sunday the pollution levels over the Netherlands are much lower than on a working day, i.e. Saturday or Monday. The TRAQ mission will assess the air quality changes at global and regional scales, determine the strength and distribution of the sources and sinks of trace gases and aerosols influencing air quality and help in the understanding of the role of tropospheric composition in global change.
Saturday 15 October 2005 Sunday 16 October 2005 Monday 17 October 2005 Figure 1: Day-to-day NO 2 variation over Europe seen by OMI (courtesy KNMI/NASA/NIVR) Preliminary Mission Concept A new synergistic sensor concept is proposed to address the TRAQ mission objectives: a high spectral resolution pushbroom short wave spectrometer (SWS) in the range from ultraviolet (UV), visible (VIS) to near-infrared (NIR); a high spectral resolution across-track scanning long wave spectrometer (LWS) in the thermal infrared (TIR) with an embedded cloud imager; a multi-view polarization-resolving pushbroom radiometer (MPR). A short-wave infrared (SWIR) band is also required and its implementation in either the short- (SWS) or the long-wave (LWS) spectrometer will be subjected to dedicated implementation trade-offs. The cloud imager is a dual band infrared instrument that will optimize in real time the pointing direction of the mono-pixel TIR spectrometer over clear area. As an alternative, a fully imaging TIR spectrometer is being considered. The mission will offer a spatial resolution of about 10 km for the spectrometers and 1-4 km for the imagers. A non-sunsynchronous orbit with an inclination of about 50 degree will optimise the diurnal sampling frequency at mid-latitudes. TRAQ will be the first mission fully dedicated to air quality and to the science issues related to tropospheric composition and global change. PREMIER Overview and Mission Objectives The primary aim of PREMIER is to explore processes controlling the composition of the mid/upper troposphere and lower stratosphere. PREMIER will observe trace gas, particulate and temperature distributions in this region down to finer scales than any previous satellite mission, which will also facilitate integration with measurements made at higher resolution at particular locations and times by ground-based and airborne instruments. PREMIER focuses on processes linking trace gases, radiation, chemistry and climate in the atmosphere. Specific science objectives include Convective transport, thin cirrus, tropical tropopause layer Stratosphere-troposphere exchange, pyroconvection, long-range transport of pollutants Gravity waves and their impact on global circulation
Upper troposphere humidity and cirrus (water vapour climate feedback) Radiative forcing by tropospheric O 3 and CH 4 and by stratospheric O 3 and water vapour Chemistry-climate interaction (OH chemistry) Processes linking clouds and aerosols to atmospheric chemistry and global climate. For many of these issues the key altitude range is around the tropopause where the radiative effects of H 2 O and clouds have their maximum, the chemical regime changes and gravity waves are often generated. This region is characterised by small-scale processes which have not been resolved by previous missions. The primary aim of PREMIER is therefore to explore processes controlling the composition of the mid / upper troposphere and lower stratosphere with high horizontal and vertical resolution. This will be achieved by two limb-sounding instruments: an imaging infrared spectrometer that will observe a larger set of chemical species at high spectral resolution to address atmospheric chemistry issues, or a reduced set at low spectral but very high spatial resolution to address small-scale processes of predominantly dynamical character; a millimetre-wave push-broom spectrometer providing key species at high horizontal resolution, even in presence of most cirrus clouds. Clouds in the field of view will be imaged at even higher spatial resolution to support the retrieval of the infrared spectra and to provide additional characterisation of upper tropospheric cloud. The second aim of PREMIER is to explore processes controlling the composition of the lower troposphere / boundary layer and links to higher layers, e.g. related to air quality and climate forcing by trace gases. Although PREMIER does not perform measurements in this altitude range, it will add information on vertical distribution of trace gases observed by GOME-2 and IASI on the Metop platform. PREMIER will fly in tandem with Metop for an optimum spatial and temporal co-registration of the measurements. Figure 2 shows a sketch of the processes in the upper troposphere / lower stratosphere and the concept used for observing them with PREMIER. Preliminary Mission Concept The mission objectives call for 3-D sounding of the mid/upper troposphere and stratosphere by two complementary limb-sounders in the infrared and mm/sub-mm-waves. The limb will be imaged at high spatial resolution for cloud decontamination and derivation of the cloud/aerosol properties. The infrared sounder concept is based on a imaging Fourier transform spectrometer operated either in a high spectral resolution mode, optimized for observation of minor trace gases (atmospheric chemistry mode) or in a high spatial resolution mode, optimized to resolve atmospheric structure (atmospheric dynamics mode). The mm/sub-mm-wave sounder is a limb-sounding heterodyne receiver with a channel in 320-360 GHz range for measurements of the main target species H 2 O, O 3 and CO (or with a slight spectral shift to observe HCN, HDO, N 2 O, ClO, HNO 3 ) in the UT/LS altitude range and a channel in 488-504 GHz range with improved sensitivity to stratospheric target species (e.g. ClO, N 2 O, H 2 O and isotopes). The instrument will be developed in the frame of a national Swedish contribution to the PREMIER mission.
Figure 2: PREMIER - Process exploration through measurements of infrared and millimetre-wave emitted radiation; the figure depicts the Upper Troposphere / Lower Stratosphere exchange as well as the PREMIER implementation concept PREMIER will be the first mission to explore the processes which link atmospheric composition and climate globally from space in unprecedented detail and, for the first time, with sufficient resolution. A-SCOPE Overview and Mission Objectives A-SCOPE, like other missions to observe CO 2 from space, aims to provide an innovative source of data to advance understanding of the carbon cycle and to validate inventories of emissions of greenhouse gases. A-SCOPE will provide global coverage with good temporal resolution. The mission addresses the important scientific objective of mapping the sources and sinks of CO 2 on a scale of 500 km or better. Although the duration of the satellite mission is short compared with the time series of observations at ground stations and although the accuracy of individual measurements is lower than at ground stations, it is expected that A-SCOPE data will provide regionally resolved maps of CO 2 sources and sinks when the data are assimilated by a transport model. It suffices to focus on CO 2, the principal objective of A-SCOPE, as factors such as the diurnal variation, isotopic composition, concentrations of related trace gases and detection of secular trends can be measured more effectively at surface stations than from space. A-SCOPE would be the first mission dedicated to measuring CO 2 with a DIAL sensor. The active technique proposed should provide better accuracy and better spatial and temporal coverage than the planned mission embarking passive sensors, namely OCO (NASA) and GOSAT (JAXA). A-SCOPE would eliminate three important sources of bias that may be problematic for OCO and GOSAT: Firstly, the uncertainty due to atmospheric scattering will be greatly reduced as A-SCOPE will differentiate the ground echo and the atmospheric contribution; Secondly A-SCOPE will measure both by night and by day (sampling time bias); Thirdly, A-SCOPE will measure at high latitudes (spatial bias); The mission would complement missions dedicated to land and vegetation, and would provide continuity to OCO and GOSAT datasets for long term estimates of CO 2 fluxes. Based on the measurement technique, there are benefits expected in terms of spin-off products like aerosols, clouds and surface reflectivity. These parameters are important for the radiation balance of the Earth.
Figure 3 shows the steady increase of carbon dioxide with time from different observing stations. Figure 3: The figure shows the carbon dioxide curves as measured at various carbon dioxide observing stations showing a steady increase with time (courtesy: Dr. Pieter Tans, NOAA ESRL GMD Carbon Cycle, Boulder/CO, http://www.cmdl.noaa.gov/ccgg) Preliminary Mission Concept The core element of the A-SCOPE mission will be a nadir-viewing DIAL instrument measuring the reflected (scattered) radiation from Earth s surface and clouds tops, which are illuminated by laser pulses at slightly different wavelengths. From the ratio of the Lidar echoes the optical depth is calculated first, then the total CO 2 column and finally the path-integrated averaged CO 2 mixing ratio. The mission aims at reaching CO 2 mixing ratio measurement precision of 1.5 parts per million volume (ppmv) as a threshold, with a goal of 0.5 ppmv. Suitable absorption lines for CO 2 concentration measurements with respect to absorption cross-section and temperature insensitivity can be found in the near infrared (NIR) around 1.6 µm and 2.1 µm. In both spectral regions, temperature cross-sensitivity and line interference with water vapour or other atmospheric trace gases can be minimized by dedicated selection of the on- and off-line positions in the vicinity of the absorption line. In order to improve the sensitivity of the measurement in the lower troposphere and in the planetary boundary layer, appropriate absorption lines around 2 µm are preferred. This allows sounding in the far line wing with the corresponding weighting function peaking near the surface. An imaging camera operating in the visible (VIS), near infrared (NIR), and thermal infrared (TIR) with a narrow swath width of 50 km will provide contextual information and a broader view on clouds and Earth surface texture. The A-SCOPE mission will provide new insight into carbon cycle aspects not accessible to existing and currently planned satellite missions. THE EARTH EXPLORER CORE MISSION SELECTION PROCESS The current cycle of Earth Explorer Core Missions will lead to the selection of the 7 th Earth Explorer mission in the 2009/-10 time-frame, due for launch in 2014/2015. As for the previous cycles, the selection process is articulated along 4 steps. The first step (mission assessment Phase 0) was initiated in 2006 with the nomination of the Mission Assessment Groups that will provide independent advice to the Agency for the definition of the detailed scientific objectives of the missions, the
consolidation of the mission requirements, the definition of required scientific support activities and the production of the Reports for Assessment. The selection process is depicted in Figure 4. Figure 4: The Earth Explorer Core Mission selection process The other major element of the mission assessment step consists of the industrial studies at phase 0 level. They were initiated in spring 2007 with the objective of identifying suitable end-to-end implementation concepts for each mission as well as their preliminary feasibility both in terms of required technology and of compliance to the relevant programmatic constraints. The assessment step will be concluded with a User Consultation Process that will review the results of the mission assessment and propose a short-list of candidates to enter the next (mission feasibility Phase A) step. At the end of Phase A, a second User Consultation Process will recommend the final candidate to enter the development phases (B/C/D) and finally join the Earth Explorer family as the 7 th mission of the series. REFERENCES The ESA Living Planet Programme, http://www.esa.int/livingplanet