A-SCOPE: Objectives and concepts for an ESA mission to measure CO 2 from space with a lidar

Transcription

1 A-SCOPE: Objectives and concepts for an ESA mission to measure CO 2 from space with a lidar Yannig Durand, Jérôme Caron, Paolo Bensi, Paul Ingmann, Jean-Loup Bézy, Roland Meynart European Space Agency esa.int CLRC XV Toulouse, June

2 CLRC XV Toulouse, June Outline Earth Explorer missions selection process A-SCOPE mission objectives Measurement principles A-SCOPE instrument concepts Expected performances

3 ESA s Living Planet Programme Earth Explorer Research driven Earth Watch Service driven Core Missions Opportunity Missions EUMETSAT GMES GOCE 2009 ADM-Aeolus 2011 CryoSat SMOS 2009 MSG MTG Metop/EPS Post-EPS Sentinels 1-5 EarthCARE 2013 Swarm 2010 Earth Explorer Objective: Research in Earth Sciences and demonstration of new Earth Observation techniques CLRC XV Toulouse June 2009 Page 3

4 The next Earth Explorer Science Priorities in the Call for Ideas The Global Water Cycle The Global Carbon Cycle Atmospheric Chemistry and Climate The Human Element and the Impact on above Topics 24 proposals evaluated 6 Candidate missions selected for phase 0 study o FLEX: FLuorescence Explorer o TRAQ: TRopospheric composition and Air Quality o BIOMASS: A BIOMASS Monitoring Mission for Carbon Assessment o PREMIER: PRocess Exploration through Measurements of Infrared and millimetre-wave Emitted Radiation o A-SCOPE: Advanced Space Carbon and Climate Observation of Planet Earth o CoRe-H2O: Cold Regions Hydrology High-resolution Observatory CLRC XV Toulouse June 2009 Page 4

5 ESAC recommendations for A-SCOPE Therefore, quantifying carbon fluxes, especially over land, represents one of the biggest scientific challenges of our time. Regrettably, as stated in the Report for Assessment, the active technology is not at a state such that the mission can be flown in the 2016 time frame. However, ESAC views the A-SCOPE approach as offering the potential of an enormous step forward in our understanding of the carbon budget which is not obtainable by other means. Therefore ESAC very strongly recommends the following: further development of the measurement and sensor concept and technology, as spelled out by the evaluation panel report ESA/ESAC(2009)1, Att of ESA/PB-EO(2009)28, Paris, 3 February 2009 CLRC XV Toulouse June 2009 Page 5

6 Carbon cycle: Scientific and societal context SOCIETAL/POLITICAL OBJECTIVE: Sarmiento and Gruber (2002, updated) To stabilize the concentration of greenhouse gases at a level that prevents dangerous interference with the climate system UN Framework Convention on Climate Change CO 2 most important anthropogenic greenhouse gas. Emissions from burning of fossil fuel & landuse change have caused atmospheric CO 2 to increase by more than 30% over the last 250 years. This CO 2 increase accounts for the majority of the change in radiative forcing. The current network is sparse, leaving large gaps in many key areas (tropics) CLRC XV Toulouse June 2009 Page 6

7 Existing satellite observations of CO 2 Atmospheric CO 2 is the most integrative measurement to assess the state and dynamics of the global carbon cycle. Multi-purpose missions: TOVS, AIRS, IASI: emission spectroscopy in thermal infrared SCIAMACHY: absorption solar spectroscopy Provide the first global distribution of CO 2 Suffer from insufficient precision, low sensitivity in the boundary layer, biaises Snapshot of model simulation for June 2005 Model simulated CO 2 (S. Houweling, 2008) Dedicated mission to measure CO 2 : OCO: Orbiting Carbon Observatory by NASA: launch failure GOSAT: Greenhouse gases Observing SATellite by JAXA: launch CLRC XV Toulouse June 2009 Page 7

8 Challenge of passive missions: scattering layers Dedicated CO 2 Passive Instrument Fourier Transform Spectrometer OCO GOSAT A-SCOPE A-SCOPE Solar spectrometer measures the depth of CO 2 absorption lines Challenges: Scattering layer F.-M. Breon (pers. Comm) Sampling impacted by clouds, no sampling in winter in high latitudes Need to correct for atmospheric scattering (aerosols & clouds) CLRC XV Toulouse June 2009 Page 8

9 Mission objectives of A-SCOPE Overarching objective: To improve our understanding and to better quantify the global carbon cycle. Specific objectives: To observe the spatial and temporal gradients of atmospheric CO 2 significantly better than achieved with the current observation network or the planned space-borne systems. To determine the sources and sinks globally within an uncertainty smaller than 0.02 Pg C yr -1 at the scale of 1000 km x1000 km. Data products: Level 2: Column weighted dry-air mixing ratio of CO 2 (XCO 2 ) Level 3: Global carbon fluxes at a spatial resolution of 1000 km x 1000 km CLRC XV Toulouse June 2009 Page 9

10 Measurement principle: Integrated Path Differential Absorption Lidar The differential absorption between the on- and the offline pulse is directly related to the column density of CO 2, from which the column averaged mixing ratio of CO 2 can be computed. λ on Surface backscattering λ off Pulsed laser Direct detection Distance between measurements ~ 150 m Laser footprint on ground < 100 m CLRC XV Toulouse June 2009 Page 10

11 Observation principle: Laser absorption spectroscopy 2 suitable sets of wavelengths (1.57 µm and 2.05 µm) Low sensitivity to temperature Laser linewidth << CO 2 absorption lines On-line detuning w.r.t. line centre: Optical Depth ~ 1 Increase sensitivity in the boundary layer Minimize interference with other trace gases CLRC XV Toulouse June 2009 Page 11

12 Uniqueness of A-SCOPE Better sampling An order of magnitude better sampling than current surface network, substantially better sampling than OCO and GOSAT, i.e. twice a day, during all seasons at high latitudes. Higher data quality Superior data quality relative to existing missions, especially because of its much lower sensitivity to aerosols, i.e. less sensitive to biases; Complementarity to the existing surface network which provide higher accuracy and precision Better surface fluxes and process understanding A-SCOPE will constrain the surface fluxes of CO 2 to the desired accuracy, permitting the identification of the underlying key processes determining the exchange of CO 2 between atmosphere, terrestrial biosphere and oceans. CLRC XV Toulouse June 2009 Page 12

13 How good do the measurements have to be? Uniform weighting function Precision requirement Pressure scaled weighting function Flux requirement A-SCOPE MAG Report (2008) Using inverse modeling, a flux uncertainty of 0.02 Pg C yr -1 requires an atmospheric CO 2 measurement precision of about 0.5 ppm. Because the impact of measurement biases on the inversely estimated fluxes is large, the requirements for the systematic error need to be stringent. E.g.: 0.1 ppm bias in the north-south gradient of atmospheric CO 2 would lead to an error in the spatial allocation of the carbon flux of 0.25 Pg C yr-1. Error Target (ppm) Threshold (ppm) Random Systematic CLRC XV Toulouse June 2009 Page 13

14 Instrument overview: design drivers Low random error: 0.5 ppm wrt 380 ppm (0.13 % of DAOD) for ~ 350 measurements high laser power telescope aperture low detector noise maximize the on and off-lines pulses overlap on ground Very low systematic error: 10 % of random error for 1000 x 1000 km 2 laser spectral stability and knowledge laser spectral purity stability of power monitoring of emitted laser pulses stability and knowledge of the S/C pointing CLRC XV Toulouse June 2009 Page 14

15 Instrument overview: Direct detection lidar in bistatic configuration Data Processing & Instrument Control Unit Proximity Electronics Detector Pointing Control Electronics Frequency Reference Unit Seeder On Seeder Off Frequency Reference Subsystem Receiver Subsystem Transmitter Electronics Power Laser Head Transmitter Subsystem Pointing & Power monitoring Subsystems Receiver Telescope Transmitter Telescope CLRC XV Toulouse June 2009 Page 15

16 Instrument overview: Opto-mechanical layout and instrument budgets Payload budgets Mass kg Power 500 W Data rate Mb/s 1 m 1.2 m CLRC XV Toulouse June 2009 Page 16

17 Instrument architecture: transmitter Concepts: 1.57 µm: Nd:YAG pumped Optical Parametric Oscillator and Amplifier Seeders: External Cavity Laser Diodes 2.05 µm: Thulium pumped Holmium Power Oscillator Seeders: Tm:Ho stable resonator Specifications: 2 pulses of 50 mj separated by 200 µs 50 Hz pulse repetition frequency Spectral purity > % within 1 GHz Spectral drift < 100 khz for 3 years CLRC XV Toulouse June 2009 Page 17

18 Instrument architecture: receiver Concepts 1.57 µm: Free beam propagation 2.05 µm: Optical fibres 1 m class Cassegrain-like telescope Field of View 100 m Detector: single pixel ( µm) low noise Avalanche PhotoDiode (APD) high stability of linear response CLRC XV Toulouse June 2009 Page 18

19 Instrument architecture: Spectral and radiometric management Two concepts of frequency management via Optical Frequency Comb (OFC) locked to GPS signal stabilisation of on-line seeder based on beating with the OFC measurement of on-line pulses based on high-resolution wavemeter calibrated with the OFC; stabilisation to CO 2 gas cell Pointing control via position sensitive detectors monitors each outgoing pulse stabilises the transmit-receive path Accurate relative power monitoring opto-mechanical design insensitive to change of beam parameters, components ageing integrating sphere CLRC XV Toulouse June 2009 Page 19

20 Performance: random errors At observation level (50 km 350 measurements) 450 ppm CO 2 assumed (worst case) 1.57 µm 2.05 µm Threshold requirements are met for all geophysical assumptions CLRC XV Toulouse June 2009 Page 20

21 Performance: systematic errors Instrument requirements to keep the allowed temporal and spatial varying biases below the threshold 1.57 µm Biases ~ < µm Biases ~ < CLRC XV Toulouse June 2009 Page 21

22 Conclusion Global carbon cycle research is severely limited by the lack of observations. Atmospheric CO 2 is the most integrative measurement to assess the state and dynamics of the global carbon cycle. A-SCOPE provides an opportunity to observe atmospheric CO 2 from space for the first time with the accuracy and the spatio-temporal sampling needed to globally determine the sources and sinks of CO 2 at the regional scale. CLRC XV Toulouse June 2009 Page 22