Cal & VAL for Greenhouse gases observation spectrometers CEOS WGCV-42 Working Group on Calibration & Validation May 28, 2017, Sioux Falls, SD Cal & VAL for Greenhouse gases observation spectrometers

TANSO is the only FTS for GHG measurements from space SCIAMACHY (2002 - 2012) GOSAT (2009 - ) OCO-2 (2014-) OCO-3 (2018) GHGsat (2016-) TanSat (2016-) TROPOMI (2017-) GOSAT-2 (2018-) MERLIN (2020) GeoCARB MicroCarb (2020) ASCENDS (2021) CarbonSat (2022) GOSAT-3 G3E Lab model test 2

International effort to demonstrate the effectiveness of satellite GHG observation. Japanese GOSAT and US OCO-2 have different observation strategies. TANSO-FTS onboard GOSAT has wide spectral coverage from SWIR to TIR and an agile pointing system at the expense of spatial context, while OCO-2 targets CO2 with higher spatial resolution using imaging grating spectrometers. Since the early phase of the two projects, both teams have worked in calibration and validation to demonstrate the effectiveness of satellite greenhouse gases observation.

Inter-comparison between GOSAT and OCO-2 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Radiometric calibration Prelaunch X-CAL Annual Vicarious Calibration at the desert playa in Nevada CO2 & CH4 profile In situ CO2 and CH4 on AJAX XCO2 & XCH4 Dec, 2008＠TKSC Difference 1.59%,1.1%, 1.4% GOSAT OCO-2 Column with EM-27 FTS Coincident Target Retrieved Parameter Comparison over match up points Calibrated GOSAT and OCO-2 radiance spectra agrees within 5% for all bands. Retreived XCO2 bias is much less than 0.5ppm over match up observations points. 4

Path 36 Path 37 from East from West Calibration and Validation at RRV, Nevada Every June near summer solstice (9th in 2017) Path 36 from East Path 37 from West 33.0deg 25deg 19deg 19.9deg TOA Spectral radiance High altitude Horizontal CO2　 CH4 Vertical CO2　 CH4 O3　 Surface Thermal radiation Surface and Profile of Pressure, Temperature, Humidity Aerosol Optical Depth Surface CO2　CH4 CO O 3 Wind speed Surface Spectral Reflectance UV-SWIR BRDF　 Variability 5 1

Calibration, Retrieval, and Validation Common standards Should be calibrated and validated in each level. H2O CO2 CH4 Measurement Level 1: spectra Level 2: retrieved CO2 GOSAT/TANSO-FTS and OCO-2 observe sunlight reflected from the Earth’s surface and retrieve atmospheric carbon dioxide (CO2) with different observing geometries and ground track repeat cycles. Most measured spectral radiances (Level 1) agree within 5% for all bands shared by the two instruments Retrieved CO2 densities also agree within +/- 4ppm (1%). Level 1 Level 2

Level 1: Cross calibration of Radiance spectra FTS-based GOSAT and Grating base OCO: other than Railroad Valley Average Difference after BRDF correction O2A band @ 0.76 μm : 4% <% Weak CO2 @1.6 μm : 2% <% Strong CO2 @1.6 μm : 3% The above level differences do not affect XCO2 retrieval as CO2 is retrieved with differential absorption

UV imager vicarious calibration (2017 campaign) V+UV V+SWIR UV V+UV V+SWIR Sand-blasted Aluminum Lower reflectivity Spectrally flat Robust Spectralon Reflectance degraded in UV when contaminated. Diffusive (Ideal Lambertian) Assuming sand-blasted Aluminum BRDF has low spectral dependence. Larger than 5%

Leve 2: CO2 cross-validation GOSAT/ACOS –OCO2 Level 2 XCO2 FTS ⊿XCO2 (ACOS-OCO2) error bar: ⊿XCO2 deviation within GOSAT-FOV ● Temporal : GOSAT – OCO2 time difference : +/- 1 hour ● Spatial : Select OCO2 data within 10.5 km GOSAT footprint Sep2014 Feb2016 Grating [Data period] 2014/09～2016/02 ; Level2 matchup : 715 points Agreement: <0.14ppm over Ocean

Geometry Dependency (forward vs backward) Related retrieved parameters: aerosol optical depth Uniform within GOSAT 10.5 km footprint ACOS B7.3 sometimes miss small cloud fractions Forward is weaker but less geometry sensitive Correlation of forward cased is much better Path 36 Forward Path 37 backward 10 B7.3 AOD vs. Aeronet located at the center of the playa

XCO2, XCH4, and XCO Validation with mobile FTSs Direct solar light observation for column-averaged dry air mole fractions of CO2 and CH4 Just before the campaign The campaign at Railroad Valley EM27-NIR TCCON EM-27 @Caltech 2 InGaAs detectors O2, CO2, and CH4 Weak CO and CH4 at 2.3 μm Thermoelectric cooler From sunrise to sunset Intercomparison with high spectral resolution FTS and moderate res. FTS. The Total Carbon Column Observing Network (TCCON) EM27-MIR Single InSb detector Extended to 4.7 μm for strong CO band Mechanical cooler

(not matured yet, but one of our goals) Flux estimation (not matured yet, but one of our goals) Satellite data alone cannot provide flux information. Surface wind speed information is needed. Simplest case: No influx Point source Column averaged 10.5 km diameter GOSAT can target center of the emission source (accuracy of 0.5 km) Emission t/hour=△XCH4(ppb)*0.495t*3600s*V(m/s)/5000m

Common Standards and Data for Calibrations and Retrieval   Common standards and data Notes Calibration Prelaunch Cross-calibrate radiometers Viewing common radiometric standard Onboard Solar data Vicarious CEOS site (RRV etc.) Annual campaign Retrieval Algorithm Parameters Molecular spectroscopy A priori model (Aerosol) Input Calibrated spectra at 0.76, 1.6, 2.0μm Observation geometry and condition Level 1 Output Retrieved column amount GHG Simultaneously retrieved parameters (aerosol optical depth, Solar-induced plant chlorophyll fluorescence (SIF)) Level 2 Validation Ground Site High resolution spectrometer mobile spectrometer TCCON Air plane In situ measured data CONTRAIL etc. Match up Land type and Ocean Viewing Common sites

Data products Different satellite data are available from the same site https://co2.jpl.nasa.gov/

Future plan 9th GOSAT –OCO-2 joint vicarious calibration campaign at RRV, NV (June, 2017) On June 28 both GOSAT and OCO-2 target form the east viewing toward the sun. Coincident spiral flight wit NASA Ames AJAX to measure in-situ CO2, CH4 and O3 density. (2) CH4 intercomparison with airborne LIDAR Coincident flight with airborne model CHARM of MERLIN August 2017 Small campaign, Spring 2018, CoMET HALO (3) CAL-VAL method and data Open web site in JAXA EORC GOSAT web page (2017) “The cross-calibration of spectral radiances and cross-validation of CO2 estimates from GOSAT and OCO-2.” for MDPI remote sensing special issue for satellite (2017) (4) Estimated Flux comparison