GOME2 Error Study: Column Retrieval

1/ [email protected]

GOME2 Error Study: Column Retrieval

Overview of Final Presentation (IUP Contribution):

Main contribution from IUP Radiative Transfer Modelling and Data Simulation Error analysis for GOME2 trace gas column retrieval

Contributors Rüdiger de Beek (retrieval, error analysis, RTM database) Vladimir and Alexei Rozanov (RTM development) Andreas Richter (DOAS settings, GOME1 error assessment) Marco Vountas (Ring effect) Mark Weber (project management, error analysis) John Burrows (PI University of Bremen, GOME1 lead scientist)


GOME2 Error Study: Column Retrieval

Topics: Task Report 1: Tool Adaptation and Definition of Data

DOAS trace gas column retrieval (WP120, M. Weber) RTM and spectral simulation of Input Datasets (WP130, M. Weber) Overview of error sources (WP150, M. Weber) Basic SNR error (WP 150, R. de Beek)

Task 2: Analysis of Error Sources Spatial Aliassing (WP 210, M. Weber) Spectral Resolution and Undersampling (WP 230, R. de Beek) RTM Assumptions and Earth Curvature (WP250, R. de Beek) BRDF (WP 260, M. Weber) Pointing and Geolocation (WP 270, R. de Beek)

Task 3: Optimal Operational Settings and Error Mitigation (M. Weber) Overall Error Budget Recommendations Future Work


WP120: Column Retrieval Technique

WP 120: Tools adaptation A) Trace gas column retrieval Retrieval Techniques:

DOAS = Differential Optical Absorption spectroscopy (Platt and Perner 1994) Basic assumptions:

weak trace gas absorption (10-4-10-2) negligible T-dependence of x-sections slow variation in Rayleigh- and aerosol scattering contribution with

condition fulfilled for NO2, OClO, and BrO, but O3 is a strong absorber!

Standard DOAS (Two step retrieval) slant column fit: linear fit to match superpositions of X-sections to observed differential optical

depth conversion to vertical columns via AMF calculation by RTM

Although modified DOAS (Diebel et al. 1996) or weighting function DOAS (Buchwitz et al., 2000) may be more appropriate for ozone, standard DOAS was used in this study for all trace gases as done in the current operational GOME1 retrieval

AMF error is dominated by a-priori assumptions (beyond scope of this assessment), here focus on slant column retrieval


WP 130: Acquisition of Input Data

Spectral fitting windows Recommendation based upon GOME1 experience O3 VIS as option investigated in selected cases

Analysis of data DOAS Algorithm: KVANT (Fortran 90, M. Eisinger) Linear mapping of errors (see WP150)



B) Simulation of GOME2 Spectra

Radiative Transfer Model Full spherical model CDIPI (Combined Differential Integration with Picard Iteration)

(Rozanov et al. 2001) Arbitrary viewing geometries (SZA<98°)

Limb Nadir

Spectral range: 240-2380 nm (SCIAMACHY range) IR: line-by-line, correlated-k Accuracy (UV/VIS): <1% (SZA<90°), <3% (SZA>90°)

Approximate spherical model CDI (Rozanov et al. 2000) Pseudo-spherical source functions, i.e. CDIPI w/o PI Accuracy: <2% in limb geometry above ~35 km tangent height Pseudo-spherical version as option (SCIATRAN/GOMETRAN compatible)

CDI sufficient for non-limb geometry as is the case for GOME2



Comparison between CDI and CDIPI

CDI sufficient for non-limb geometry as is the case for GOME2


Viewing and solar angles in a topocentric coordinate system (tcs) Satellite (SAT) Top of atmosphere (TOA) surface (GRD)

Plan-parallel atmospheres use only one fixed set of angles

Modifications to CDI during this study: BDRF (RPV formalism, ocean glint) Refraction Weighting function


Choices of origins of tcs: SAT, TOA, GRD

Snow BDRF 300-330nmSZA=40°



GOME2 spectral simulation Viewing geometries (ERS Propagator)

Jan, April, July, October Latitudes: 5N, 55N, 75N, 75S High (0.8) and low (0.05) albedo 24 trace gas scenarios

Scan Simulation zenith line-of-sight angles

- 46.5° to 46.5° Scan time = 4.5 sec. Sampling time = 0.01875 sec # of line-of-sights / = 240

IT=0.1875s 10 LOS (24 ground pixels per forward scan)

Table: 24 atmospheric scenarios

Month Latitude

deg SZA Deg

Rel. az. Deg

(east/west)

Albedo %

1 January 5 N 46.5 23.3/156.7 5 2 80 3 55 N 79.8 46.8/133.2 5 4 80 5 April 5 N 36.7 22.0/158.0 5 6 80 7 55 N 49.5 39.6/140.4 5 8 80 9 75 N 65.0 50.4/129.6 5

10 80 11 July 5 N 40.4 40.0/140.0 5 12 80 13 55 N 39.0 31.8/148.2 5 14 80 15 75 N 53.4 48.5/131.5 5 16 80 17 October 5 N 35.8 10.0/170.0 5 18 80 19 55 N 66.3 49.4/130.6 5 20 80 21 75 N 83.4 54.3/125.7 5 22 80 23 75 S 76.0 55.0/125.0 5 24 80



Realistic tracegas scenarios

SLIMCAT 3D CTM stratospheric profiles

Tropospheric modifications

Constant tropospheric O3 number density profile (all)

Biomass burning/ biogenic emission (5N, July) H2CO 2ppb < 3 km (may affect BrO fit) NO2 taken from MPI-2D CTM O3 doubled < 5 km

Free tropospheric BrO (55N, April) BrO 1ppt < 10 km (Fitzenberger et al., 2001)

PBL BrO explosion (75S, October, „ozone hole“) BrO 50ppt < 2 km O3 0ppm < 2 km

MPI 2D

Albedo dependent photochemical activity

BrO, April, 55°N



Comparison between MPI 2D (GOME1 tracegas climatology) to modified SLIMCAT

Ozone hole, 75S, Oct PBL low ozone event

high OClO, 75S, Oct chlorine activation

MPI 2D

MPI 2D as used in GOME1 V2.7 LV2 retrieval is outdatedGOME1 V3 climatology based upon TOMS V7 (O3) and US standard (NO2)


WP150: Overview of column errors

Overview of potential error sources

in trace gas column retrieval

diffuser plate spectral signaturediffuser plate spectral signature Spectral interference pattern

from sanded Al surface in solar irradiance

GOME2 currently uses the same diffuser plate

Errors of 50% and 70% in NO2 and BrO VC density, respectively

Without improving diffuser in GOME2 minor tracegas column retrieval not possible

O3 UV retrieval errors are ~0.3%

Richter and Wagner, 2001



Dichroic features in Channel 3Dichroic features in Channel 3 No specific investigation for GOME2 SCIAMACHY investigation on combined polarisation correction and dichroics

effect on O3 VIS retrieval reported in Appendix of Final Report

GOME1: no reliable O3 VIS retrieval & shift of NO2 fitting window to 425-450nm Different polarisation state measurements and polarisation correction scheme

in SCIAMACHY and GOME2 make a translation of the error to GOME2 difficult Dichroics are reduced in GOME2 as

compared to GOME1

Error in GOME2 polarisation correctionError in GOME2 polarisation correction Direct O3UV fitting of simulated error spectra

provided by SRON

O3 UV column error on the order of 0.3%

Richter and Wagner



Scan mirror degradationScan mirror degradation Different UV degradation rate for

irradiance and nadir spectra after 1999

No systematic trend in GOME1 V2.7 total ozone observed after 1999

Decrease in SNR due to blackening of the mirror effect column retrieval

DOAS retrieval is robust against instrumental degradation, indirect effect due to SNR changes, however, increases retrieval error

Bramstedt et al., 2002

Comparison of GOME1 total ozone with NH Dobson stations



Wavelength calibration errorWavelength calibration error Air-vacuum effect and outgassing

see dichroics radiometric calibration error (wavelength shift in key parameters)

Doppler shift in solar irradiance (0.008 nm) See undersampling error in WP230

DOAS retrieval does not require absolute radiometric calibration, however steep gradients in key parameter (dichroics) and noise due to interpolation (undersampling) error can introduce unwanted spectral artefacts

Wavelength shifts without secondary effects (see above) can be corrected using shift and squeeze to align radiances and x-section spectra

Dichroics are much reduced in GOME2 as compared to GOME1



Error sources investigated in Task 1 and Task 2Error sources investigated in Task 1 and Task 2 SNR (WP 150) baseline error/reference case Spatial Aliassing (WP 210) Spectral Resolution (WP 230)

Open Slit Defocussing Undersampling (interpolation) error

RTM assumption (WP 250) Refraction Pseudo-spherical approximations (SAT, TOA, GRD)

BRDF and ocean glint (WP 260) Geolocation and pointing error (WP 270)

Documents

GOME2 Error Study: Column Retrieval