Intercomparison of OMI NO2 and HCHO

air mass factor calculations: recommendations and best practices

A. Lorente, S. Döerner, A. Hilboll, H. Yu and K. F. Boersma

19th OMI Science Team Meeting, KNMI, De Bilt. 2nd September 2015

What is needed?• Although satellite retrievals have improved over the last decades,

there is still a need to better understand the uncertainties with every retrieval step

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QA of ECV retrieval algorithm

step by step

QA of ECV retrieval algorithm

step by step

QA of complete

ECV record

Provide quality assured long term climate data record

QA of independent

reference data

Harmonise, improve and quality assure independent validation data

Community best practice retrieval algorithm will be applied on NO2 and HCHO data records

Traceability chains available at: www.qa4ecv.eu

What does QA4ECV do?

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Retrieval uncertainty dominated by AMF uncertainties

• Traceability chains of ECV productions• Best practices to facilitate harmonization, evaluation and

demonstration of traceability

What is this work about?

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QA of ECV retrieval

algorithm step by

step

QA of ECV retrieval

algorithm step by

step

Community best practice retrieval algorithm will be applied on NO2 and HCHO data records

Retrieval uncertainty dominated by AMF uncertainties

www.qa4ecv.eu/ecv

What is this presentation about?

• We evaluate different approaches to calculate AMFs by different scientific groups– Altitude dependent AMFs LUT

– Tropospheric total and clear-sky AMFs for specific OMI orbit• Common settings

• Preferred settings

• The objectives are:– Establish the error of forward models for UV/Vis retrievals.

– Test and improve box AMF LUT for HCHO and NO2 for QA4ECV and

TROPOMI.

– Overall measure of uncertainty on AMF calculation

• Recommendations and best practices for AMF calculation in the DOAS NO2 and HCHO algorithm

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AMF LUT

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• Altitude dependent AMFs • at 440nm, with NO2 absorption• at 338nm with HCHO absorption

• Commons settings • Several values of surface albedo and surface height • Wide range of viewing and solar geometry• Clear sky: no aerosols• Rayleigh scattering, O3 absorption.

New NO2 AMF LUT

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• New NO2 LUT that will be used in QA4ECV retrieval algorithm, DOMINO v3 and

TROPOMI.• Significant increase in reference points: chosen according to sensitivity

studies of RTMs and box AMFs.From 13 x 9 x 10 x 25 x 15 x 24…… to… 16 x 11 x 10 x 26 x 14 x 174

Pressure levels 174

Surface albedo 26

NO2 AMF LUT

Correct vertical shapethroughout the atmosphere

Mean relative differences between DAK-SCIATRAN-LIDORT models within 2%

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Statistical noise due to Monte Carlo simulations Higher rel. differences for extreme geometries

HCHO AMF LUT

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• Good agreement between box AMFs, although relative differences are somewhat higher than for NO2

• Comparison of TOA reflectances showed higher differences at 340 nm – Rayleigh scattering dependency with wavelength

Correct vertical shapethroughout the atmosphere

Mean relative differences betweenmodels within 6%

NO2 tropospheric AMFs

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• Common settings with choice for ancillary data from DOMINO V2: • surface albedo (Kleipool et al. 2008)• cloud information from OMI cloud retrieval• temperature profile and NO2 a priori profile (TM4)• terrain height (Global 3km Digital Elevation Model data (DEM_3KM))

• Cloud correction via IPA• Mcl = cloudy AMF; Mcr = clear sky AMF; w = cloud rad. fraction

• Temperature correction

• Four specific pixels:• Pixel I and pixel II: polluted regions

• Southern Beijing, South Korea• Pixel III and IV: clean, remote locations

• Pacific Ocean

Agreement in clean pixels is better (0.2%) than for polluted pixels (~1.4%)

TOTAL TROPOSPHERIC AMFs

NO2 tropospheric AMFs

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• Agreement is the same with and without temperature-Applying temperature correction is more realistic

Pixel I: southern Beijing area

Tropospheric NO2 AMFs: full orbit

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• First results: IASB-BIRA vs. WUR tropospheric AMFs using

common settings

Slope: 1.0087Correlation: 0.9998Points: 55343

Tropospheric NO2 AMFs:

Clear sky vs. total AMFs

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Total AMF Clear sky AMF DifferenceTotal – clear sky AMF

Cloud fraction < 0.2

Tropospheric NO2 AMFs: Clear sky vs. total AMFs

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Higher differences for higher cloud fraction

As a function of cloud fraction:

Total AMFs are higher : albedo effect

Clear sky AMFs are higher: screening effect

As a function of cloud pressure:

Tropospheric NO2 AMFs:

Preferred settings

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• Each group calculates AMFs with their preferred settings• Which are these preferred settings?

RTMsRTMsSurface albedo

Surface albedo

Kleipool et al. 2008Kleipool et al. 2008

MODIS climatology BRDFMODIS climatology BRDF

Cloud correction

Cloud correction

Terrain height

Terrain height

Temperature profile

Temperature profile

DEM 3kmDEM 3km

Cloud information

Cloud information

FRESCO +FRESCO +

O2-O2 retrievalO2-O2 retrieval

DAKDAK

LIDORTLIDORTSCIATRANSCIATRAN

McArtimMcArtimIPAIPA

Cloud screeningCloud screening

A priori profile

A priori profile

TM4TM4

IMAGES CTMIMAGES CTM

ECMWF ERA-interimECMWF ERA-interim

GEOS-ChemGEOS-Chem

ECMWF ERA-interimECMWF ERA-interim

Tropospheric NO2 AMFs:

Preferred settings

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• Each group calculates AMFs with their preferred settings• Project partners : WUR (KNMI), MPIC, IUP-UB, IASB-BIRA• Other international groups have been invited

Round robin exercise to encourage their active participation

Asses similarities and differences between AMF calculation approaches

Already received positive response!!

Summary and conclusions

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• Uncertainty assessment in every step of the retrieval algorithm is needed

• After this comparison exercise, we are able to asses best practices on NO2 and HCHO box AMFs LUT

• Good agreement between box AMFs calculated by different groups (within 2% for NO2, 6% for HCHO)

• Improvement of LUT for current and future missions

• NO2 tropospheric AMFs:

• Recommendations• Ancillary data: reasonable mature products• Include temperature correction

• Other choices for cloud correction (via IPA or cloud screening), aerosol treatment are important (still under investigation, more statistics with more orbits are needed)

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Thank you!

Back up – Reflectances • TOA Reflectance as a function of:

a/ Wavelength b/ cosine of solar zenith angle

Very good agreement between RTMs!19

• Differences are generally within 1% for relevant scenarios• Exception occurs for large SZA

Back up – Reflectances • TOA Reflectance as a function of:

a/ Wavelength b/ cosine of solar zenith angle

Very good agreement between RTMs!20

• Differences are generally within 1% for relevant scenarios• Exception occurs for large SZA

Back - up slides

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REFLECTANCES• Processes that might be causing differences have been investigated:

• GENERAL CONCLUSIONInfluence of all processes Layering

SphericityRayleigh scattering

Intrinsic (light path) differences between RTMs are the most likely cause the higher differences

Back – up slide. Box AMFs at 950 hPa• Differences dependency with different parameters

90% of the retrievalsare done here!

Relative differences within 1.5%

• Minor dependence of box AMFsdifferences with geometry parameters

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