Article Index

Validation Summary

The Sentinel-5p TROPOMI L2_NO2 data (tropospheric, stratospheric, and total NO2 column) are in good overall agreement with correlative ground-based measurements (NDACC MAX-DOAS, NDACC ZSL-DOAS, and PGN direct-sun DOAS) and with the OMI QA4ECV NO2 satellite data.  Bias estimates meet the mission requirements of <50% for tropospheric NO2 and <10% for stratospheric NO2. For stratospheric NO2, the dispersion estimate meets the mission requirement of <0.5 Pmolec/cm2. For tropospheric Pmolec/cm2 the dispersion estimate exceeds the mission requirement of <0.7 Pmolec/cm2, which can however be attributed partly to comparison errors (irreducible co-location mismatch, and smoothing differences).

 

Product ID Stream Product Bias Dispersion Special features
 L2_NO2 NRTI NO2 troposphere -22% 3.6 Pmolec/cm2 Total NO2 bias varies with column amount:
positive bias over low pollution,
negative bias over high pollution,
with 7 Pmolec/cm2 as a threshold.
NO2 stratosphere -9% 0.4 Pmolec/cm2
NO2 total 0±50% -
OFFL
RPRO
NO2 troposphere -22% 3.6 Pmolec/cm2
NO2 stratosphere -6% 0.3 Pmolec/cm2
NO2 total,low +5% 1 Pmolec/cm2
NO2 total,high -22% 3 Pmolec/cm2

 

Below is an overview of the processor versions for the NRTI, OFFL and RPRO streams.

Product ID  Stream  Version Orbits
L2_NO2        NRTI 01.04.00   16259-current
01.03.02   9159-16258
01.03.01  7999-9158
01.03.00  7519-7999
01.02.02  5931-7517
01.02.00  5336-5929
01.01.00   3947-5333
01.00.02   3745-3946
01.00.01 2955-3364
OFFL 01.04.00 16211-current
01.03.02 8815-16210
01.03.01 7907-8814
01.03.00 7425-7906
01.02.02 5840-7424
01.02.00 5236-5832
RPRO 01.02.02 2836-5235


A detailed description of the method and a comprehensive discussion of validation results can be found in the Sentinel-5p Quarterly Validation Report #11: April 2018 - June 2021

 

Latest Quarterly Validation Report Product info  Current Processor Version
 #11: April 2018 - June 2021  See sentinel.esa.int

  02.02.00


 

The graph below shows up-to-date time series of the difference between TROPOMI and NDACC ZSL-DOAS stratospheric NO2 column.Vertical markers (black solid, dashed, and dotted lines) indicate major events, respectively: the TROPOMI processor switch from version 1.2.2 to 1.3.0 in March 2019, the activation of the finer horizontal resolution of TROPOMI in August 2019, and the processor switch to 1.4.0 in December 2020. (© Update April 6, 2021, of Verhoelst et al., AMT 2021). Stations are ordered by latitude.

NO2 strato mozaic

 


 

The graph below shows up-to-date time series of the difference between TROPOMI and NDACC MAX-DOAS tropospheric NO2 column. Vertical markers (black solid, dashed, and dotted lines) indicate major events, respectively: the TROPOMI processor switch from version 1.2.2 to 1.3.0 in March 2019, the activation of the finer horizontal resolution of TROPOMI in August 2019, and the switch to processor version 1.4.0 in December 2020. (© Update April 6, 2021, of Verhoelst et al., AMT 2021). Stations are ordered by median tropospheric column value.

NO2 tropo mosaic


 

The graph below show up-to-date time series of the difference between TROPOMI and PGN/Pandora total NO2 column data. Vertical markers (black solid, dashed, and dotted lines) indicate major events, respectively: the TROPOMI processor switch from version 1.2.2 to 1.3.0 in March 2019, the activation of the finer horizontal resolution of TROPOMI in August 2019, and the switch to processor version 1.4.0 in December 2020. (© Update April 6, 2021, of Verhoelst et al., AMT 2021). Stations are ordered by median total column.

NO2 total mozaic


The retrieval of Sentinel-5p TROPOMI NO2 data involves the successive determination of three geophysical quantities: the stratospheric, tropospheric and total column amount of NO2.  These three quantities are validated as follows: 

  • The stratospheric NO2 column is compared to ground-based zenith-scattered twilight DOAS measurements (ZSL-DOAS) of the NDACC network.  The zenith-sky twilight geometry increases the ground-based sensitivity to the stratosphere.  To account for the often large difference in solar local time between the satellite (afternoon) and ground-based (twilight) observations, a diurnal cycle correction is applied based on the PSCBOX 1D stacked-box photochemical model initiated by SLIMCAT CTM fields (Errera and Fonteyn, 2001,Hendrick et al., 2004).
  • The tropospheric NO2 column is compared to ground-based multi-axis DOAS measurements (MAX-DOAS).  The multi-axis geometry can increase the ground-based sensitivity to the troposphere.  
  • The total NO2 column is compared to ground-based direct-sun measurements from Pandora spectrometers of the PGN network.  Direct-sun measurements are sensitive to the entire column.

 

Details on the methodology and results are described in Verhoelst, et al. (2021).

Sentinel-5p TROPOMI NO2 data are also compared to alternative retrievals, and to corresponding satellite data from the Aura OMI instrument processed with the QA4ECV community method (Boersma et al., 2018, EC FP7 QA4ECV project).

References

Verhoelst, T. et al., Ground-based validation of the Copernicus Sentinel-5p TROPOMI NO2 measurements with the NDACC ZSL-DOAS, MAX-DOAS and Pandonia global networks, Atmos. Meas. Tech., v14, 2021, p481-510 doi.org/10.5194/amt-14-481-2021.

Errera, Q. and Fonteyn, D. Four-dimensional variational chemical assimilation of CRISTA stratospheric measurements
J. Geophys. Res., 2001 , 106, 12253-12265, doi:10.1029/2001JD900010

Hendrick, F. et al., Retrieval of nitrogen dioxide stratospheric profiles from ground-based zenith-sky UV-visible observations: validation of the technique through correlative comparisons
Atmos. Chem. Phys., 8, 2004, 2091-2106, 10.5194/acp-4-2091-2004


Correlative measurements used as reference for satellite validation are collected from several ground-based monitoring networks contributing to WMO's Global Atmosphere Watch:

  • Zenith-Scattered-Light Differential Optical Absorption Spectroscopy UV-Visible spectrometers (ZSL-DOAS): About 30 ZSL-DOAS instruments perform network operation from the Arctic to the Antarctic, in the framework of the Network for the Detection of Atmospheric COmposition Change (NDACC).
  • Multi-Axis Differential Optical Absorption Spectroscopy UV-Visible spectrometers (MAX-DOAS): Several of those instruments perform network operation in the framework of the Network for the Detection of Atmospheric COmpoisition Change (NDACC).
  • Direct-sun Pandora spectrometers, also based on Differential Optical Absorption Spectroscopy (DOAS), perform network operation in the framework of the Pandonia Global Network (PGN).

networks_ZSLDOAS_MAXDOAS_directsun

Geographical distribution of the UV-visible DOAS spectrometers contributing NO2 correlative measurements to the validation of Sentinel-5p: 26 NDACC ZSL-DOAS (in green), 19 MAX-DOAS (in blue), and 25 PGN instruments (in red). (© Update of Verhoelst et al., 2021.)

Correlative data is obtained via the following channels:

  • ESAs FRM programme and LuftBlick/U. Innsbruck (A. Cede, M. Gebetsberger and M. Tiefengraber) for rapid data delivery from the Pandonia Global Network,
  • the SAOZ_RT processing facility at IPSL/UVSQ/UPMC/CNRS LATMOS (A. Pazmino, A. Bazureau, F. Goutail, and J.-P. Pommereau) for rapid data delivery from the NDACC UV-Vis/SAOZ network,
  • S5PVT AO projects CESAR (ID #28596, PI A. Apituley, KNMI) and NIDFORVAL (ID #28607, PI G. Pinardi, BIRA-IASB) for rapid data delivery from NDACC MAX-DOAS and ZSL-DOAS stations,
  • the NDACC Data Host Facility, mirrored at EVDC and in BIRA-IASB's CORR-2 database for validation purposes,
  • the PGN data archive, mirrored at EVDC and in BIRA-IASB's CORR-2 database for satellite validation purposes,

 


The Sentinel-5p Mission Performance Centre provides an operational validation service relying on the VDAF Automated Validation Server (VDAF-AVS).  This fully automated data analysis system collects correlative measurements from quality-controlled monitoring networks and compares them to Sentinel-5p data using community endorsed protocols.  It outputs a suite of traceable quality indicators enabling users to judge the fitness-for-purpose of the Sentinel-5p data.

The illustration given hereafter shows the automated comparison of Sentinel-5p TROPOMI total NO2 column data with respect to correlative measurements aquired by a Pandora UV/Visible spectrometer of the Pandonia Global Network.  More TROPOMI NO2 validation results are available at http://mpc-vdaf-server.tropomi.eu/no2 .

 

The VDAF Automated Validation Server in brief:

 


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Quarterly Validation Report of the Copernicus Sentinel-5 Precursor Operational Data Products #11: April 2018 - June 2021

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