Calibration/Validation

The Sentinel-5 Precursor (S5P) mission represents the first in a series of atmospheric observing systems within Copernicus. The S5P mission is a single-payload satellite in a low Earth orbit that provides daily global information on concentrations of trace gases and aerosols important for air quality, climate forcing, and the ozone layer.
The payload of the mission is the TROPOspheric Monitoring Instrument TROPOMI, which has been jointly developed by the The Netherlands and ESA, and consists of a spectrometer with spectral bands in the ultraviolet, the visible, the near-infrared and the shortwave infrared.
The S5P mission was launched on the 13^th of October 2017 and has been injected into a near-polar, near sun-synchronous orbit by a ROCKOT launcher. The initial 6 months of in-orbit operation covered spacecraft, TROPOMI and ground segment level commissioning activities (Phase E1). Since the 30^th of April 2018 the instrument is measuring in the nominal operations phase E2, due to last 6.5 years.
We report on the inflight calibration status of TROPOMI as derived during the first year in-flight and on updates to the L1b processor.

Launched on October 13, 2017, ESA’s Sentinel-5 Precursor (Sentinel-5p) is the first atmospheric composition satellite of European Union's Copernicus Earth Observation programme. On board, the UV/VIS/NIR/SWIR spectrometers of TROPOspheric Monitoring Instrument (TROPOMI) measure on the global scale, on a daily basis, and at unprecedented horizontal resolution the atmospheric abundance of species related to air quality, climate forcing, ozone, UV radiation and volcanic hazards: O₃, NO₂, HCHO, SO₂, CO, CH₄, clouds, aerosols...

Developed with joint support from ESA and from The Netherlands, Germany, and Belgium, the Sentinel-5p Mission Performance Centre (MPC) operates the Validation Data Analysis Facility (VDAF), on which is based the routine TROPOMI validation service to ESA, Level-2 data developers, Copernicus services and other data users. Building upon a two-decade heritage of geophysical validation research with precursor instruments (GOME, SCIAMACHY, OMI, GOME-2) and on recent advances in QA practices and validation systems, VDAF has been tailored to the TROPOMI and Copernicus needs, with special emphasis on operational aspects. It ingests Fiducial Reference Measurements (FRM) archived at ESA’s Validation Data Centre (EVDC) and other validation data collected from ground-based monitoring networks (WMO's GO₃OS, NDACC, SHADOZ, TCCON, TOLNET…), and it compares those data to TROPOMI data following community-endorsed protocols in an automated environment.

In this paper we report on the operational geophysical validation of TROPOMI data, with highlights on both the heritage and advances implemented in the MPC VDAF system, and with TROPOMI validation results illustrating the first months of operation to date.

Current facilities in Romania (South of Bucharest in Magurele) include active and passive remote sensing instruments (a multi-wavelength Raman depolarization lidar system, ceilometers, a sun-lunar CIMEL photometer, a double spectrometer PANDORA instrument, a FTIR, microwave radiometer) measuring various properties of aerosols, trace gases and clouds, but also an research aircraft(Swing and in situ cloud measurements), UAVs, and ground-based mobile platforms (trace gases concentrations). In situ instrumentation is dedicated also to aerosol chemical characterization(ACSM, SMPS, PM). Many of the instruments participated in the Quality Assurance programs of various continental networks such as EARLINET, AERONET, MWRNET, ACTRIS.

The Romanian Atmospheric Mobile Observation System-RAMOS is under implementation and will include new cutting-edge airborne and ground based instruments for trace gases monitoring. Due to these technological improvements RAMOS will be able to assess the required confidence of the different data products of various satellite missions (Sentinel-5P, ADM-Aeolus, EarthCARE).

Since 2011 Romanian team have been participated (datasets submitted) in several international campaigns such as CALIPSO-EARLINET CAL-VAL, EMEP winter (Jan.-Feb. 2013), ACTRIS ACSM (Jun. 2012 – May 2013), AROMAT-1 (Sep. 2014) AROMAT-2 (Aug-Sep. 2015 and May 2016), Finokalia PRE-TECT (April 2017). Through the synergy of measurements and algorithms we were able to identify mixture of aerosols including dust advections, long range transported and/or smoke from forest fires and strong anthropogenic contributions and few relevant study cases will be presented.

New state of art instruments (cloud and rain RADAR, scanning wind lidar, new multiwavelength multi-depolarization lidar, atmospheric radiation station) will be added and implemented on a new facility MARS- Magurele center for Atmosphere and Radiation Studies therefore the datasets will continuously expands offering the opportunity for validating satellites aerosol, cloud CCI, winds and radiation retrievals over Eastern Europe.

Efficient use of Earth Observation-EO data relies on multi source data access, interoperability, long-term data preservation, and definition standards. To address the societal benefits and scientific objectives set forth by the satellite atmospheric missions, the downstream services and GEOSS, coordinated development of synergistic ground-based measurements techniques and algorithms are needed.

Today, the technology and the know-how are concentrated in several observation networks (e.g. in Europe AERONET, EARLINET, PANDONIA, etc.), each with its own strategy regarding instruments and data handling procedures. Initiatives to join and harmonize such networks are now ongoing, e.g. ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure) which was included in 2016 in the ESFRI roadmap and is currently in the preparatory phase. ACTRIS is covering ground-based in situ and remote sensing of the short-lived species in the atmosphere.

For the moment, access to ground-based data is granted through the different data centers independently, ACTRIS DC being one of the most complex and comprehensive. In most of the cases, the time delay between the collection of the data and the availability of the data products is of the order of months, as the data processing and submission relies on the individual stations. In the framework of several EU projects, ACTRIS has developed and improving GARRLiC (Generalized Aerosol Retrieval from Radiometer and Lidar Combined data). However, except AERONET, which developed a simple Data synergy tool to display in parallel sunphotometer products, HYSPLIT backtrajectories and MODIS/AQUA images, no synergetic approach has been coherently made up-to-now to link ground-based and satellite observations, and to provide composite data products. Validation of satellite products is therefore a difficult and time-consuming process, supported by research projects. Partially, this is due to the unavailability (in a suitable timeframe) of centralized independent data, but partially is because of the lack of appropriate validation tools, i.e. IT platforms allowing user-defined data mining, processing, re-gridding and visualization.

DIVA project (pilot) is about setting up a hub to collect, handle, archive, and exploit in a synergetic way observational data from ground and space, either for the validation of ESA and Copernicus missions, or for scientific purposes. The backbone of this synergy is GRASP (Generalized Retrieval of Aerosol and Surface Properties). The system is versatile and integrates ground-based (lidar, sun/lunar photometer and spectrometer), satellite and model data, stand-alone and synergetic algorithms for advanced data products, using combined data from different platforms and sensors, as well as innovative data mining and data visualization tools. A prototype software is developed to process combined data in a unified approach, and the software components are integrated in a backend processing infrastructure which may be further used as a platform for remote sensing data validation. The software is developed distributed, but integrated in one platform. Webservices and tools are implemented to integrate, run, monitor, distribute, and visualize the applications.

DIVA approaches the end of its phase 1, including preparation of inputs (lidar, photometer, spectrometer), development of the GRASP prototype, and setup of the processing environment.

Sentinel-5 Precursor (S-5P) is the first of a series of atmospheric chemistry missions within the European Commission’s Copernicus Programme, launched successfully in October 2017. S-5P entered tits operational Phase at the end of April 2018 and provides continuity in the availability of global atmospheric data products between its predecessor missions SCIAMACHY (Envisat) and OMI (AURA) and the future Sentinel-4 and -5 series. S-5P delivers unique data regarding the sources and sinks of trace gases with a focus on the lower Troposphere including the planet boundary layer due to its enhanced spatial, temporal and spectral sampling capabilities as compared to its predecessors.

The S-5P satellite carries a single payload, namely TROPOMI (TROPOspheric Monitoring Instrument) that was jointly developed by The Netherlands and ESA. Covering spectral channels in the UV, visible, near- and short-wave infrared, it measures various key species including tropospheric/stratospheric ozone, NO2, SO2, CO, CH4, CH2O as well as cloud and aerosol parameters.

The geophysical validation and characterization of the TROPOMI Level 1 and Level 2 data products during the phase E2 is conducted by ESA at different levels. The so-called Mission Performance Center carries out the routine validation throughout the mission life-time and rely on the availability of independent data sets for example from ground based measurements or the so-called Fiducial Reference Measurement data sets, as well as the contributions from independent national Validation Teams coordinated by ESA under the Sentinel 5 Precursor Validation Team (S5PVT).

The overall ESA S5P Validation approach during the operational phase E2 will be presented in this paper.

Uncertainty estimates of Earth observation data are considered adequate if they explain the differences found between the results of different measurement systems. While methodologies exist to intercompare datasets of different measurement geometry and vertical resolution, the natural variability along with less than perfect collocations of observations add another component to the differences, which is not explained by the error budgets. We develop a parametrizaton which will allow estimation of the variability-induced uncertainty as a function of the mean spatial and temporal distance of the measurements. To estimate the natural variability fields, high-resolved data from the BASCOE Model are used. The model run is driven by ERA interim analysis data, with an altitude resolution according to ERA interim. 28 gases and temperatures are included in the package. The statistics how the typical vmr-difference increases with spatial/time difference are calculated. This allow to build a regression function of the type Δvmr = f(Δl;Δt).