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The QA4ECV (Quality Assurance for Essential Climate Variables) version 1.1 stratospheric and tropospheric NO2 vertical column density (VCD) climate data records (CDRs) from the OMI (Ozone Monitoring Instrument) satellite sensor are validated using NDACC (Network for the Detection of Atmospheric Composition Change) zenithscattered light differential optical absorption spectroscopy (ZSL-DOAS) and multi-axis DOAS (MAX-DOAS) data as a reference. The QA4ECV OMI stratospheric VCDs have a small bias of 0:2 Pmolec:cm-2 (5 % 10 %) and a dispersion of 0.2 to 1 Pmolec:cm-2 with respect to the ZSLDOAS measurements. QA4ECV tropospheric VCD observations from OMI are restricted to near-cloud-free scenes, leading to a negative sampling bias (with respect to the unrestricted scene ensemble) of a few peta molecules per square centimetre (Pmolec:cm-2) up to -10 Pmolec:cm-2 (-40 %) in one extreme high-pollution case. The QA4ECV OMI tropospheric VCD has a negative bias with respect to the MAX-DOAS data (-1 to -4 Pmolec:cm-2), which is a feature also found for the OMI OMNO2 standard data product. The tropospheric VCD discrepancies between satellite measurements and ground-based data greatly exceed the combined measurement uncertainties. Depending on the site, part of the discrepancy can be attributed to a combination of comparison errors (notably horizontal smoothing difference error), measurement/retrieval errors related to clouds and aerosols, and the difference in vertical smoothing and a priori profile assumptions.

International audience; This paper assesses the quality of IASI (Infrared Atmospheric Sounding Interferometer)/Metop-A (IASI-A) and IASI/Metop-B (IASI-B) ozone (O3) products (total and partial O3 columns) retrieved with the Fast Optimal Retrievals on Layers for IASI Ozone (FORLI-O3; v20151001) software for 9 years (2008–July 2017) through an extensive intercomparison and validation exercise using independent observations (satellite, ground-based and ozonesonde). Compared with the previous version of FORLI-O3 (v20140922), several improvements have been introduced in FORLI-O3 v20151001, including absorbance look-up tables recalculated to cover a larger spectral range, with additional numerical corrections. This leads to a change of ∼ 4% in the total ozone column (TOC) product, which is mainly associated with a decrease in the retrieved O3 concentration in the middle stratosphere (above 30hPa/25km). IASI-A and IASI-B TOCs are consistent, with a global mean difference of less than 0.3% for both daytime and nighttime measurements; IASI-A is slightly higher than IASI-B. A global difference of less than 2.4% is found for the tropospheric (TROPO) O3 column product (IASI-A is lower than IASI-B), which is partly due to a temporary issue related to the IASI-A viewing angle in 2015. Our validation shows that IASI-A and IASI-B TOCs are consistent with GOME-2 (Global Ozone Monitoring Experiment-2), Dobson, Brewer, SAOZ (Système d'Analyse par Observation Zénithale) and FTIR (Fourier transform infrared) TOCs, with global mean differences in the range of 0.1%–2% depending on the instruments compared. The worst agreement with UV–vis retrieved TOC (satellite and ground) is found at the southern high latitudes. The IASI-A and ground-based TOC comparison for the period from 2008 to July 2017 shows the long-term stability of IASI-A, with insignificant or small negative drifts of 1%–3%decade−1. The comparison results of IASI-A and IASI-B against smoothed FTIR and ozonesonde partial O3 columns vary with altitude and latitude, with the maximum standard deviation being seen for the 300–150hPa column (20%–40%) due to strong ozone variability and large total retrievals errors. Compared with ozonesonde data, the IASI-A and IASI-B O3 TROPO column (defined as the column between the surface and 300hPa) is positively biased in the high latitudes (4%–5%) and negatively biased in the midlatitudes and tropics (11%–13% and 16%–19%, respectively). The IASI-A-to-ozonesonde TROPO comparison for the period from 2008 to 2016 shows a significant negative drift in the Northern Hemisphere of −8.6±3.4%decade−1, which is also found in the IASI-A-to-FTIR TROPO comparison. When considering the period from 2011 to 2016, the drift value for the TROPO column decreases and becomes statistically insignificant. The observed negative drifts of the IASI-A TROPO O3 product (8%–16%decade−1) over the 2008–2017 period might be taken into consideration when deriving trends from this product and this time period.

International audience; In recent years, many analyses of space- and ground-based data records reported signs or evidence of increasing ozone concentrations in the extrapolar upper stratosphere since the late 1990s. However, the magnitude and significance of the trend estimates vary from one study to another, prompting the ozone research community to further investigate the causes of these differences. A broader consensus has emerged in the past year, placing the positive trend in the upper stratosphere on solid ground and heralding the start of an observation-based exploration of the recovery of stratospheric ozone. More accurate trend estimates are needed to identify the geophysical processes contributing to the recovery and their relative importance. Uncovering seasonal and spatial trend patterns will be key in reaching this objective, not just in the extrapolar upper stratosphere but elsewhere as well.However, at the moment, it remains unclear whether current ozone profile observing systems are able to provide this information. We address this question with an exploration of the capabilities and limitations of current data records in space (limb/occultation sounders) and on the ground (NDACC/GAW/SHADOZ-affiliated sonde, stratospheric lidar and microwave radiometer sites) to infer decadal trends and their vertical, latitudinal and seasonal patterns. We focus on long-term stability, one of the key drivers of the ability to detect trends. We present updated results of a comprehensive analysis that allowed us to quantify the drift of satellite data relative to the ground-based networks (Hubert et al., 2016). In a companion analysis we exploited the satellite data to uncover temporal and spatial inhomogeneities in the ground-based time series, some of which were traced to known changes occurring at different moments across the network. These changes add to the challenge to derive unbiased ozone trends from ground-based observations and they impede our ability to constrain satellite drift to the level required for current and future ozone trend assessments. We conclude that ongoing efforts to homogenise the ground-based data records are essential.

International audience; Monitoring of and research on air quality, stratospheric ozone and climate change require global and long-term observation of the vertical distribution of atmospheric ozone, at ever-improving resolution and accuracy. Global tropospheric and stratospheric ozone profile measurement capabilities from space have therefore improved substantially over the last decades. Being a part of the space segment of the Copernicus Atmosphere and Climate Services that is currently under implementation, the upcoming Sentinel-5 Precursor (S5P) mission with its imaging spectrometer TROPOMI (Tropospheric Monitoring Instrument) is dedicated to the measurement of nadir atmospheric radiance and solar irradiance in the UV-VIS-NIR-SWIR spectral range. Ozone profile and tropospheric ozone column data will be retrieved from these measurements by use of several complementary retrieval methods. The geophysical validation of the enhanced height-resolved ozone data products, as well as support to the continuous evolution of the associated retrieval algorithms, is a key objective of the CHEOPS-5P project, a contributor to the ESA-led S5P Validation Team (S5PVT). This work describes the principles and implementation of the CHEOPS-5P quality assessment (QA) and validation system. The QA/validation methodology relies on the analysis of S5P retrieval diagnostics and on comparisons of S5P data with reference ozone profile measurements. The latter are collected from ozonesonde, stratospheric lidar and tropospheric lidar stations performing network operation in the context of WMO's Global Atmosphere Watch, including the NDACC global and SHADOZ tropical networks. After adaptation of the Multi-TASTE versatile satellite validation environment currently operational in the context of ESA's CCI, EUMETSAT O3M-SAF, and CEOS and SPARC initiatives, a list of S5P data Quality Indicators (QI) will be derived from complementary investigations: (1) data content and information content studies of the S5P data retrievals; (2) traceable preparation of the S5P data and correlative measurements in view of data comparisons (co-location studies, unit and representation conversions, handling of smoothing and sampling issues, independent estimate of tropopause altitude, (sub-)column integration...), with associated error propagation; (3) data comparisons leading to statistical estimates of the systematic bias and random difference between S5P and reference network data as a function of latitude, their cycles, their long-term evolution, and their dependences on influence quantities (e.g., clouds, solar zenith angle, and slant column density); (4) and finally the assessment of compliance with user requirements as formulated, e.g., by Copernicus Atmosphere and Climate services and by GCOS.

International audience; Tropospheric ozone plays a key role in air quality and has a significant impact on the radiation budget of the Earth, both directly and through its chemical influence on other trace gases. Assessments of atmospheric composition change and of associated climate change therefore demand accurate observations of the tropospheric ozone, both on the global and regional scales and both in the longer and shorter term. Such observations have been provided by two series of European nadir-viewing ozone profilers, namely the scattered-light UV-visible spectrometers of the GOME type, launched regularly since 1995 (GOME, SCIAMACHY, OMI, GOME-2 on MetOp-A/B/C, and the upcoming S-5p TROPOMI and Sentinel-5 missions), and the thermal infrared emission sounders of the IASI type, launched regularly since 2006 (IASI on MetOp platforms and IASI-NG on MetOp-SG). In particular, several tro- pospheric ozone data products have been improved and harmonised in the context of the European Space Agency’s Climate Change Initiative (ESA CCI) on ozone (www.esa-ozone-cci.org). To verify their fitness-for-purpose, those tropospheric ozone datasets must undergo a comprehensive quality assessment (QA), including (a) detailed iden- tification of their geographical, vertical and temporal domains of validity, (b) quantification of their potential bias, noise and drift and their dependences on influence quantities, and (c) assessment of the mutual consistency of data from different sounders. For this purpose we have applied to the aforementioned Ozone CCI datasets a versatile QA/validation system developed over years in the context of ESA’s Multi-TASTE and CCI projects, EUMETSAT’s O3M-SAF, and the European Commission’s GEOmon and QA4ECV. For both GOME- and IASI-type climate data records we report on data content studies, information content studies, and comparisons with co-located reference observations from the well established NDACC, SHADOZ, and GAW ozonesonde networks. Dependence of the tropospheric ozone data quality on major influence quantities and perspectives for the future Sentinel missions are discussed.

International audience

International audience; Research addressing air pollution, the maintenance of the stratospheric ozone layer, and global climate change requires global and long-term monitoring of the vertical distribution of atmospheric ozone at ever-improving resolution and accuracy. Global tropospheric and stratospheric ozone profile measurement capabilities from space have therefore improved substantially over the last two decades, among others with new generation hyperspectral instruments measuring backscattered UV-visible sunlight (GOME, SCIAMACHY, OMI, GOME-2) and thermal emission (TES, IASI) at the nadir of the satellite. Enhanced versions of those instruments are now being developed within EU’s Copernicus Earth Observation programme: the UV-visible-NIR TROPOMI instrument on board of the mid-afternoon satellite Sentinel-5P, to be launched in 2016, and both UV-visible and infrared instruments of the GOME and IASI types on board of the geostationary Sentinel-4 and polar orbiting Sentinel-5, to be launched at the end of the decade. Additionally, stringent climate research user requirements like e.g. the Global Climate Observing System (GCOS) targets call for continuous quality assessment and evolution of ozone data and their associated retrieval algorithms over the whole relevant spatial domain, vertical range, and mission lifetime. The fitness-for-purpose of tropospheric ozone column and ozone profile data products must thus be warily verified by means of in-depth QA/validation studies of the satellite data and associated retrieval algorithms before being used in scientific research and operational applications.To that purpose, an extensive validation system has been developed on the heritage of various validation activities, starting in the 1990s with the first GOME ozone profile validations and progressively extending up to the current ESA Multi-TASTE Phase F data evolution and Ozone_cci production of multi-mission climate data records on ozone. Currently this validation system is being further consolidated with metrological traceability practices and with generic QA guidelines established within the FP7 QA4ECV project. The end-to-end approach of this system combines preliminary QA/QC procedures, data content studies, in-depth information content studies, information-content based co-location procedures, data homogenisation, and the more traditional data comparisons with respect to reference measurements acquired by ground-based networks of ozonesondes and lidars (NDACC, SHADOZ, WMO GAW). An OSSE system with detailed metrology of the remote sensing data is thereby used to assess the propagation of errors associated with differences in smoothing and with mismatches in space and time between the various measurements.In this paper we briefly describe the principles and implementation of this QA/validation system, now in pre-operational phase. Through illustrative evaluation activities from ESA’s Ozone_cci project we demonstrate its broad applicability to virtually all ozone (partial) column and profile datasets. We conclude with a perspective on current developments of this system required to address the specific challenges of the upcoming TROPOMI ozone data validation as envisaged in the S5PVT AO project CHEOPS-5p (Validation of Copernicus HEight-resolved Ozone data Products from Sentinel-5P TROPOMI).

International audience; Numerous vertical ozone profile data records collected over the past decades from space-based platforms have the potential to allow the ozone and climate communities to tackle a variety of research questions. A prime topic is the study and documentation of long-term changes in the vertical distribution of atmospheric ozone, as targeted by the recent SPARC/IO3C/IGACO-O3/NDACC Initiative (SI2N) and WMO’s ozone assessment. Such studies typically require data records with documented mutual consistency in terms of bias and long-term stability. Ground-based networks play a pivotal role in evaluating which satellite records comply with end-user requirements and are fit for their purpose. They provide high-quality, independent measurements on a pseudo- global scale from the ground up to the stratosphere.Here, we present an assessment of the long-term stability and mutual consistency of fourteen limb/occultation ozone profile data records, using NDACC/GAW/SHADOZ ozonesonde and NDACC lidar network data as reference standards. We show how a harmonized analysis framework and robust statistical methods allow us to derive reliable estimates of the drift, bias, and short-term variability of each satellite data record. We examine the dependence of these parameters on altitude and, whenever feasible, on latitude and season. The analysis is furthermore performed in four different ozone profile representations, as it turns out that auxiliary data used for unit and representation conversions can impact data quality. We discuss the mutual consistency and compliance of satellite data sets with respect to specific user requirements from GCOS and from climate research groups. We conclude by reflecting on the implication of our results for trend assessments on recently merged ozone profile records (Ozone_CCI, GOZCARDS, SWOOSH, ...)

Over the past 30 years various space-based limb-looking instruments have recorded complementary data on the distribution of ozone in the Earth's atmosphere. The synergistic use of those data records will improve, on the global scale and in the long term, our understanding of interactions between changes in ozone, ultraviolet radiation and climate. Several international efforts were recently launched in this context, like the SPARC/IO3C/IGACO-O3/NDACC Initiative on Past Changes in the Vertical Distribution of Ozone (SI2N), ESA's Climate Change Initiative project on ozone (Ozone_cci), and NASA's Global OZone Chemistry And Related trace gas Data records for the Stratosphere (GOZCARDS). Each ozone vertical profile data record has its particular features determined by the instrument and retrieval characteristics of the measurement system, like e.g. its sampling properties, viewing geometry, spectral range and degradation. Therefore, assessing the mutual consistency and long-term stability of the complementary data records is a prerequisite for the data merging process aimed at by the aforementioned initiatives. In support to the latter, we present such a systematic evaluation of the operational ozone profile data records from the following fourteen limb/occultation profilers: ERBS SAGE-II, UARS HALOE and MLS, SPOT-3 POAM-II and SPOT-4 POAM-III, Odin OSIRIS and SMR, Meteor-3M SAGE-III, Envisat GOMOS, MIPAS and SCIAMACHY, SCISAT-1 ACE-FTS and ACE-MAESTRO, and EOS-Aura MLS. The ground-based networks of ozonesonde and lidar stations affiliated with the Network for the Detection of Atmospheric Composition Change (NDACC) and WMO's Global Atmosphere Watch (GAW) act as a suitable standard transfer of well established quality between the space-based data records. The ground-based transfer enables us to carry out a systematic evaluation of the data quality of each of the fourteen satellite missions using a common comparison methodology. The temporal behavior (long-term drift and annual cycles) of the bias is investigated using robust regression methods. Studies of the vertical and meridian structure of the bias and associated noise yield a threshold altitude below which the satellite data quality degrades rapidly. Our results conclude with a comprehensive overview of the mutual consistency between and the long-term stability of the various limb profilers, and apply directly to the merging activities upon which the different initiatives rely.