richter_acve_2013_extended_abstract_formatted_131220 OK ANNALS OF GEOPHYSICS, 56, FAST TRACK-1, 2013; 10.4401/AG-6335   1   Validation strategy for satellite observations of tropospheric reactive gases ANDREAS RICHTER*, MARK WEBER*, JOHN P. BURROWS*, JEAN-CHRISTOPHER LAMBERT+, ANNE VAN GIJSEL# *Institute of Environmental Physics, University of Bremen +Institut d'Aéronomie Spatiale de Belgique (IASB) #Royal Netherlands Meteorological Institute (KNMI) richter@iup.physik.uni-bremen.de Abstract Satellite observations of tropospheric reactive gases are an integral part of the earth observing system but require continuous validation by independent measurements. For short-lived tropospheric species, the large variability in space and time results in specific challenges for validation, often combined with the scarcity of appropriate validation data. In this paper, the need for validation is discussed, previous work on validation of satellite observations is briefly reviewed, and the challenges and possible approaches for cur- rent and future validation networks are evaluated. I. INTRODUCTION ver the last two decades, satellite ob- servations of tropospheric composi- tion have become possible using nadir viewing spectrometers operating in the UV, visible, near infrared, and thermal infrared spectral range. Using measurements from instruments such as GOME, SCIAMACHY, OMI, GOME-2, IASI, TES, and MOPITT, global maps of the spatial distribution of many of the most important tropospheric re- active gases including O3, H2O, CO, NO2, SO2, HCHO, CHOCHO and BrO can be re- trieved. These data for the first time provide a global observational view of tropospheric chemistry, and have led to important in- sights into the relevance of trace gas emis- sion sources, the transport and chemical transformation of reactive species in the at- mosphere and the temporal and spatial scales involved. They have also been applied in studies on air pollution, the assignment of emissions sources and strengths and their changes over time [e.g. Martin, 2008, Wagner et al., 2008, Burrows et al., 2011]. With im- proving spatial resolution of the sensors and maturity of the retrieval algorithms, applica- tions to chemical weather forecast and rou- tine air quality monitoring will soon become possible. As any remote sensing observation, satellite data on tropospheric species needs to be val- idated using independent data with known and documented uncertainties, in order to understand and characterise the capabilities and uncertainties of the satellite measurement, assess data quality, and pro- vide users with quality indicators enabling them to judge the fitness of the data for their purpose. For satellite observations of strato- spheric composition, in particular for ozone, validation has been performed on a routine O ANNALS OF GEOPHYSICS, 56, FAST TRACK-1, 2013; 10.4401/AG-6335   2   basis for several decades, and in many cases, both the independent measurements and the methods applied are close to being mature. However, the situation for tropospheric spe- cies in general and in particular for the reac- tive trace gases is not as advanced, and in fact, many of the relevant data products in this field are still poorly validated. In this manuscript, an attempt is made to discuss the challenges and limitations of validating satellite measurements of reactive tropospheric gases, to review the achieve- ments reached so far, to investigate possibili- ties to overcome the current limitations, and to formulate recommendations for a valida- tion strategy for the current and future suite of European tropospheric space sensors. The discussion will mainly focus on data prod- ucts from UV/vis nadir sounders but the concepts are also applicable to tropospheric NIR and TIR data sets II.  THE CHALLENGE When trying to validate satellite observa- tions of tropospheric reactive gases, a num- ber of challenges become apparent that make this a more difficult task than expected. The first characteristic of reactive gases is their large variability in space and time. This is the direct result of their reactivity which leads to a short atmospheric lifetime and this, in combination with often strongly lo- calised sources, results in highly varying at- mospheric fields. This is true for both the horizontal and the vertical direction. For in- stance, species such as NO2 are mainly resid- ing in the boundary layer of polluted regions and will not even be well mixed within this layer. A large spatial variability in combina- tion with atmospheric transport also leads to concentrations rapidly changing in time and as such poses a problem for validation as lo- calised validation measurements are not rep- resentative for larger areas and time differ- ences between satellite and validation meas- urement have to be small to ensure compa- rability. A direct result of the large variability is the presence of strong concentration gradients, which complicate validation as the exact po- sition and time of validation measurement has a large impact on the result. In contrast, the satellite measurements usually average over larger areas, smoothing the gradients and not reproducing the variability of the reference measurement. The short atmospheric lifetime in combina- tion with active photochemistry also leads to diurnal variations of the atmospheric con- centrations of some species such as NO2, of- ten enhanced by diurnal variations in emis- sions for example during rush hour or be- cause of the higher probability of thunder- storms and lightning, in the afternoon. Again, this necessitates a good temporal co- incidence of satellite and validation meas- urements. In contrast to the stratosphere, the spatial distribution of tropospheric species is strongly influenced by the distribution of emission sources such as cities, power plants, biogenic sources, wild fires, sea ice etc., which are very inhomogeneous and of- ten in regions not well accessible. A valida- tion network well representing the variabil- ity of atmospheric conditions therefore has to be wide spread, including also difficult to probe regions such as rain forests, cities or the polar sea ice region. The special characteristics of tropospheric retrievals also have important impacts on the validation needs. In most cases, the sensitiv- ity of the retrieval varies strongly with alti- tude, usually with lowest values close to the surface where validation measurements are often located. As a result, the retrieval de- pends critically on a-priori data including the vertical profiles of the species of interest and of temperature, surface reflectance and emissivity, and also the presence of clouds and aerosols. For proper validation, ideally ANNALS OF GEOPHYSICS, 56, FAST TRACK-1, 2013; 10.4401/AG-6335   3   these input data will also have to be vali- dated in order to be able to decide if any dif- ference observed is linked to the measure- ment itself or to the ancillary data used. An additional challenge is the small signal often obtained for tropospheric species, ei- ther because their abundances are small or because it is difficult to separate the tropo- spheric from the stratospheric signals. In many cases, the validation measurements themselves are also not as accurate and pre- cise for these small signals as one would like, adding the uncertainty of the validation data to that of the satellite measurement. Considering all the above points, an ideal validation measurement for tropospheric species should provide the vertical profile of the species at different times of the day, for all seasons, at good spatial sampling and covering an area typical for a satellite obser- vation. It should also provide a good cover- age of all atmospheric situations, have de- cent observation statistics and sufficient ac- curacy (bias, precision) and cover also the quantities needed as additional input in the retrievals. Unfortunately, the typical valida- tion measurement falls short in one or even many of these aspects, and in some cases, there exists nearly no independent validation data to compare with. III. CURRENT VALIDATION WORK In spite of the difficulties, many studies have been performed validating tropospheric sat- ellite products. These studies have been doc- umented in the literature, for example in the SCIAMACHY book [Gottwald and Bov- ensmann, 2011], the ACCENT-AT2 book on Remote Sensing of Tropospheric Composi- tion from Space [Burrows et al., 2011] and the AURA validation collection [Schoeberl et al., 2008], but also in many individual arti- cles which are too numerous to be referenced here. Arguably the best situation exists for tropo- spheric ozone validation, where data from the ozone sonde network as well as lidar profiles mainly from the ground but also from aircraft and in-situ aircraft measure- ments can be applied [e.g. Verstraeten et al., 2013]. While these data provide vertical pro- files at relatively high frequency and good systematic error and precision, they still lack coverage in the southern hemisphere and at low latitudes, in spite of specific programs like SHADOZ [Thompson et al., 2007] which in recent years have increased the number of sonde stations in the tropics. For CO, in-situ aircraft observations are the main source of validation which is assisted by ground-based Fourier Transform Spec- trometer (FTS) observations providing col- umn data in cloud free conditions [e.g. Em- mons et al., 2009, Kerzenmacher et al., 2012]. Acquired at a limited number of stations set up initially for stratospheric monitoring, these data sets are sparse and usually do not include pollution hot spots or biomass burn- ing areas. Also, regions with challenging re- trieval conditions (dark surfaces) are not covered adequately. However, due to the relatively long lifetime of CO, coincidence criteria do not have to be very strict and sat- ellite data can be linked to validation obser- vations by using backward trajectories. For tropospheric NO2, different validation approaches have been taken. A large number of surface in-situ measurements of NO2 are taken within national and local air quality networks. By using assumptions on the ver- tical distribution of NO2, these data can be converted to tropospheric columns to be used for validation of satellite measurements [e.g. Ordóñez et al., 2006, Boersma et al., 2009]. While providing good statistics at least locally, such comparisons suffer from the large uncertainty introduced by the con- version from surface mixing ratios to col- umns, from the lack of accuracy of the in- struments employed and from sampling is- sues when using extremely localised road side measurements. Tropospheric columns ANNALS OF GEOPHYSICS, 56, FAST TRACK-1, 2013; 10.4401/AG-6335   4   (and coarse vertical profiles) can be meas- ured by Multi-Axis Differential Optical Ab- sorption Spectroscopy (MAX-DOAS) in- struments from the ground, and these data are well suited for validation [e.g. Irie et al., 2008]. However, the number of such stations is small and many of them are located in clean air regions. Although DOAS measure- ments average in the vertical and to a lesser degree also in the horizontal direction, their measurement volume in the troposphere is still much smaller than that of current space instruments. This lack of spatial representa- tiveness and the fairly large uncertainty of individual observations is a problem for validation unless many instruments are dis- tributed over a larger area. The validation of stratospheric columns, that are used to infer the tropospheric column from satellite total column data, can be done using zenith-sky twilight DOAS measurements [e.g. Ionov et al., 2008, Peters et al., 2012]. The spatial vari- ability has been addressed by airborne ob- servations using both in-situ and DOAS re- mote sensing measurements [e.g. Heue et al., 2005, Martin et al., 2004] but these are very limited in number. Recently, the use of car mounted MAX-DOAS has also been demon- strated for validation [Shaiganfar et al., 2011], providing spatial coverage at low cost. Vertical profile information is currently mainly available from dedicated aircraft flights using in-situ instruments, comple- mented by MAX-DOAS which mainly re- solves the lowest layers. For other absorbers such as BrO, SO2, HCHO, CHOCHO and IO, even less valida- tion is available, and it is nearly completely limited to a small number of in-situ observa- tions on the ground and in aircraft and to a few active and passive DOAS and FTS ob- servations [e.g. Vigouroux et al., 2009, Heue et al., 2011, Choi et al., 2012, Dix et al., 2013]. For SO2, a more extensive network of DOAS instruments has been set-up in the EC project NOVAC [Galle et al., 2009] for monitoring volcanic emissions, and this could possibly be used to validate satellite data. IV. STRATEGY FOR THE FUTURE Any strategy for the future has to address the gaps identified in the current validation activities. The requirements for a good vali- dation strategy are simple – continue acquir- ing new data, go to the right places, take the right measurements at the right time, accu- mulate enough data, include validation of ancillary data and facilitate data access. Sev- eral actions can and should be taken to move into this direction. Most importantly, continued operation of existing networks such as the DOAS, MAX- DOAS, lidar, FTS, and ozone sonde networks needs to be secured and maintained. Unfor- tunately, many of the stations in these net- works do not have secure funding and the number of measurements taken is currently declining, further limiting our ability to vali- date tropospheric satellite observations. As many of the stations are in clean air regions as was appropriate for their original purpose of upper atmospheric observations in the context of the Network for the Detection of Atmospheric Composition Change, NDACC (previously known as Network for the De- tection of Stratospheric Change, NDSC) or the Global Atmospheric Watch programme of the World Meteorological Organisation, WMO-GAW, they should be complemented by stations in regions where validation of tropospheric species is needed, for example in pollution hot-spots, biomass burning re- gions, or areas with large biogenic emissions. Also, there is a lack of stations in the South- ern Hemisphere. In some cases, existing networks such as WMO-GAW could be augmented by additional instrumentation such as MAX-DOAS instruments to make them (more) useful for validation while mak- ing use of existing infrastructure and experi- ence. ANNALS OF GEOPHYSICS, 56, FAST TRACK-1, 2013; 10.4401/AG-6335   5   One interesting possibility is the establish- ment of a small number of end-to-end refer- ence sites (or so-called primary sites), dedi- cated to the validation of tropospheric data and the intermediate steps of their produc- tion. These stations should be equipped to provide high quality measurements of all the quantities needed for validation including the input quantities for the retrievals under different conditions. They should be oper- ated having validation in mind by taking year-round measurements at the right times of day and with a perspective of long-term operation. By strategically placing them on different continents (US, Europe, Asia), such stations could be used to validate both Low Earth Orbit (LEO) satellites and the new generation of geostationary (GEO) observa- tories to be launched in the coming years. By ensuring that the same LEO satellites are val- idated by all stations, these validated LEO instruments could serve as transfer stan- dards between the three GEO satellites which do not have overlapping measure- ments. While networks of stations provide good sta- tistic and long-term validation, campaign based validation is essential for more de- tailed analysis. These campaigns should take place in regions close to relevant observa- tions which are not yet validated, for exam- ple regions with large spatial gradients, pol- lution transport, biogenic emissions, ship- ping emissions, biomass burning, lightning, and bromine explosions. Sometimes, ex- periments of opportunity such as emission reductions for Olympic Games [e.g. Mijling et al., 2009] or changes in legislation affecting pollutant emissions [e.g. Kim et al., 2006] can also be performed. Often, such campaigns can combine different aims such as address- ing a science question (e.g. TRANSBROM campaign [Krüger and Quack, 2013]), an in- strument intercomparison (e.g. CINDI [Piters et al., 2012]) or a multi-platform experiment such as DISCOVER-AQ (http://discover- aq.larc.nasa.gov/) with validation, while other campaigns are fully dedicated to vali- dation (e.g. SCIAVALUE [Fix et al., 2005]). However, it is essential to ensure that the needs for validation are addressed from the planning to the execution of measurements and finally the analysis of data. In this con- text, campaigns already planned by other groups can be used for validation purposes by adding instrumentation for example for column measurements or profiling and en- suring that validation aspects are taken into account when designing the measurement programme. In some cases, new developments with po- tential applications for validation can be supported. Examples are the recent construc- tion of NO2 sondes [Sluis et al., 2010] to be used in a similar way as O3 sondes that can fill an important gap in the atmospheric ob- servation system from ground. The Pandora systems of small and flexible remote sensing instruments [Herman et al, 2009] could be used to extend MAX-DOAS networks. Re- cent developments of highly sensitive Cavity Enhanced or Cavity Ring Down Spectros- copy (CRDS) instrumentation promise better detection limits for a number of species, and smaller and cheaper lidar and FTS instru- ments could facilitate larger numbers of ob- servations albeit at reduced accuracy. A more radical approach could integrate a large number of small and cheap sensors, usually based on solid-state detectors for crowd measurements with low precision but excellent sampling statistics. A large potential for validation measure- ments lies in the use of existing platforms such as cars, trains, ships or commercial air- crafts. By mounting small and automated in- struments to these vessels, good spatial cov- erage and statistics can be achieved without having to cover the large costs of vehicle op- eration. The usability of such platforms for validation measurements is limited by the constraints imposed by the platform opera- ANNALS OF GEOPHYSICS, 56, FAST TRACK-1, 2013; 10.4401/AG-6335   6   tors which are often not in line with valida- tion requirements, but as an additional data source, such measurements can provide an important contribution. In addition to existing platforms, unconven- tional platforms can become very useful for validation measurements, for example ultra- light aircraft, unmanned aircraft, zeppelins, tethered balloons or buoys in the ocean and sea ice. All these platforms have the potential to extend the range of validation measure- ments either vertically or to regions not usu- ally accessible by other observations. One approach to validation which is not linked to independent measurements is the use of chemical data assimilation systems to assess the consistency of a satellite data set. As the data assimilation system implements the chemical and dynamical processes in the atmosphere, it can be used to detect internal biases, spatial offsets and temporal changes in the assimilated data sets. For example, degradation in instrument performance lead- ing to bias in a data product can be picked- up in the quality control of a data assimila- tion system. This has been successfully used in the past for CO observations from MOPITT and IASI in the MACC assimilation system, and can potentially be extended to other species. In addition, the use of chemi- cal transport models in general and data as- similation systems in particular can increase the number of co-locations usable for valida- tion as the model effectively interpolates in time and space [e.g. Klonecki et al., 2012]. Such techniques work best for long-lived species which are better constrained by transport and chemistry in the model, limit- ing the applicability of data assimilation for species such as NO2 or BrO. Finally, com- parisons with model data should always be used with care, and can only complement, not replace validation based on real observa- tional data. As important as collecting new data is facili- tating access to existing data. This includes data sets assembled for validation purposes as well as data from campaigns and observa- tional networks. Ideally, all these data sets should be available for validation through a unified portal, providing a uniform interface and data protocol as well as consistent meta- data and formats. Such activities are cur- rently pursued in the context of several ini- tiatives and programs including GEOMS (Generic Earth Observation Metadata Stan- dard, see http://avdc.gsfc.nasa.gov/GEOMS), NDACC (http://www.ndacc.org), NORS (Demonstration Network Of ground-based Remote Sensing Observations in support of the Copernicus Atmospheric Service, see http://nors.aeronomie.be), the ESA and NASA validation data centres (EVDC (http://nadir.nilu.no/calval) and AVDC (http://avdc.gsfc.nasa.gov)), as well as the CEOS Cal/Val Portal (Committee on Earth Observation Satellites, see http://calvalportal.ceos.org/). Finally, an essential part of a validation strategy is also to ensure the conservation of know-how. As space-borne missions and validation projects come and go, the team of validation scientists is constantly changing, and there is a component of periodic rein- vention of the wheel as new generations of scientists encounter the challenges of satellite data validation. It is, therefore, important to provide consistent guidelines to validation groups including information on validation aims, reporting strategies, documentation, fitness for purpose considerations as well as display and interpretation of validation re- sults. In addition, common language and approaches based on metrology should be enforced in validation for the treatment of vertical and horizontal resolution, the treat- ment of time mismatches, the correct use of appropriate terminology and standards, the differentiation between type A and type B errors, and error reporting in general. ANNALS OF GEOPHYSICS, 56, FAST TRACK-1, 2013; 10.4401/AG-6335   7   V. CONCLUSIONS Validation of satellite observations of tropo- spheric reactive trace gases is a challenging task, both because of the intrinsic variability in the atmospheric fields to be observed and because of the peculiarities of remote sensing of the troposphere. It is an on-going activity that needs continuous support for long-term measurements, campaigns, data analysis, and development of new capabilities. For tropospheric species in particular, it is crucial for validation measurements to have good and adaptive spatial coverage, cover- age of the right quantities, and a combina- tion of long-term observations and dedicated campaigns for specific atmospheric events. A multi-tiered approach is needed to im- prove on the current situation where there are by far not enough validation means available for tropospheric species. This can be only overcome by substantial investment in money and time to establish a more tropo- sphere oriented and more comprehensive validations system. Otherwise, we will not have the infrastructure, people, and data needed for tropospheric validation of the many current and upcoming European mis- sions such as GOME-2, IASI, Sentinel-5P, Sentinel-4, and Sentinel-5. VI. 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