Progress in remote sensing of atmospheric composition and more specifically of tropospheric constituents has been significant in the past two decades (see e.g. Burrows et al., 2011). Since the launch of the Global Ozone Monitoring Experiment (GOME, Burrows et al. 1999) in April 1995, the ideas originally developed in Europe and the US in the late seventies (e.g. Noxon 1975; Perner and Platt, 1976; Platt and Perner, 1980) to measure atmospheric trace gases by UV-visible remote-sensing using the Differential Optical Absorption Spectroscopy (DOAS) technique were progressively developed in space experiments as part of a number of successive missions. Currently in operation are the OMI/AURA instrument, OMPS/NPP and the two GOME-2 instruments onboard of the MetOp-A and MetOp-B platforms. These will be followed soon by a third GOME-2 on MetOp-C (2017) and more importantly by Sentinel-5 Precursor (S5P), which will be the first of a series of atmospheric missions to be launched within the European Commission’s Copernicus (former GMES) Programme. With a nominal lifetime of 7 years, S5P will provide global atmospheric data products at an unprecedented horizontal nadir resolution of 3.5x7 km2 and will ensure the necessary continuity between its predecessors and the future Sentinel-4 and -5 series to be launched in the 2020-2021 timeframe. 


The main goal of S5P is to provide the necessary space measurements in support of the development of information services on air quality, climate and the ozone layer such as developed within the Copernicus Atmospheric Monitoring Service (CAMS). To ensure that products delivered by the atmospheric Sentinels match user requirements in terms of accuracy, precision and fitness for purpose, it is essential to establish a strong validation programme relying on an extensive community of expert scientists and well established fiducial reference measurements. 


In contrast to stratospheric ozone and related gases for which dedicated measurement systems and networks have been developed along well-established procedures, the validation of tropospheric reactive trace gases poses a number of additional challenges (see e.g. Piters et al., 2011, Richter et al., 2013). They are characterized by high variability in time and space and, as a result, their distributions are affected by strong gradients and are strongly depend on local emission sources. Also the retrieval of tropospheric species critically depends on a-priory information sources (e.g. the shape of the vertical distribution of the species), which need to be assessed along with the products themselves. In some cases, the tropospheric signal is small either due to the low concentration or to a comparatively large stratospheric signal (e.g. ozone). Since the retrievals are generally complex and rely on several a-priori assumptions, validation approaches must be developed that take into account this complexity. 


A number of different sources of correlative measurements can be used to validate tropospheric data products (Piters et al., 2011). Among them, the Multi-Axis Differential Optical Absorption Spectroscopy (MAXDOAS) is a relatively new approach which has been developed in the years 2000 to provide information on the vertical distribution of trace gases in the boundary-layer and, when combined with twilight zenith-sky measurements, also in the stratosphere (e.g. Hönninger and Platt 2002; Hönninger et al. 2004; Wittrock et al. 2004). The principle of the MAXDOAS technique relies on DOAS observations being performed in different viewing directions, which - through application of suitable inversion techniques - provide vertical profile information on gases and aerosols present in the lower troposphere. This vertical profile information improves the accuracy of the primary measurement quantity (i.e. the tropospheric column), but it also provides highly valuable information for satellite validation and more generally for atmospheric chemistry applications. 


Since, they are based on optical remote-sensing in the UV-Vis region like nadir backscatter satellite measurements, MAXDOAS measurements provide access to most of the products measured by the atmospheric Sentinels, i.e. NO2, HCHO, SO2, ozone, glyoxal, etc. In addition they give access to ancillary information on aerosol extinction profiles that can be used for advanced validation studies. Currently, MAXDOAS measurements are being conducted by various research groups in several places in the world, however except for the recent Pandonia initiative, no coordination has been established yet to harmonize and standardize these measurements in a traceable network configuration. 


The present activity intends to go one step further in the direction of establishing a coordination for MAXDOAS operation and data processing as further described in this proposal.