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We compare tropospheric column densities (verticallyintegrated concentrations) of NO<sub>2</sub> from three data sets for themetropolitan area of Paris during two extensive measurement campaigns (25 daysin summer 2009 and 29 days in winter 2010) within the European researchproject MEGAPOLI. The selected data sets comprise a regional chemicaltransport model (CHIMERE) as well as two observational data sets: ground-based mobile Multi-AXis-Differential Optical Absorption Spectroscopy(car-MAX-DOAS) measurements and satellite measurements from the OzoneMonitoring Instrument (OMI). On most days, car-MAX-DOAS measurements werecarried out along large circles (diameter ~ 35 km) aroundParis. The car-MAX-DOAS results are compared to coincident data from CHIMEREand OMI. All three data sets have their specific strengths and weaknesses,especially with respect to their spatiotemporal resolution and coverage aswell as their uncertainties. Thus we compare them in two different ways:first, we simply consider the original data sets. Second, we comparemodified versions making synergistic use of the complementary informationfrom different data sets. For example, profile information from the regionalmodel is used to improve the satellite data, observations of the horizontaltrace gas distribution are used to adjust the respective spatial patterns ofthe model simulations, or the model is used as a transfer tool to bridge thespatial scales between car-MAX-DOAS and satellite observations. Using themodified versions of the data sets, the comparison results substantiallyimprove compared to the original versions. In general, good agreementbetween the data sets is found outside the emission plume, but inside theemission plumes the tropospheric NO<sub>2</sub> vertical column densities (VCDs). are systematicallyunderestimated by the CHIMERE model and the satellite observations (comparedto the car-MAX-DOAS observations). One major result from our study is thatfor satellite validation close to strong emission sources (like power plantsor megacities), detailed information about the intra-pixel heterogeneity isessential. Such information may be gained from simultaneous car-MAX-DOASmeasurements using multiple instruments or by combining (car-) MAX-DOASmeasurements with results from regional model simulations

atmospheric measurement techniques

New concepts for the comparison of tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; column densities derived from car-MAX-DOAS observations, OMI satellite observations and the regional model CHIMERE during two MEGAPOLI campaigns in Paris 2009/10