Abstract. We present an approach to reduce topographyrelated errors of vertical tropospheric columns (VTC) of NO 2 retrieved from the Ozone Monitoring Instrument (OMI) in the vicinity of mountainous terrain. This is crucial for reliable estimates of air pollution levels over our particular area of interest, the Alpine region and the adjacent planes, where the Dutch OMI NO 2 product (DOMINO) exhibits significant biases due to the coarse resolution of surface parameters used in the retrieval. Our approach replaces the coarsegridded surface pressures by accurate pixel-average values using a high-resolution topography data set, and scales the a priori NO 2 profiles accordingly. NO 2 VTC reprocessed in this way for the period 2006-2007 suggest that NO 2 over the Po Valley in Italy and over the Swiss plateau is underestimated by DOMINO by about 15-20% in winter and 5% in summer under clear-sky conditions (cloud radiance fraction <0.5). A sensitivity analysis shows that these seasonal differences are mainly due to the different a priori NO 2 profile shapes and solar zenith angles in winter and summer. The comparison of NO 2 columns from the original and the enhanced retrieval with corresponding columns deduced from ground-based in situ observations over the Swiss Plateau and the Po Valley illustrates the promise of our new retrieval. It partially reduces the underestimation of the OMI VTCs at polluted sites in winter and fall and generally improves the agreement in terms of slope and correlation at rural stations. It does not solve, however, the issue that the OMI DOMINO product tends to overestimate very low columns observed at rural sites in spring and summer.

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Abstract.A recent development in ground-based remote sensing of atmospheric constituents by UV/visible absorption measurements of scattered light is the simultaneous use of several directions with small elevation angles in addition to the traditional zenith-sky pointing. The different light paths through the atmosphere enable the vertical distribution of some atmospheric absorbers such as NO 2 , BrO or O 3 to be retrieved.In this study, the amount of profile information that can be retrieved from such measurements on aircraft is investigated for the trace gas NO 2 . A Sensitivity study on synthetic data is performed for a combination of four lines of sight (LOS) (0 • (nadir), 88 • , 92 • , and 180 • (zenith)) and three wavelength regions [center wavelengths: 362.5 nm, 437.5 nm, and 485.0 nm]. The method used in this work is a combination of two previously established methods described in Petritoli et al. (2002) and Wang et al. (2004). The investigation presented here demonstrates the potential of this LOS/wavelengths setup to retrieve a significant amount of profile information from airborne multiax is differential optical absorption spectrometer (AMAXDOAS) measurements with a vertical resolution of 3.0 to 4.5 km in the lower troposphere and 2.0 to 3.5 km near flight altitude. Above 13 km the profile information content of AMAXDOAS measurements is sparse. The retrieval algorithm used in this work is the AMAXDOAS profile retrievalalgorithm (APROVAL).Further, retrieved profiles with a significant amount (up to 3.2 ppbv) of NO 2 in the boundary layer over the Po-valley (Italy) are presented. Airborne multiaxis measurements are thus a promising tool for atmospheric studies in the troposphere.

Abstract. The AMAXDOAS instrument is an airborne multi-axis DOAS instrument covering the spectral range from 300 to 600 nm. During one flight of the SCIAVALUE campaign on 19 March 2003, the AMAXDOAS onboard the DLR Falcon detected tropospheric NO 2 over Europe under both cloudy and cloud free conditions. By combining the measurements in nadir and zenith direction, and analysing the spectra in the UV and the visible spectral region, information was derived on where the bulk of the observed NO 2 was located. Vertical columns of up to 5.7×10 16 molec cm −2 were observed close to Frankfurt, with a distribution peaking in the boundary layer and in good agreement with surface measurements. On several occasions, strong tropospheric NO 2 signals were also detected when flying above clouds. The ratio of zenith and nadir measurements indicates that the NO 2 observed was located within the cloud, and assuming the same profile as for the cloud free situation the NO 2 vertical column was estimated to be 5.0 × 10 16 molec cm −2 . The results are relevant for the retrieval of tropospheric NO 2 columns from space-borne instruments in cloudy situations.