SCIAMACHY tropospheric NO 2 over the Alpine region and importance of pixel surface pressure for the column retrieval

This study evaluates NO2 vertical tropospheric column densities (VTCs) retrieved from measurements of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) above Switzerland and the Alpine region. A clear relationship between a spatially and temporally highly resolved Swiss NOx emission inventory and SCIAMACHY NO2 columns under anticyclonic meteorological conditions supports the general ability of SCIAMACHY to detect sources of NOx pollution in Switzerland. Summertime NOx lifetime estimates derived from this relation agree reasonably with values from literature. A further evaluation of the SCIAMACHY data is based on the comparison with NO2 VTCs retrieved from the Global Ozone Monitoring Experiment (GOME). The annual mean NO2 VTCs calculated from both data sets clearly show the advantage of the improved SCIAMACHY pixel resolution for qualitatively estimating the NOx pollution distribution in a small country such as Switzerland. However, a more quantitative comparison of seasonally averaged NO2 VTCs gives evidence for SCIAMACHY NO2 VTCs being systematically underestimated over the Swiss Plateau during winter. A possible explanation for this problem (not reported in earlier literature) is the use of inaccurate satellite pixel surface pressures derived from coarsely resolved global models in the retrieval. The marked topography in the Alpine region can lead to deviations of several hundred meters between the assumed and the real mean surface height over a pixel. A sensitivity study based on selected clear sky SCIAMACHY NO2 VTCs over the Swiss Plateau and two fixed a priori NO2 profile shapes indicates that inaccurate pixel surface pressures have a considerable effect of up to 40% on the retrieved NO2 columns. For retrievals in the UV-visible spectral range with a decreasing sensitivity towards the earth's surface, this effect is of major importance when the NO2 resides close to the ground, which occurs most pronounced during the winter season.