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SCIAMACHY LIMB-Measurements

NO2

NO2 in the stratosphere is one of the most important trace species because of its large impact on the ozone budget: For the altitude range from 24 to 36 km the NOx-cycle is the major loss mechanism for ozone (Wennberg et al., 1994). And via the coupling with the reaction cycles of hydrogen and halogen compounds, stratospheric NO2 also strongly effects the ozone in the lower stratosphere (Solomon et al., 1999).

NO2 profiles can be determined from the SCIAMACHY limb observations by combining absorption spectroscopy and radiative transfer modelling. The retrieved profiles are applied in our group for studies on stratospheric chemistry, correlation with other trace gas profiles and meteorological parameters, validation studies by balloon measurements, and comparison to results from model calculations. In addition the retrieval of accurate stratospheric profiles is necessary to allow for a correct retrieval of the tropospheric column.

For the profile retrieval we use a two step approach: First, Slant Column Densities (SCDs) of the respective absorber are determined in the UV/VIS spectral range by Differential Optical Absorption Spectroscopy (DOAS). Second, inversion of the retrieved SCDs (as function of tangent height) yields vertical profiles of the trace gas concentration (as function of altitude). For that purpose, we either apply an optimal estimation method (constraining the inversion by a priori information) or invert the measurements by a least squares approach (independently from a priori). For both approaches box air mass factors calculated by the full spherical radiative transfer model McArtim (Deutschmann et al., 2008) are utilized as weighting functions. For a detailed description of the retrieval algorithm please refer to Kühl et al., 2008 and Pukite et al., 2008.


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Figure 1: Example for a DOAS-Analysis of SCIAMACHY limb spectra for NO2. The black curve is the retrieved optical depth of NO2, i.e. the absorption cross section scaled by the SCD retrieved. The red curve is the black curve plus the residual, i.e. noise and structures that are not related to the spectra included in the fit. The example is typical for an analysis between 20 and 30 km.



About 100 spectra (measured at tangent heights from approx. 0 to 150 km with a vertical step of ~ 3 km ) are evaluated to retrieve the profile for a certain location (one SCIAMACHY state).
Figure 2 shows examples for profile retrievals on the 7th of January 2008 for different latitudes: 64° N, 44° N, 15° S and 44° S. The corresponding SCIAMACHY limb measurement states are shown in the left side of the figure. It can be seen that the magnitude of the NO2 number density and also the altitude of the profile peak vary strongly with latitude.

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Figure 2: Examples for retrieved vertical profiles of NO2 number densities for the 7th of January, 2008. The red curve shows the retrieval by optimal estimation constrained with a priori knowledge, the blue curve the profile retrieved by the least squares approach. For comparison, the a priori profile is plotted in green. The geolocation of the respective SCIAMACHY measurements is displayed in the map on the left side, kindly provided by DLR (http://atmos.caf.dlr.de/projects/scops ).


Due the impact of transport and photochemistry the NO2 profile for a given place also changes considerably during the year. The figure below shows the temporal evolution of the NO2 profile, as observed by SCIAMACHY, for the different latitudes:

NO2_longmeans_limb (166K)
Figure 3: NO2 number density as function of altitude and latitude, zonally averaged, for the 15th of March, June, September and December, 2007. The seasonal variability is largest for mid- and high latitudes, while NO2 values at the equator are small throughout the year. The altitude of the peak ranges from 25 km at the poles to about 32 km at the equator. Maximum NO2 concentrations are observed at the summerpole, while the minimum is found during polar spring. Also note that the latitudinal extension of the measurements is determined by the position of the sun in the respective month (approx. 55° S to 80° N in June and 80° S to 65° N in December).