Derivation of Tropospheric NO2 by Synergistic Use of Satellite Observations and Chemical Transport Modelling

Kurzfassung

When deriving the NO2 content of the troposphere from satellite borne spectrometers using DOAS two key problems arise: the quantification of the stratospheric contribution and the calculation of the tropospheric air mass factor (hereafter: AMF). This work aims at combining satellite observations and chemical transport modelling to overcome these problems and to gain accurate tropospheric NO2 columns. To take into account the stratospheric (zonal) variability we derive the stratospheric NO2 column at the instrument’s overpass time by means of a stratospheric CTM (ROSE/DLR) driven by meteorological wind- and temperature fields. To avoid a bias, the resulting stratospheric NO2 analysis is scaled to “clean” observation conditions at a reference sector. The stratospheric NO2 slant column density (hereafter: SCD) is derived by applying a geometric AMF. The tropospheric SCD is then derived by simply subtracting the stratospheric SCD from the total SCD derived from SCIAMACHY observations. In order to consider the variability of tropospheric NO2 and to determine a profile dependent tropospheric AMF, the air quality model EURAD-CTM is applied. The tropospheric AMF is derived by weighting the height dependent air mass factors with the relative NO2 concentrations. This allows computing the air mass factor as a function the forecasted NO2 profile shape. In addition, surface albedo, temperature, cloud fraction and cloud top height are considered. Surface reflectivity and the presence of clouds are taken form satellite observations. In an accompanying study we investigated the stratospheric NO2 content using data assimilation and two different MIPAS observational data sets.