A method for retrieving the spatial distribution of trace gases using measurements of three ground-based MAX-DOAS instruments

Within the project VINDOBONA (VIenna horizontal aNd vertical Distribution OBservations Of Nitrogen dioxide and Aerosols), a method was developed to retrieve the spatial distribution of trace gases using data from three ground based MAX-DOAS instruments and was applied on the example of NO₂. At three different locations in Vienna (Austria) MAX-DOAS instruments were installed performing measurements in the visible and UV spectral range. Currently, each instrument is set up to determine the column densities in different azimuthal directions and low elevation angles within approximately a horizontal plane. The different lines of sight of the three instruments intersect horizontally and can be used to estimate the horizontal spatial distribution of trace gases. With the knowledge of vertical profiles, even the vertical distribution can be estimated using this method. 

The intersections of the different lines of sight define segments along the slant columns for which the mass concentrations can be estimated. Knowledge about the vertical profiles for a chosen trace gas can be used to correct the retrieved trace gas concentration to specific altitudes above ground. Such corrections are also required since the three instruments were set up at different heights above ground, at different altitudes relative to sea level and with different elevation angles of the lowest viewing direction. One open issue for the retrieval process is the terrain in Vienna in combination with the prevailing wind condition that impacts the horizontal and vertical trace gas distribution and make the retrieval challenging.