Horizontal Gradients of Aerosols and Trace Gases: Insights from MAX-DOAS Measurements in Mainz

This study investigates horizontal gradients of aerosols and trace gases using a rooftop-mounted MAX-DOAS instrument located at the Max Planck Institute for Chemistry in Mainz. Horizontal inhomogeneities, especially in urban areas or near strong emission sources, can significantly affect the accuracy of MAX-DOAS profile retrievals, as these typically assume horizontally homogeneous atmospheric conditions. The experimental setup allows for measurements in two azimuthal directions (possibly extended to 4 azimuth directions), focusing on gradients between opposing directions (180° apart). By analyzing the retrieved differential slant column densities (dSCDs) at low elevation angles horizontal gradients will be identified and quantified. They are compared to the temporal variability of the dSCDs at high elevation angles, taking into account also wind speed and direction. From the MAX-DOAS observations also trace gas and aerosol profiles are retrieved.  Retrievals under conditions of strong and weak gradients are contrasted to assess their influence on the atmospheric profile retrievals. As a result, we give recommendations which situations might be favorable or not favorable for MAX-DOAS profile inversions, particularly relevant in areas influenced by complex emission sources.