MAX-DOAS measurements of tropospheric NO₂ and HCHO in Munich and the comparison to OMI and TROPOMI satellite observations

<p>We present two-dimensional scanning Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of nitrogen dioxide (<span class="inline-formula">NO₂</span>) and formaldehyde (HCHO) in Munich. Vertical columns and vertical distribution profiles of aerosol extinction coefficient, <span class="inline-formula">NO₂</span> and HCHO are retrieved from the 2D MAX-DOAS observations. The measured surface aerosol extinction coefficients and <span class="inline-formula">NO₂</span> mixing ratios derived from the retrieved profiles are compared to in situ monitoring data, and the surface <span class="inline-formula">NO₂</span> mixing ratios show a good agreement with in situ monitoring data with a Pearson correlation coefficient (<span class="inline-formula"><i>R</i></span>) of 0.91. The aerosol optical depths (AODs) show good agreement as well (<span class="inline-formula"><i>R</i> = 0.80</span>) when compared to sun photometer measurements. Tropospheric vertical column densities (VCDs) of <span class="inline-formula">NO₂</span> and HCHO derived from the MAX-DOAS measurements are also used to validate Ozone Monitoring Instrument (OMI) and TROPOspheric Monitoring Instrument (TROPOMI) satellite observations. Monthly averaged data show a good correlation; however, satellite observations are on average 30&thinsp;% lower than the MAX-DOAS measurements. Furthermore, the MAX-DOAS observations are used to investigate the spatiotemporal characteristic of <span class="inline-formula">NO₂</span> and HCHO in Munich. Analysis of the relations between aerosol, <span class="inline-formula">NO₂</span> and HCHO shows higher aerosol-to-HCHO ratios in winter, which reflects a longer atmospheric lifetime of secondary aerosol and HCHO during winter. The analysis also suggests that secondary aerosol formation is the major source of these aerosols in Munich.</p>