Improved TROPOMI HCHO Column Validation Using Dual-Scan MAX-DOAS Retrievals

Formaldehyde (HCHO) is the most abundant aldehyde in the atmosphere. HCHO ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements have been performed in Uccle, Belgium during one year and a half from May 2018 to December 2019. The MAX-DOAS instrument was measuring in a dual-scan viewing mode consisting of one scan in elevation in a main azimuthal direction followed by an azimuth scan at a fixed low elevation angle. Aerosols and HCHO vertical profiles are retrieved in the main azimuthal direction by applying a profile inversion algorithm based on the Optimal Estimation Method. With the aid of these vertical profiles and the dual-scan MAX-DOAS retrieval strategy, near-surface HCHO concentrations and columns are retrieved in all directions of the azimuth scan. This ground-based dataset is then used for two main purposes: (1) the characterization of the HCHO horizontal distribution around the station and (2) the validation of HCHO columns measured by the TROPOMI satellite instrument over the Brussels area. This study reveals that (1) the HCHO horizontal distribution differs per season and (2) the use of dual-scan MAX-DOAS measurements improves the TROPOMI HCHO column validation results.