Tomographic reconstruction of NO2 over Montevideo using ground-based MAX-DOAS observations

MAX-DOAS techniques provide reliable ground-based remote sensing of trace gas abundances by exploiting the spectral absorption signatures of atmospheric compounds in the UV and visible spectral ranges. From the integrated concentration along light paths corresponding to different viewing directions at a single observation site, vertical profiles can be retrieved. When measurements from multiple ground-based locations are combined, tomographic reconstruction techniques can be applied to estimate the spatial distribution of gas concentrations, accounting for horizontal inhomogeneities.

In this work, we combine measurements from two ground-based instruments scanning a vertical plane across a large urban region. Building on previous studies, we apply inversion algorithms that explicitly account for the sparse and inhomogeneous sampling of the vertical plane, allowing retrieval of the best-estimate concentration distribution consistent with the observations, with an effective horizontal resolution on the order of 50 m. At this spatial scale, previously used simplifying assumptions are no longer justified. Instead, we model the gas distribution as the superposition of a smoothly varying vertical background profile and localized fluctuations within the region of interest. Preliminary results are presented for the reconstruction of NO₂ over Montevideo, Uruguay, covering several kilometres in a vertical plane over the city, based on measurements of scattered sunlight acquired between July 2024 and March 2025.