A discrete-wavelength imaging instrument for the remote sensing of nitrogen dioxide from space at high spatial and temporal resolution

Air pollution is a major environmental issue, causing millions of premature deaths worldwide and billions of pounds in economic losses every year. Nitrogen dioxide (NO2) is a key air pollutant and a good proxy for air quality. Reliable NO2 monitoring is key to understanding and tackling air pollution, and can be achieved at a global scale using satellite remote sensing. However, current and upcoming instruments do not provide the necessary spatio-temporal resolution to resolve fine-scale sources. This thesis explores a novel instrument concept, the High-resolution Anthropogenic Pollution Imager (HAPI), for use in a constellation of small satellites as a low-cost solution to the observational data gap. HAPI has only ten narrow spectral bands and uses a novel Discrete-Wavelength variant of the well-established Differential Optical Absorption Spectroscopy (DW-DOAS). This work evaluates a HAPI airborne demonstrator built at the University of Leicester during the OmniSat-HAPI project. Laboratory, field and flight tests were performed which showed consistently poor retrieval results. An investigation was conducted which confirmed that the cause of the poor retrieval performance was a suboptimal choice of wavelengths for NO2 nadir observations in combination with a DOAS-like method. A new set of wavelengths was identified and found to perform remarkably well. The robustness of DW-DOAS with the new wavelengths was evaluated by means of a sensitivity analysis, based on which new instrument performance requirements were defined for the next iteration of HAPI. While there are a few technical challenges to overcome, the OmniSat-HAPI constellation has the potential to provide NO2 observations at unprecedented spatial and temporal resolution, and would be a valuable complement to the existing air quality monitoring landscape.