Urban pollution monitoring with the AOTF-based NO ₂ camera: validation with other DOAS instruments

Abstract. Elevated surface concentrations of nitrogen dioxide (NO2) are associated with poor air quality, making its detection and monitoring important for human health and the environment. Existing instruments such as the TROPOMI satellite currently deliver daily global maps of NO2 tropospheric columns, and the future Sentinel-4 instrument will return hourly maps. While areas of strong concentrations (cities, large industries) can be detected in these satellite observations, their spatiotemporal resolution remains too coarse to capture local hot spots and quick variations. In the context of urban air quality monitoring, we present a new type of remote sensing instrument capable of observing spatial and temporal gradients in the NO2 field, which is not currently possible with either space instruments or from the routine operations of conventional diffraction grating and other ground-based remote sensing instruments. This novel instrument is based on an acousto-optical tunable filter (AOTF) located at the heart of a telecentric imaging system. The instrument acquires spectral images in the region 430–455 nm, where NO2 exhibits strong absorption features. A dense spectral sampling was commanded in order to enable the application of the DOAS (differential optical absorption spectroscopy) method in the processing of the spectra measured by each detector pixel. In March 2024, the instrument was deployed at the BAQUNIN supersite for atmospheric research, located in the center of Rome. In order to validate the NO2 camera measurements, coincident acquisitions by a MAX-DOAS and a Pandora spectrometer were performed. The results show very good agreement among the three instruments. They also illustrate the additional capabilities of the NO2 camera in observing the spatial and temporal variability of the urban NO2 field.