AOTF-based NO2 camera: CINDI-3 results

In recent years, a novel type of passive remote sensing instrument has been under development at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB). The aim of this instrument, known as the NO2 camera, is to improve the spatio-temporal resolution of ground-based-observed NO2 slant column densities which are usually measured by azimuth and elevation scanning spectrometers. As a result, NO2 emitted in polluted environments can be better characterized. The working principle of this so-called NO2 camera is based on an acousto-optical tunable filter (AOTF), a crystal whose lattice is modulated by a propagating acoustic wave. The elasto-optic effect taking place in the crystal allows the selection of any single wavelength from the incoming light bundle with a spectral resolution between 0.6 and 0.8 nm. Within this imaging optical setup, the AOTF records spectral images of a scene (in this case the atmosphere) in successive wavelengths. For our instrument, a crystal operating in the 435 to 455 nm band was selected. The resulting hyperspectral images are then processed using the DOAS technique.Between May 21 and June 24, 2024, the BIRA-IASB NO2 camera participated in the Third Cabauw INtercomparison of DOAS-type Instruments (CINDI-3), providing an opportunity to benchmark its performance against state-of-the-art instruments. During the campaign, the camera operated in close synchronization with other participating DOAS-type instruments. Additionally, outside of the prescribed campaign protocol, the camera sampled the NO2 field in the direction of Rotterdam and Utrecht. We present the first results of these measurements, discuss their performance and highlight preliminary findings.