the aotf-based no₂ camera

The abundance of NO₂ in the boundary layer relates to air quality and pollution source monitoring. Observing the spatiotemporal distribution of NO₂ above well-delimited (flue gas stacks, volcanoes, ships) or more extended sources (cities) allows for applications such as monitoring emission fluxes or studying the plume dynamic chemistry and its transport. So far, most attempts to map the NO₂ field from the ground have been made with visible-light scanning grating spectrometers. Benefiting from a high retrieval accuracy, they only achieve a relatively low spatiotemporal resolution that hampers the detection of dynamic features.

We present a new type of passive remote sensing instrument aiming at the measurement of the 2-D distributions of NO₂ slant column densities (SCDs) with a high spatiotemporal resolution. The measurement principle has strong similarities with the popular filter-based SO₂ camera as it relies on spectral images taken at wavelengths where the molecule absorption cross section is different. Contrary to the SO₂ camera, the spectral selection is performed by an acousto-optical tunable filter (AOTF) capable of resolving the target molecule's spectral features.

The NO₂ camera capabilities are demonstrated by imaging the NO₂ abundance in the plume of a coal-fired power plant. During this experiment, the 2-D distribution of the NO₂ SCD was retrieved with a temporal resolution of 3 min and a spatial sampling of 50 cm (over a 250 × 250 m² area). The detection limit was close to 5 × 10¹⁶ molecules cm⁻², with a maximum detected SCD of 4 × 10¹⁷ molecules cm⁻². Illustrating the added value of the NO₂ camera measurements, the data reveal the dynamics of the NO to NO₂ conversion in the early plume with an unprecedent resolution: from its release in the air, and for 100 m upwards, the observed NO₂ plume concentration increased at a rate of 0.75–1.25 g s⁻¹. In joint campaigns with SO₂ cameras, the NO₂ camera could also help in removing the bias introduced by the NO₂ interference with the SO₂ spectrum.