A two-camera instrument for highly resolved Gas Correlation Spectroscopy measurements of NO2

Imaging of atmospheric trace gases is becoming an increasingly important field of remote sensing. Conventional methods (like imaging-DOAS) typically use dispersive elements and wavelength mapping (at moderate to high spectral resolution) and need intricate optical setup. Therefore, they are limited in spatio-temporal resolution.

Some atmospheric trace gases can, however, be detected only by using a few carefully selected spectral channels, specific to the selected trace gas. These can be filtered using non-dispersive spectral filters without spatial mapping of continuous spectra, vastly increasing the spatio-temporal resolution. This has become a routine in volcanic SO₂ flux analysis, where band-pass filters provide the spectral filtering.

We propose fast imaging of spatial Nitrogen Dioxide (NO₂) distributions employing Gas Correlation Spectroscopy (GCS) in the visible wavelength range. Two spectral channels are used, one with a gas cell that is filled with a high amount of NO₂ in the light path and one without. An additional band-pass filter preselects a wavelength range containing structured and strong NO₂ absorption (e.g. 430 - 450 nm). The NO₂ containing gas cell serves as a NO₂ specific spectral filter, almost blocking the light at wavelengths of the strong NO₂ absorption bands within the preselected wavelength range. Absorption by atmospheric NO₂ has therefore a lower impact on the channel with gas cell compared to the channel without gas cell. This difference is used to generate NO₂ images.

NO₂ plays a major role in urban air pollution, where it is primarily emitted by point sources (power plants, vehicle internal combustion engines), before undergoing chemical conversions. The corresponding spatial gradients can neither be resolved with the established in-situ techniques nor with the widely used DOAS remote sensing method.

Recent advances in the physical implementation of a GCS-based NO₂ camera suggest, that the quality of the measurement may be vastly enhanced in a two-detector (two-camera) set-up. Here, individual cameras are used for the two spectral channels. Not only does this double the photon budget available, but it also allows for synchronized exposure in both channels. This is critical for the quality of the measurement, since dynamic gas or intensity features on time scales smaller than the exposure delay of a one-camera system can induce strong false signals.

A proof of concept measurement was carried out, where test cells with NO₂ column densities ranging from 1E16 to 4E18 molecules cm^-2 were measured both with DOAS and our camera. The results coincided within their uncertainties and allow for camera calibration based on an instrument forward model.