A novel approach to imaging NO2 in the atmosphere

Conventional spectroscopic methods have proven to be reliable and of high selectivity by utilizing the characteristic spectral absorption signature of a wide range of atmospheric trace gases, such as NO₂. However, they typically lack the spatio-temporal resolution required to resolve fast processes, such as NO₂ emissions from stacks or other point sources.

We present a novel fast imaging instrument for NO₂: the NO₂ camera based on Gas Correlation Spectroscopy (GCS) in the blue spectral range. Two gas cells (cuvettes) are placed in front of two camera modules. One gas cell is empty, while the other is filled with a high concentration of the target gas (i.e. NO₂). The filled gas cell operates as a non-dispersive spectral filter to the incoming light, maintaining the two-dimensional imaging capability of the sensor arrays. NO₂ images are generated on the basis of the light intensity ratio between the two images in the spectral window between 430 and 445 nm, where the NO₂ absorption cross section has strong spectral structures. We report measurements taken at a large power plant, the Großkraftwerk Mannheim (GKM) in Baden-Württemberg, Germany. NO₂ column densities in the stack plume of a GKM chimney are quantified at a spatio-temporal resolution of 1/12 frames per second (FPS) and 0.92m x 0.92m. A detection limit of 2·10¹⁶ molec cm^-2 was reached. An NO₂ mass flux of (7.4 ± 4.2) kg h^-1 was estimated based on momentary wind speeds obtained from consecutive images using optical flow estimation. By comparison with a well-established model for plume chemistry (Janssen model), we demonstrate that the NO₂/NO_x ratio of stack plumes can be investigated using an imaging instrument like ours. The instrument prototype is highly portable and cost-efficient at building costs of below 2,000 Euro.