NO2 videos from a measurement at the Großkraftwerk Mannheim

These three videos show image sequences recorded with an NO₂ camera based on gas correlation spectroscopy. The frame rate is 0.2 Hz and the resolution is 0.92 x 0.92 square meters (i.e. one pixel width and height corresponds to 0.92 meters).

The measurement principle goes as follows:

Two cameras are placed behind glas cells, one is empty and one is filled with a high column density of NO₂. If light is scattered into the two cameras and traverses no atmospheric NO₂ on its way, there will be a significant difference in measured intensity due to the attenuation in the cell filled with NO₂. Whereas, if light reaching the instrument traverses a bit of NO₂ in the atmosphere, it will arrive pre-attenuated and the cell will make a smaller difference. Thus, the (logarithmic) intensity ratio can be used as a measure for the atmospheric column density.

The images show such ratios $\ln (I_2/I_1)$, where $I_2$ and $I_1$ are the intensities measured by the pixels of two imaging sensors. A reference image against the zenith sky and dark images and a signal offset are subtracted.

The three videos show differently postprocessed versions of the image sequence:

1. raw-video: Just the logarithmic intensity ratio as explained above.

2. 4-video: The same, but with postprocessing applied in order to reduce the background signal. The top right shows the sequence with a broadband surface fit subtracted from each individual image. The bottom left shows the sequence with the time-minimum of each pixel subtracted. The bottom right shows the combination of both methods.

3. corr-video: Just the bottom right sequence of 4-video.