based on my cursory look into this, I think the NOx ‘measurement’ is a secondary derivative of UV-Vis satellite detection of ozone. If that’s the case, then I wouldn’t be surprised if the apparent drop is due to a sharp decline in air traffic.
NOx is chemically reactive so it would need to be constantly generated in order to be maintained in the atmosphere. So if the measurements are an accurate measure, then the absence would indeed indicate that the source of the NOx is not operating. Re-emergence of a steady atmospheric NOx concentration would be a instantaneous indicator of source reactivation.
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