Reading the annotations on each of the plots, these are all average NOx concentrations of the labelled year, not point measurements - as you are insinuating. Furthermore, the color scales match up too. (edit) Do note: the more recent plot is zoomed in though and scales are much less clear!
I think it is fair to say that recent policy changes, like rejecting private vehicles from the center and promoting active transportation work. Though aftereffects of COVID (work-from-home) may be a contributor too.
Thunderbird has RSS integrated, which could be quite neat once that synchronizes.