Can you provide a citation? I live in a city center Warsaw, and co2 outside is similar to anywhere else - around 420-450 max at all times. Perhaps it's due to a good city design with multiple open spaces though.
https://www.researchgate.net/figure/The-maps-of-global-histo...
Older study https://www.sciencedirect.com/science/article/abs/pii/S13522...
... tldr a higher planetary boundary layer over cities can form under various weather conditions. That reduces mixing, and cities are large sources of CO2.
So I can say - I stand corrected, and TIL :)
Anywhere that traps smog should show similar correlations with CO2.