I know developed countries have a very different understanding of the word "clean", but in my city -- which is stuck in the 18th century -- the difference between winter and summer months is extreme. 500-1000 µg/m³ of PM2.5 in winter is the usual deal. 1500-2000 µg/m³ are not unheard of. Yet in summer it's often only 5-10 µg/m³, with spikes of no more than 50 µg/m³ in the evenings due to -- again -- coal burning.

And we have a lot of traffic, regular traffic jams. The average age of a typical car is older than 10 years, according to government data. Most of them are used cars with 100k miles (or more) on them imported from western Europe or the US.

Still, the difference in particulates in summer vs winter is literally hundreds of times.