Stockholm is built on islands that helps, and the rains are not frequent but heavy enough to clean the city a bit, waste management is good. Thera are also fewer people are active in the center of Stockholm compared to Paris; Stockholm 5 214 pop/km², Paris 20 909 pop/km² (this disregards important factors about how those numbers work, but it's an interesting indication.)

I notice it's worse in cities with extremely dense urban cores, like NYC. There was a distinct smell (not just garbage, but something I basically call 'the NYC smell' in my head) in many parts. Other cities never seemed quite so bad; at least not bad enough I can still remember that smell.

Maybe Stockholm has plenty of wind? Big cities tend to stink. Didn't bother me until my 30s, now I can barely tolerate the smell, not just the fact that it kills us.

But hey, people want cars. Lots of dirty but cheap cars. Almost nobody seems to care. It's just the nature of living in a poor country I guess.