Another outstanding example is Beijing, widely studied and appreciated by UN agencies and other international agencies. About a decade or two ago, Beijing had the dubious distinction of consistently ranked the most polluted city in the world, with PM 2.5 well in excess of 100. Its smog was notorious, driving many multinational companies and diplomatic missions to seriously consider and publicly speak about relocating out of Beijing. Even though China as a whole ranked quite poorly in international pollution rankings, it put in highly focused and major efforts into tackling air pollution in Beijing. It moved all coal-based power plants and industries out of the city, it ensured phasing out of older and more polluting vehicles, and introduced low-Emission Zones in the city where only the cleanest or electric vehicles were allowed, an idea also enforced in London and other European cities. Beijing has been transformed from a car-centred city to what agencies have described as an example of sustainable mobility, expanding urban rail, bicycle and pedestrian mobility. A major afforestation effort was also taken in the northern regions from where recurring dust-storms bringing fine dust into Beijing were curbed. Widespread use of domestic coal-burning stoves was also curbed. As a result of all these measures, Beijing’s air pollution levels have been reduced by almost half its earlier levels, also bringing down pollution in the huge extended tri-city “megapolis” area.

From https://peoplesdemocracy.in/2023/1112_pd/annual-air-pollution-festival

It was really bad at one point but it has improved a lot in the decade. This is because of a concerted effort to limit car proliferation and focus on renewable sources of energy. Here in India on the other hand the air in the cities is nigh toxic and there are no signs of improving. Where I live there were signboards installed that showed the air quality index. Over the past year or so they have stopped showing this index and only use it to show the weather because it used to say “very dangerous almost all the time”.

Edit: this is only Beijing though. I will try to find some more info.