In 2017, a dense haze occured over northern India. The cause of haze invloved involved both production of air pollution and the weather pattern.weather patterns. During this event, some of the record highs of PM2.5 occurred 999 ug/m3 (over 10x various WHO and USEPA standards).
Particles came from crop burning, urban activity, and many other sources.
Likely low-level temperature inversion prevented the pollution from leaving the city.
https://www.nytimes.com/2017/11/07/world/asia/delhi-pollution-gas-chamber.html
Here's the aerosol optical depth over that time period (from NOAA)
Visual satellite during that time.
Mark Goldman