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My guess is manufacturing plants and burning of farm fields.

Reason is i am not seeing other big cities like shanghai/Bombay etc. which would have comparable number of automobiles but smaller cities which are known to have big factories.

In many cities, cars contribute relatively less pollution compared to factories and power plants, but because it’s localised along city streets where people live and work, it can be just as dangerous.

There are multiple sources:

Automobiles (including highly polluting two-stroke rickshaws and motorbikes), coal-fired power plants, industrial emissions, domestic cooking fires, and agricultural burn-offs.

I am also wondering does source of pollution is always local ?

Not always, wind has a huge impact