Can sinusitis be caused by air pollution?

Yes. If the air pollution causes inflammation of the nasal cavity and sinuses... Then yes it can cause sinusitis. It is not a typical cause and if you are having recurrent episodes, I would explore other causative factors.
Absolutely. Any airborne irritant, including allergens and pollutants can trigger a sinus infection. If you live in an area with bad pollution, consider using saline rinses in you nose daily or more often.