Parked car with idling engine

At about 5.15pm and again a few minutes later, I passed a car parked at the kerb with its engine audibly running. On the second occasion, I asked the driver to turn his engine off and told him that this is a crime because it makes the air around the car so much worse for pedestrians passing by etc. He refused, demanding to know if I were a police officer and clearly not inclined to do anything without a police warning.