Pollution from Nitrogen dioxide (NO2) has plummeted due to the lockdown. NO2 is one of the key pollutants, whose levels regularly breach European and UK health standards. (The other main pollutant is ‘small particulates’ or PM).
Comparing figures for March-May in 2019 and 2020, falls of 35% to 50% have been seen at the sites with the highest pollution. See Stop Heathrow Expansion web site for more information.
General road traffic was much lower in 2020 but aircraft flights and airport-related road traffic fell even more. So there can be no doubt that much of the reduction in air pollution is due to the fall in air traffic.