It's not just aircraft that pollute when they're flying.  Airports directly and indirectly help to add to pollution levels with almost everything they do.
 

Airports generate air pollution from a number of sources including on-site power and heating, equipment to service aircraft, on-site vehicles, airport-related traffic on surrounding roads (staff, passengers and freight) as well as aircraft.
 

The main pollutant of concern around airports is nitrogen dioxide (NO2). NO2 is formed by nitrogen oxide (NOx) emissions from surface traffic, aircraft and airport operations.  PM2.5 is also of concern, since particulate emissions from jet exhausts are almost all in this fine fraction.
 

NOx in the lower atmosphere contributes to the production of ozone; ozone in the lower atmosphere is a pollutant, and contributes to global warming. Nitrogen oxides from high-altitude supersonic aircraft are thought to damage the stratospheric ozone layer, the protective layer that filters out harmful radiation from the sun.