When airports track their carbon footprint it is concentrated to sources on the ground airports can control, typically the airport's use of energy. Reducing footprints are generally done by expanding the portfolio of renewable energy sources.  Phoenix Sky Harbor International Airport has a sustainability program in place to reduce airport CO2 emissions.  Measures some airports have taken is electrification of ground support equipment and help airlines become less dependent on the use of fuel driven Auxiliary Power Units (APUs) on the ground by providing preconditioned air for the aircraft cabins at the gates.  Emissions from airport ground operations are modeled for valley airports in periodic emission inventories based on landing and take-off volumes.  Emission inventories are available on the Maricopa County web site. Airport ground operations are classified as a non-road mobile source.