Quantifying the air quality-CO₂ tradeoff potential for airports

Aircraft movements on the airport surface are responsible for CO₂ emissions that contribute to climate change and other emissions that affect air quality and human health. While the potential for optimizing aircraft surface movements to minimize CO₂ emissions has been assessed, the implications of CO₂ emissions minimization for air quality have not been quantified. In this paper, we identify conditions in which there is a tradeoff between CO₂ emissions and population exposure to O₃ and secondary PM_2.5 - i.e. where decreasing fuel burn (which is directly proportional to CO₂ emissions) results in increased exposure. Fuel burn and emissions are estimated as a function of thrust setting for five common gas turbine engines at 34 US airports. Regional air quality impacts, which are dominated by ozone and secondary PM_2.5, are computed as a function of airport location and time using the adjoint of the GEOS-Chem chemistry-transport model. Tradeoffs between CO₂ emissions and population exposure to PM_2.5 and O₃ occur between 2-18% and 5-60% of the year, respectively, depending on airport location, engine type, and thrust setting. The total duration of tradeoff conditions is 5-12 times longer at maximum thrust operations (typical for takeoff) relative to 4% thrust operations (typical for taxiing). Per kilogram of additional fuel burn at constant thrust setting during tradeoff conditions, reductions in population exposure to PM_2.5 and O₃ are 6-13% and 32-1060% of the annual average (positive) population exposure per kilogram fuel burn, where the ranges encompass the medians over the 34 airports. For fuel burn increases due to thrust increases (i.e. for constant operating time), reductions in both PM_2.5 and O₃ exposure are 1.5-6.4 times larger in magnitude than those due to increasing fuel burn at constant thrust (i.e. increasing operating time). Airports with relatively high population exposure reduction potentials - which occur due to a combination of high duration and magnitude of tradeoff conditions - are identified. Our results are the first to quantify the extent of the tradeoff between CO₂ emissions and air quality impacts at airports. This raises the possibility of reducing the air quality impacts of airports beyond minimizing fuel burn and/or optimizing for minimum net environmental impact.