Tradeoff between Contrail Reduction and Emissions in United States National Airspace Neil Y. Chen * and Banavar Sridhar † NASA Ames Research Center, Moffett Field, CA 94035-1000 Hok K. Ng ‡ University of California, Santa Cruz, Moffett Field, CA 94035-1000 This paper describes a class of strategies for reducing persistent contrail formation with the capability of trading off between contrails and aircraft induced emissions. The concept of contrail frequency index is defined and used to quantify the contrail activities. The contrail reduction strategies reduce the contrail frequency index by altering aircraft’s cruising altitude with consideration to extra emissions. The strategies use a user-defined factor to trade off between contrail reduction and extra emissions. The analysis shows that contrails can be reduced with extra emissions and without adding congestion to airspace. For a day with high contrail activities, the results show that the maximal contrail reduction strategy can achieve a contrail reduction of 88%. When a trade-off factor is used, the strategy can achieve less contrail reduction while emitting less emissions compared to the maximal contrail reduction strategy. The user-defined trade-off factor provides a flexible way to trade off between contrail reduction and extra emissions. Better understanding of the trade-offs between contrails and emissions and their impact on the climate need to be developed to fully utilize this class of contrail reduction strategies. The strategies provide a starting point for developing operational policies to reduce the impact of aviation on climate. I. Introduction Aircraft induced environmental impact has drawn attention in recent years. 1 The three largest emission impacts include direct emissions of greenhouse gases such as CO 2 , emissions of NOx, and persistent contrails. Contrails are clouds that are visible trails of water vapor made by the exhaust of aircraft engines. Contrails form when a mixture of warm engine exhaust gases and cold ambient air reaches saturation with respect to water, forming liquid drops which quickly freeze. They persist if the aircraft is flying in certain atmospheric conditions. Persistent contrails reduce incoming solar radiation and outgoing thermal radiation in a way that accumulates heat. 2 The global mean contrail cover in 1992 was estimated to double by 2015, and quadruple by 2050 due to an increase in air traffic. 3 Studies suggest that the environmental impact from persistent contrail is estimated to be three to four times, 4 or even ten times 5 larger than the aviation induced emissions. Therefore, methods to reduce aircraft induced persistent contrails are needed to minimize the impact of aviation on climate. Efforts have been made in the past to reduce the persistent contrail formation. Gierens 6 and Noppel 7 reviewed various strategies for contrail avoidance. Mannstein 8 proposed a strategy to reduce the climate impact of contrails significantly by only small changes in individual flight altitude. Campbell 9 presented a methodology to optimally reroute aircraft trajectories to avoid the formation of persistent contrails with the use of mixed integer programming. Both methodologies require onboard contrail detection system and flight rerouting. Fichter 10 showed that the global annual mean contrail coverage could be reduced by downshifting the cruise altitude. Williams 11, 12 proposed strategies for contrail reduction by identifying fixed and varying * Research Aerospace Engineer, Systems Modeling and Optimization branch, MS 210-10, Senior Member. † Senior Scientist for Air Transportation Systems, Aviation Systems Division, MS 210-10, Fellow. ‡ Senior Software Engineer, University Affiliated Research Center, MS 210-8, Member. 1 of 14 American Institute of Aeronautics and Astronautics