Aviation and Chemistry and Transport Processes in the Upper Troposphere and Lower Stratosphere

E xhaust emissions from aircraft modify the chemical balance in major flight corridors, with significant direct and indirect impacts on global climate. It is estimated that the number of f lights will double from present-day rates by about 2025 (ICAO 2007). Unless major changes to combustion systems can be implemented, aircraft emissions can also be expected to double by 2025, along with their attendant consequences on the atmosphere. To determine the effects of current aviation operations on the upper troposphere and lower stratosphere (UTLS) and reliably predict future impacts, we must first understand the fundamental chemical and dynamical processes that occur in this region of the atmosphere. This contribution summarizes the main conclusions that emerged out of a group discussion of two white paper reviews of chemistry and transport (Toohey et al. 2008; McConnell et al. 2008) that were presented at the Aviation Climate Change Research Initiative (ACCRI) Science Meeting (held in Virginia Beach, Virginia, on 25–27 February 2008). The most important products of combustion of aircraft fuel (e.g., kerosene) are carbon dioxide (CO2), water vapor (H2O), NOx, soot, and oxides of sulfur (SOx). Through their interaction with infrared and visible light, CO2, H2O, and soot serve to warm or cool the planet. SOx, NOx, and soot alter the nature and radiative properties of particulate matter (aerosols and clouds) or promote formation of new particles by changing the extent of supersaturation through their influence on temperature and water vapor abundances. These three minor products of combustion can also affect the natural abundances of the radiatively important gases ozone (O3) and methane (CH4) or cause the vertical redistribution of H2O and nitric acid (HNO3) via sedimentation of large cirrus cloud particles. Through such alteration of the radiation balance, aircraft emissions influence atmospheric transport, especially between the troposphere and stratosphere. Water is also important for formation of contrails and cirrus, which influence atmospheric composition by providing surfaces for heterogeneous reactions that enhance ozone-depleting forms of the halogens chlorine and bromine. AVIATION AND CHEMISTRY AND TRANSPORT PROCESSES IN THE UPPER TROPOSPHERE AND LOWER STRATOSPHERE