Atmospheric Effects of Aviation The impact of aircraft operations on air quality in the local airport environment has long been recognized. In order to minimize this impact, a set of emission standards for aircraft during landing and takeoff has been adopted by the international community. Much less attention has been directed to aircraft emissions at cruise altitudes and there are currently no regulations on this facet of aircraft operation. At cruise altitudes, aircraft emit nitrogen oxides (NOx), which are key participants in ozone photochemistry. Aircraft also emit other chemicals that influence the EarthÕs radiation budget, namely, water, carbon dioxide, and soot/aerosols. Aircraft exhaust emitted at cruise altitudes persists far longer than that emitted near the ground, making cruise emissions an issue on a global scale. Concern over the potential atmospheric impacts of aircraft have been heightened by aircraft industry projections of a substantial increase in air traffic over the next 20 years. In response to this anticipated increase in demand, larger subsonic aircraft are being designed and the groundwork is being laid for a new generation of supersonic passenger planes.
Preliminary estimates indicate that future subsonic and planned supersonic aircraft operations will increase concentrations of nitrogen species in the troposphere and stratosphere by more than 10% and 100%, respectively. The response of ozone to these perturbations is highly uncertain, but present model simulations indicate ozone concentrations will increase in the troposphere and decrease in the stratosphere. Water vapor and sulfur oxide concentrations are expected to increase significantly, perhaps as much as 40%, in the stratosphere. The current best estimate of climatic impacts is that the positive radiative forcing due to the release of NOx from aircraft may be of similar magnitude to the effect of CO2 released from aircraft.
Several major airborne field campaigns, including one directed at sampling the exhaust of a supersonic jet aircraft, have already addressed important aspects of the chemistry associated with aircraft exhaust. However, in order to develop a sound scientific basis for assessing aircraft impacts, a host of issues remain involving the dispersion of exhaust into the atmosphere, possible effects of aircraft contrails on the stability and radiative properties of existing cirrus clouds, and the role of exhaust particulate matter as a catalyst for atmospheric chemical reactions. Information obtained on these issues will allow scientists to make better predictions about the possible environmental impacts of engines that could be used to power high-speed civil transport to be certified in about 2005, and to provide a credible basis for formulation of possible cruise emission standards by the International Civil Aviation Organization.