Assessment of Potential Impact of Stratospheric Flight on Earth's Ultraviolet Irradiance

Abstract

The UV radiation increase at the earth's surface depends primarily on the magnitude of the emission index of NO X (i.e., NO and NO2 in g of NO 2/kg of fuel) in the engine effluents. In contrast with tropospheric flight experience, the physical factors that produce the UV radiation problem are: 1) Ozone has very strong absorption characteristics in the wavelength region below 320 nm. Therefore, it is one of the most important substances in the atmosphere for determining the level of UV radiation reaching the earth's surface; 2) A large fraction of the unperturbed atmospheric ozone column is stratospheric ozone, which is a trace gas with a concentration relative to stratospheric air of the order of 10~ 6; 3) The residence times of NO X effluents in the stratosphere are very long compared with those in the troposphere, a result that stems from the effect, on vertical transports, of the stratospheric temperature inversion from ozone heating; 4) The catalytic role of NOX in the chemical destruction of ozone is significant. The NOX in the unperturbed stratosphere is also a trace gas with a concentration relative to stratospheric air of the order of 10'9 The stratosphere is the atmospheric layer extending from the tropopause to about 50 km altitude. The average tropopause height is a function of latitude, i.e., it varies from about 16 km at low latitudes to about 8 km at the poles. The formation of ozone is the result of mutual interactions among the solar radiation, photochemistry, chemical kinetics, and dynamics in the stratosphere. Therefore, the assessment of the potential UV radiation increase due to the effect of NO X effluents on ozone requires an understanding of these complex interactions in the stratosphere. In fact, predictions of the potential UV radiation increase from NOX engine effluents can only be based on the use of models of the stratospheric processes, because of the following two factors: 1) Proper simulation of the relevant couplings of stratospheric phenomena in the laboratory is not feasible, because of the large number of degrees of freedom in the complex stratospheric phenomena; and 2) The potential UV radiation increases would take place in a rather distant future after the initial operation of a worldwide fleet of aircraft in the lower stratosphere. The time scale for the UV radiation increase is controlled by a) the advent of a large-scale operation of supersonic aircraft in the lower stratosphere at altitudes between about 17 and 20 km (56-66 kft), and b) the time lag for an equilibrium response of the ozone decrease to an initial and continuous large-scale injection of NOX effluents in the lower stratosphere. The equilibrium response would tend to duplicate the ozone decrease seasonally, assuming that the operation of aircraft in the lower stratosphere remains constant. The time to reach the equilibrium response of the zone decrease is calculated to be of the order of 5-7.5 yrs. Because of this long time lag, even an immediate escalation in the operation of aircraft in the lower stratosphere would not cause an early equilibrium (i.e., maximum) perturbation of the UV radiation at the earth's surface. Only valid models of the couplings among radiative, chemical, and dynamical processes in the stratosphere can, therefore, provide correct and timely identification of potential adverse effects on the biosphere from the operation of aircraft in the lower stratosphere. The importance of this timing is emphasized further by the long time scales that are usually associated with the development and flight qualification of aircraft engine subsystems. Basic requirements for the validation of these models are then as follows: 1) the models must at least simulate accurately the observed latitudinal and monthly variations of the UV radiation at the earth's surface as well as those of the atmospheric ozone column; 2) the models must minimize the use of empirical assumptions based on observed (i.e., unperturbed) conditions, especially for the consideration of