Nitrogen oxides from high‐altitude aircraft: An update of potential effects on ozone

A sensitivity study is carried out using the Lawrence Livermore National Laboratory one‐dimensional and two‐dimensional chemical‐radiative‐transport models of the global atmosphere to examine possible effects of future aircraft NOx emissions to stratospheric ozone. A broad range in magnitude, altitude, and latitude of the assumed NOx emissions is examined. Major findings of this initial study are as follows: (1) Nitrogen oxides from the exhaust gases of stratospheric aircraft can reduce stratospheric ozone on a global basis. These calculated ozone reductions depend strongly on NOx injection altitude and magnitude. (2) The altitude at which NOx emissions cause the largest reduction in the ozone column is about 25 km according to one‐dimensional model calculations and about 28 km according to the two‐dimensional model. (3) For a given altitude and magnitude Of NOx emissions, the two‐dimensional model finds that the reduction of global ozone depends on latitude of the injections, with maximum ozone reduction for tropical injection. For a given injection, the largest ozone column reductions occur in the polar regions. (4) For very large Clx mixing ratios, NOx emissions can increase the ozone column, partially counteracting the ozone reduction caused by Clx. This effect is found only in a highly Clx perturbed stratosphere where ozone is already greatly depleted. (5) Water vapor (without nitric oxide) in the exhaust gives a small ozone reduction; water vapor and nitric oxide together give an ozone reduction less than that calculated for NO alone by factors between 0.85 and 0.97. (6) Calculated global reductions of ozone due to NOx emissions are smaller in the two‐dimensional model than in the one‐dimensional model, by factors that range from 0.66 to 0.86 for injection altitudes between 20 to 34 km. (7) On the basis of an uncertain estimate of fuel consumption by future stratospheric aircraft, calculations for a fleet of 500 aircraft operating at 22 km with NOx emission properties of 1988 subsonic commercial aircraft engines give global ozone reductions of about 20%. There is reason to believe that such nitric oxide emissions may be lowered, perhaps by a factor of 10, by advanced emission reduction technology.