Possible effects of CO2 increase on the high‐speed civil transport impact on ozone

Abstract

The role of heterogeneous chemistry on the potential impact on ozone of a commercial fleet of high speed civil transport aircraft (HSCT) has been recently studied with assessment models. Here an attempt is made to model the effects of the carbon dioxide increase which is predicted in the future atmosphere when HSCT should be operational. For this purpose we have first used a three‐dimensional model for the radiative and dynamical calculations and then a photochemical two‐dimensional model including an explicit gas‐particle interaction in the process of aerosol formation. The denoxification and denitrification associated with the formation of nitric acid trihydrate (NAT) aerosols is shown to significantly affect the partition of chemical families. The radiative perturbation introduced by the CO2 increase is shown to perturb the stratospheric dynamics in such a way that the lower stratospheric residual circulation is enhanced. This has the effect of reducing by about 15% the stratospheric residence time of odd nitrogen injected by the aircraft, so that the overall perturbation of stratospheric chemistry due to HSCT is mitigated with respect to the reference case in which CO2 is kept at the present level. Another effect is found to be produced by the stratospheric temperature cooling following the CO2 increase. Our model predicts a large enhancement of the surface area density of NAT aerosols in the arctic region, so that the additional denitrification produces a further decrease of the relative role of the NOx catalytic cycle for ozone destruction in the lower stratosphere. For this reason, the ClO and OH decreases associated with the HSCT‐induced NOx increase are found to be dominant in the ozone budget, thus producing a global ozone increase in the case of 500 ppmv CO2 (+0.27% for mach 2.4 and NOx emission index 15). The balance between the different tendencies of ClO, NOx, and OH cycles is found to be closer in the reference case of 335 ppmv CO2, where a small column ozone depletion by HSCT is predicted (−0.53% globally).