HIGH-SPEED CIVIL TRANSPORT IMPACT - ROLE OF SULFATE, NITRIC-ACID TRIHYDRATE, AND ICE AEROSOLS STUDIED WITH A 2-DIMENSIONAL MODEL INCLUDING AEROSOL PHYSICS

Recent assessment studies have shown that heterogeneous chemistry coul d have a significant role on the model-predicted ozone changes due to gas injection from high-speed civil transport (HSCT) aircraft. One maj or limitation of these numerical experiments was the highly simplified scheme adopted for the aerosol particles and, in particular, the abse nce of any explicit feedback between the gas phase chemistry included in the models and the total aerosol surface density available for hete rogeneous reactions. In this paper we describe a two-dimensional model covering the whole stratosphere and troposphere which includes photoc hemical reactions for the sulfur cycle and a microphysical code for su lfuric acid aerosols. Starting from these particles, the same code pre dicts also the size distribution for nitric acid trihydrate (NAT) and ice aerosols, covering globally a particle radius range between 0.01 m um and about 160 mum. A rather simple scheme is described for nucleati on and condensation processes leading to the formation and growth of N AT and ice particles, still using grid point temperature data taken fr om the zonally averaged climatology of the lower stratosphere. A discu ssion is made of the HSCT impact on ozone adopting different scenarios for the aerosols. Model results for the aerosol size distribution and for the available surface densities appear reasonable when compared t o satellite and balloon measurements and to independent numerical calc ulations. As pointed out also by previous research work and assessment panels, our calculation shows that the ozone sensitivity to HSCT emis sions largely decreases when heterogeneous chemistry is included with respect to a pure gas phase chemistry case. In addition, our results i ndicate that the ozone sensitivity to HSCT emission decreases even mor e when NAT and ice aerosols are present: this is a consequence of the aerosol-induced stratospheric denitrification which makes the residenc e time of the injected odd nitrogen shorter and the relative weight of the NO(x) catalytic cycle smaller. Inclusion of the sulfur dioxide fe edback with the sulfate aerosol surface does not change significantly the ozone depletion in our model simulation, at least in the pure sulf ate case. The additional ozone change due to aircraft injection of SO2 is larger when NAT and ice aerosols are allowed to form, due to the d ecreased ozone sensitivity to NO(x). In this version of the model no d irect aircraft emission of particulate has been included as a possible source for additional condensation nuclei.