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 could have a significant role on the model-predicted ozone changes due to gas injection from high-speed civil transport (HSCT) aircraft. One major limitation of these numerical experiments was the highly simplified scheme adopted for the aerosol particles and, in particular, the absence of any explicit feedback between the gas phase chemistry included in the models and the total aerosol surface density available for heterogeneous reactions. In this paper we describe a two-dimensional model covering the whole stratosphere and troposphere which includes photochemical reactions for the sulfur cycle and a microphysical code for sulfuric acid aerosols. Starting from these particles, the same code predicts also the size distribution for nitric acid trihydrate (NAT) and ice aerosols, covering globally a particle radius range between 0.01 mum and about 160 mum. A rather simple scheme is described for nucleation and condensation processes leading to the formation and growth of NAT and ice particles, still using grid point temperature data taken from the zonally averaged climatology of the lower stratosphere. A discussion 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 to satellite and balloon measurements and to independent numerical calculations. As pointed out also by previous research work and assessment panels, our calculation shows that the ozone sensitivity to HSCT emissions largely decreases when heterogeneous chemistry is included with respect to a pure gas phase chemistry case. In addition, our results indicate that the ozone sensitivity to HSCT emission decreases even more when NAT and ice aerosols are present: this is a consequence of the aerosol-induced stratospheric denitrification which makes the residence time of the injected odd nitrogen shorter and the relative weight of the NO(x) catalytic cycle smaller. Inclusion of the sulfur dioxide feedback with the sulfate aerosol surface does not change significantly the ozone depletion in our model simulation, at least in the pure sulfate 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 decreased ozone sensitivity to NO(x). In this version of the model no direct aircraft emission of particulate has been included as a possible source for additional condensation nuclei.