AEROSOL-PARTICLE EVOLUTION IN AN AIRCRAFT WAKE - IMPLICATIONS FOR THEHIGH-SPEED CIVIL TRANSPORT FLEET IMPACT ON OZONE

yPrevious calculations of the ozone impact from a fleet of high-speed civil transports (HSCTs) have been carried out by global two-dimension al (2-D) models [Bekki and Pyle, 1993; Pitari et al., 1993] which have not included explicit wake processing of sulfur species. This process ing could be important for the global sulfate aerosol and ozone pertur bations [Weisenstein et al., 1996]. For an HSCT scenario with emission indices of NOx and sulfur equal to 5 and 0.4, respectively, and a cru ise speed of Mach 2.4 [Stolarski and Wesoky, 1993b], the Atmospheric a nd Environmental Research (AER) 2-D model gives 0.50-1.1% as the range of the annually averaged O-3 column depletion at 40 degrees-50 degree s N. This range is determined by the extreme assumption that emitted S O2 is diluted into the global model grid box either as gas or as 10 nm sulfate particles. A hierarchy of models is used here to investigate the impact of processes in the wake on the calculated global ozone res ponse to sulfur emissions by a proposed HSCT fleet. We follow the evol ution of aircraft emissions from the nozzle plane using three numerica l models: the Standard Plume Flowfield-II/Plume Nucleation and Condens ation model (SPF-II/PNC), an AER far wake model incorporating microphy sics of aerosol particles, and the AER global 2-D chemistry-transport model, Particle measurements in the wake of the Concorde [Fahey et al. , 1995a] are used to place constraints on sulfur oxidation processes i n the engine and the near field. To explain the Concorde measurements, we consider cases with different fractions of SO3 (2%, 20%, and 40%) in the sulfur emissions at the nozzle plane and also the possibility o f other unknown heterogeneous or homogeneous oxidation processes for S O2 in the wake. Assuming similar characteristics for the proposed HSCT fleet, the global ozone response is then calculated by the 2-D model. Using the model-calculated wake processing of sulfur emissions under the above assumptions and constrained by the Concorde particle measure ments, the range of annually averaged O-3 column depletion at 40 degre es-50 degrees N is reduced from 0.5-1.1% to 0.75-1.0%. Our analysis sh ows that the global ozone response is more sensitive to the assumed pa rtitioning of sulfur emissions between SO2 and SO3 at the nozzle plane than to the wake dilution rate. Outstanding uncertainties and recomme ndations for further wake-sampling experiments are also discussed.