Aerosol dynamics in near‐field aircraft plumes

Abstract

A numerical model including gas phase HOx, NOx, and SOx chemistry; H2SO4-soot adsorption; binary H2SO4-H20 nucleation; aerosol coagulation; and vapor condensation is used to investigate aerosol formation and growth in near-field aircraft plumes. The plume flow field is treated using the JANNAF standard plume flow field code, SPF-II. Model results are presented for a Mach 2.4 high-speed civil transport at 18 km altitude and 85øN latitude and a subsonic Boeing 707 at 12.2 km, 47øN. The results, based on hydroxyl radical driven oxidation kinetics, indicate that 1-2% of the emitted SO2 is converted to H2SO 4 in the near-field exhaust (1-2 s) and that for typical exhaust SO2 emission indices (=1 g kg-fuel) the plume is supersaturated with respect to both the pure liquid acid and H2SO4/H20 solutions. Classical nucleation theory predicts high levels of small (0.3-0.6 nm radius) H2SO4/H20 embryos. Coagulation and gas-to-particle conversion are followed to provide estimates for the number density of activated soot particles capable of serving as condensation nuclei for contrail formation. Results are presented illustrating the dependence of water condensation on the number density and size distribution of activated exhaust soot nuclei. IntroductionAircraft aerosol emissions in heavily traversed air traffic corridors may provide a major atmospheric perturbation. Concern over the potential effects on radiative properties and ambient ozone distributions has resulted in a number of airborne measurements in aircraft wakes aimed at quantifying the levels of gas phase HO:• (OH, HO2, H20:), NO:• (NO, NO:), and SOx (SO, SO2, SO3, HSO3, H2SO4) species, as well as exhaust aerosols [Ho]•nann and Rosen, 1978; Baurngardner et al., 1991; Arnold et al., 1992; Fahey et al., 1995a, b; Busen and Schumann, 1995]. These measurements have been supplemented by numerical calculations using plume models to track the homogeneous and heterogeneous plume chemistry from the exit plane through the near-field plume and wake-vortex interaction regime [Hoshizaki et al., 1975; Miake-Lye et al., 1993; Kdircher, 1994; Anderson et al., 1996] and atmospheric models to estimate the potential effect on atmospheric chemistry [Weisenstein et al., 1991, 1996; Bekki and Pyle, 1992, 1993; Rodriguez et al., 1994; Xue et al., 1994]. These atmospheric models indicate significant potential atmospheric effects depending to some extent on the gas phase and heterogeneous chemical kinetics in the exhaust plume. The recent twodimensional modeling study of Weisenstein et al. [1996], for example, predicts an increase in the stratospheric aerosol surface area by a factor of 2 due to sulfuric acid nucleation in supersonic high-speed civil transport (HSCT) plumes if 10% of the emitted SO2 is converted to H2SO 4 shortly after emission. In this paper, a numerical model is used to study the kinetic processes governing the formation and growth of aerosols in aircraft plumes. Results are presented for a Mach 2.4 HSCT at effects have been the focus of extensive research. In this regard...

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