Emissions from Ships with respect to Their Effects on Clouds

Abstract

Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as "ship tracks." Airborne measurements are used to derive emission factors of SO_2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ~15–89 and ~2–25 g kg^(-1) of fuel burned, respectively. By contrast a steam turbine–powered ship burning high-grade navy distillate fuel had an SO_2 emission factor of ~6 g kg^(-1). Various types of ships, burning both MFO and navy distillate fuel, emitted from ~4 x 10^(15) to 2 x 10^(16) total particles per kilogram of fuel burned (~4 x 10^(15)–1.5 x 10^(16) particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (~0.03–0.05 µm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ~0.02 µm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine–powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis. The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO_2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN. The fluxes of heat and water vapor from ships are estimated to be ~2–22 MW and ~0.5–1.5 kg s^(-1), respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.

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