SUPPLEMENTARY INFORMATION Typical measurement sequenceThe nucleation rates (J cm −3 s −1 ) are measured under neutral (J n ), galactic cosmic ray (J gcr ) or charged pion beam (J ch ) conditions. For J gcr a beam stopper blocks the pions and the chamber is irradiated by GCRs together with a small parasitic component of penetrating beam muons, whereas, for J ch , the beam stopper is opened and the pion beam is normally set to a time-averaged rate of (5 − 6) · 10 4 s −1 . Neutral nucleation rates are measured

[1] Field studies show that gas phase nucleation is an important source of new particles in the Earth's atmosphere. However, the mechanism of new particle formation is not known. The predictions of current atmospheric nucleation models are highly uncertain because the models are based on estimates for the thermodynamics of cluster growth. We have measured the thermodynamics for the growth and evaporation of small cluster ions containing H 2 SO 4 and H 2 O, and incorporated these data into a kinetic aerosol model to yield quantitative predictions of the rate of ion-induced nucleation for atmospheric conditions. The model predicts that the binary negative ion H 2 SO 4 /H 2 O mechanism is an efficient source of new particles in the middle and upper troposphere. The ion-induced HSO 4 À /H 2 SO 4 /H 2 O mechanism does explain nucleation events observed in the remote middle troposphere, but does not generally predict the nucleation events observed in the boundary layer.

Light absorption by aerosols is one of the most uncertain parameters associated with the direct and indirect aerosol effects on climate and is one of the most difficult quantities to measure. This article describes the development of a sensitive method of measuring aerosol absorption at 532 nm with excellent time response (detection limit: 0.08 Mm −1 , 60 second average) using photoacoustic absorption spectroscopy. An accurate calibration method (accuracy of 1-2%) at atmospherically relevant absorption levels and independent validation of the photoacoustic technique is presented. An upper limit to the instrument precision for aerosol absorption measurement is ∼6% (2σ , 30 sec) while instrument accuracy is calculated to be ∼5%. A standard for aerosol absorption measurement techniques using well characterized absorbing aerosol is also proposed.

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, 39% OM, and 15% BC and differs from inventories that used 81%, 14%, and 5% and 31%, 63%, and 6% SO 4 2À , OM, and BC, respectively. SO 4 2À and OM mass were found to be dependent on fuel sulfur content as were SSA, hygroscopicity, and CCN concentrations. BC mass was dependent on engine type and combustion efficiency. A plume evolution study conducted on one vessel showed conservation of particle light absorption, decrease in CN > 5 nm, increase in particle hygroscopicity, and an increase in average particle size with distance from emission. These results suggest emission of small nucleation mode particles that subsequently coagulate/condense onto larger BC and OM. This work contributes to an improved understanding of the impacts of ship emissions on climate and air quality and will also assist in determining potential effects of altering fuel standards.