Abstract. The chemical composition of 15–80 nm diameter particles was measured at Mace Head, Ireland, during May 2011 using the TDCIMS (Thermal Desorption Chemical Ionization Mass Spectrometer). Measurable levels of chloride, sodium, and sulfate were present in essentially all collected samples of these particles at this coastal Atlantic site. Organic compounds were rarely detectable, but this was likely an instrumental limitation. Concomitant particle hygroscopicity observations usually showed two main modes, one which contained a large sea salt component and another which was likely dominated by sulfate. There were several occasions lasting from hours to about two days during which 10–60 nm particle number increased dramatically in polar oceanic air. During these events, the sulfate mode increased substantially in number. This observation, along with the presence of very small (<10 nm) particles during the events, suggests that the particles were formed by homogeneous nucleation, followed by subsequent growth by sulfuric acid and potentially other vapors. The frequency of the events and similarity of event particles to background particles suggest that these events are important contributors of nanoparticles in this environment.;
