Biogenic particles in the surface microlayer and overlaying atmosphere in the central Arctic Ocean during summer

ABSTRACT Transmission electron microscopy photographs of airborne particles are compared with those of particles found in the surface microlayer of the open water between ice floes in the central Arctic Ocean in summer. The similarity in morphology, physical properties, X‐ray spectra and a chemical reaction of the numerous aggregates and their building blocks predominantly smaller than 70 nm diameter, and of bacteria and other micro‐organisms found in both, strongly suggests that the airborne particles were ejected from the water by bursting bubbles. The shape of the size distribution of aggregates in the air is very similar to that in the water, each with a well‐defined Aitken mode but shifted towards smaller sizes. Diffuse electron‐transparent material joining and surrounding the heat resistant and non‐hygroscopic particulates in both the air and water is shown to have properties consistent with the exopolymer secretions (EPS) of microalgae and bacteria in the water. EPS are highly surface‐active, highly hydrated molecules that can spontaneously assemble into gels. They are broken down by ultraviolet light or acidification. These properties provide an explanation for the different resistance to dehydration of bacteria from air and water samples when subjected to a vacuum, and the apparent absence of sea salt on airborne bacteria and aggregates. The difference in size distribution between the air and water samples is also explained. The role of EPS and particulate matter from the open lead surface microlayer in the production of the airborne Aitken mode particles and cloud condensation nuclei is examined and concluded to be very important.