Nascent sea spray aerosols (SSA) are generated through bursting of air bubbles at the ocean-air interface, and display a significant size-dependent variability in their physicochemical properties such as morphology, composition, phase state, and water uptake. Furthermore, SSA can undergo atmospheric aging by reacting with various gases and oxidants to yield heterogeneously aged SSA (hetSSA) with modified physicochemical properties that may significantly impact their climate-relevant effects. Here, atomic force microscopy (AFM) technique is employed to determine the 3D morphology, mixing states, phase state and water uptake of individual submicrometer SSA and hetSSA as a function of relative humidity (RH), size, and biological activity in seawater. SSA were collected during SeaSCAPE 2019 study over the course of phytoplankton bloom in an isolated ocean-atmosphere facility containing filtered seawater with wave braking ability. To study the effect of atmospheric aging on SSA, a potential aerosol mass oxidation flow reactor (PAM-OFR) was used. Nascent SSA were reacted with higher concentration of OH radicals to produce hetSSA equivalence to 4-5 days of atmospheric aging inside the PAM-OFR. Both SSA and hetSSA were size selected and deposited on substrates, followed by AFM analysis under ambient conditions. Six main morphologies including core-shell, prism-like, rounded, aggregate, rod, and rod-shell were identified, with varying relative abundancy as a function of particle size and seawater biological activity. SSA predominantly had rounded and core-shell morphologies, while a significant increase in the fraction of core-shell particles was observed in hetSSA sample. The phase state of organic coating for hetSSA core-shell relative to SSA was found to become more semisolid and less viscous over the RH range of 20%-60%, while water uptake at 80% RH increases. Overall, the heterogeneous aging on SSA results in changes in the relative abundance of particles morphologies and their phase state, which can significantly influence their climate-relevant processes.

Effects of atmospheric heterogeneous aging on the physicochemical properties of nascent sea spray aerosols

ACS Spring 2022

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