Atmospheric oxidation impact on sea spray produced ice nucleating particles

Abstract

The ocean provides a nearly continuous source of certain primary particles emitted as sea spray from bubble bursting that are active as ice nucleating particles. They can impact ice formation, radiative transfer and precipitation processes in regions overlain by supercooled clouds. However, factors affecting their efficacy over time in the marine boundary layer are unresolved. One factor not previously evaluated is the impact of photochemical aging on the ice nucleating ability of emitted particles. We explore this factor for sea spray particles for the first time via measurements of aerosols produced in a wave flume during a mesocosm experiment and using an oxidation flow reactor for exposing particles to oxidation by hydroxyl radicals. It is found that any amount of aging (here from 3 to 8 days equivalent) can often lead to degradation in ice nucleating particles concentrations, by factors of 2 to 5 times. By using an aerosol concentrator to enhance collection of ice nucleating particles (INPs) from a continuous flow chamber for offline analyses by transmission electron microscopy, atomic force microscopy, and Raman spectroscopy, compositional and morphological analyses of particles on different days suggested that so-called core-shell type sea salt INPs are particularly sensitive to oxidation. Raman libraries of the compositions of sea spray produced INPs were augmented herein, but a comprehensive understanding of the large range of INP compositions active at different temperatures remains elusive. Losses of particles with diameters above 2 μm in the flow reactor prevented full assessment of oxidation impacts at those sizes, and these losses secondarily confirmed supermicron sea spray as a predominant INP size class. While mechanisms for oxidative losses remain to be resolved, results imply that these reduce the ice nucleating activity of sea spray produced INPs by an equivalent temperature depression of ∼2 K during times of solar insolation over oceans.