Atmospheric Oxidation Impacts on Sea Spray Produced Ice Nucleating Particles

Abstract:

The ocean provides a nearly continuous source of ice nucleating particles (INPs) through bubble bursting production of sea spray aerosols, which thereby can impact ice formation and precipitation processes in regions overlain by supercooled clouds. However, factors affecting the lifetime of INPs in the marine boundary layer are unresolved. One factor not previously evaluated is the impact of photochemical aging on their ice nucleating ability. Herein we explore this factor via measurements made using wave flume production of sea spray aerosols during a mesocosm experiment, and then exposing particles to atmospheric time-equivalent oxidation by hydroxyl radicals using an oxidation flow reactor. We find that any amount of aging (from 0.5 to 5 days equivalent) can lead to degradation in immersion freezing ice nucleation ability. Impacts are not uniform over the course of the phytoplankton bloom, and vary in different particle size ranges, presumably reflecting differences in the composition of ice nucleating entities distributed across the sea spray particle size distribution. While the origin and complexities behind these impacts remain to be fully resolved, use of two ice nucleation methods with different size ranges of assessment made it is clear that the organic ice nucleating entities in the supermicron size range were more strongly affected by oxidation than were those in the submicron particle size range. Within the submicron size range, the degree of oxidation degradation followed with the proportion of INPs containing sea salt and organics, as inferred by various analytical methods. These results imply that during times of solar insolation over remote oceans, there is likely a constant competition between production and destruction of marine INPs, a factor that requires elaborated study and to be accounted for in atmospheric modeling of this climatically important emission source.