Impact of marine biogeochemistry on the chemical mixing state and cloud forming ability of nascent sea spray aerosol

Abstract

[1] The composition and properties of sea spray aerosol, a major component of the atmosphere, are often controlled by marine biological activity; however, the scope of impacts that ocean chemistry has on the ability for sea spray aerosol to act as cloud condensation nuclei (CCN) is not well understood. In this study, we utilize a mesocosm experiment to investigate the impact of marine biogeochemical processes on the composition and mixing state of sea spray aerosol particles with diameters < 0.2 μm produced by controlled breaking waves in a unique ocean-atmosphere facility. An increase in relative abundance of a distinct, insoluble organic particle type was observed after concentrations of heterotrophic bacteria increased in the seawater, leading to an 86 ± 5% reduction in the hygroscopicity parameter (κ) at 0.2% supersaturation. Aerosol size distributions showed very little change and the submicron organic mass fraction increased by less than 15% throughout the experiment; as such, neither of these typical metrics can explain the observed reduction in hygroscopicity. Predictions of the hygroscopicity parameter that make the common assumption that all particles have the same bulk organic volume fractions lead to overpredictions of CCN concentrations by 25% in these experiments. Importantly, key changes in sea spray aerosol mixing state that ultimately influenced CCN activity were driven by bacteria-mediated alterations to the organic composition of seawater.

Primary marine aerosol emissions from the Mediterranean Sea during pre-bloom and oligotrophic conditions: correlations to seawater chlorophyll &lt;i&gt;a&lt;/i&gt; from a mesocosm study

“…The Mårtensson et al (2003) and Tyree et al (2007) studies using sintered glass filters or pore diffusers report relatively different size distributions compared to those obtained by plunging jet experiments, likely due to the bubble formation processes. Collins et al (2013) observed three lognormal modes in seawater wave channel experiments (∼ 90, 220, 1000 nm), with changing number fractions as described in the above paragraph.…”

“…In recent wave channel experiments with natural seawater, Prather et al (2013) saw the activation diameter augment from 63 to 118 nm after a 5-fold increase in bacteria abundance; the size distributions remained essentially unchanged (as did phytoplankton, chla and total organic carbon (TOC) abundances and concentrations), leading to the notion that a change in the sea spray chemical composition (the number fraction mode) must have affected the activation diameter. During the same campaign, Collins et al (2013) observed the hygroscopicity parameter kappa, κ, reduce by 86± 5 % over the same time period as the bacterial increase.…”

“…3, 4). When augmenting the bacterial abundance in seawater, Collins et al (2013) observed an increased particle fraction of the smallest lognormal mode diameter with no change to the shape or magnitude of the size distribution; this was attributed to the replacement of internally mixed salt/organic particle types by insoluble organic type particles. Previous studies have also indicated changing size distributions or mode number fractions with increasing organic material (Fuentes et al, 2010b;Sellegri et al, 2006).…”

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

“…Single-particle measurements reveal mixtures of soot, dust, and organic carbon, with secondary species such as sulfate, ammonium, nitrate, and oxidized organics (12)(13)(14). Although, in some heavily aged environments, the diversity in particle hygroscopicity is relatively small (15), other studies demonstrate clear impacts of particle mixing state on CCN behavior (16)(17)(18).…”

In situ chemical composition measurement of individual cloud residue particles at a mountain site, southern China

“…Laboratory studies have shown that low-solubility organic particles internally mixed with ammonium sulfate would suppress water uptake of mixed particles and thus might affect CCN activity (Wise et al, 2003;Svenningsson et al, 2006;Sjogren et al, 2007). An overprediction of CCN concentration by up to 35 % was estimated based on particle internal mixingstate assumptions (Medina et al, 2007;Collins et al, 2013). The influence of mixing state on aerosol CCN activity varies depending on the proximity to the pollution plume source and/or photochemical ageing activity (Ervens and Volkamer, 2010).…”