Sea spray aerosol organic enrichment, water uptake and surface tension effects

Abstract

<jats:p>Abstract. The aerosol-driven radiative effects on marine low-level cloud represent a large uncertainty in climate simulations, in particular over the Southern Ocean, which is also an important region for sea spray aerosol production. Observations of sea spray aerosol organic enrichment and the resulting impact on water uptake over the remote Southern Hemisphere are scarce, and therefore the region is under-represented in existing parameterisations. The Surface Ocean Aerosol Production (SOAP) voyage was a 23 d voyage which sampled three phytoplankton blooms in the highly productive water of the Chatham Rise, east of New Zealand. In this study we examined the enrichment of organics to nascent sea spray aerosol and the modifications to sea spray aerosol water uptake using in situ chamber measurements of seawater samples taken during the SOAP voyage. Primary marine organics contributed up to 23 % of the sea spray mass for particles with diameter less than approximately 1 µm and up to 79 % of the particle volume for 50 nm diameter sea spray. The composition of the submicron organic fraction was consistent throughout the voyage and was largely composed of a polysaccharide-like component, characterised by very low alkane-to-hydroxyl-concentration ratios of approximately 0.1–0.2. The enrichment of organics was compared to the output from the chlorophyll-a-based sea spray aerosol parameterisation suggested by Gantt et al. (2011) and the OCEANFILMS (Organic Compounds from Ecosystems to Aerosols: Natural Films and Interfaces via Langmuir Molecular Surfactants) models. OCEANFILMS improved on the representation of the organic fraction predicted using chlorophyll a, in particular when the co-adsorption of polysaccharides was included; however, the model still under-predicted the proportion of polysaccharides by an average of 33 %. Nascent 50 nm diameter sea spray aerosol hygroscopic growth factors measured at 90 % relative humidity averaged 1.93±0.08 and did not decrease with increasing sea spray aerosol organic fractions. The observed hygroscopicity was greater than expected from the assumption of full solubility, particularly during the most productive phytoplankton bloom (B1), during which organic fractions were greater than approximately 0.4. The water uptake behaviour observed in this study is consistent with that observed for other measurements of phytoplankton blooms and can be partially attributed to the presence of sea salt hydrates, which lowers the sea spray aerosol hygroscopicity when the organic enrichment is low. The inclusion of surface tension effects only marginally improved the modelled hygroscopicity, and a significant discrepancy between the observed and modelled hygroscopicity at high organic volume fractions remained. The findings from the SOAP voyage highlight the influence of biologically sourced organics on sea spray aerosol composition; these data improve the capacity to parameterise sea spray aerosol organic enrichment and water uptake. </jats:p>

Measurement report: Cloud Processes and the Transport of Biological Emissions Regulate Southern Ocean Particle and Cloud Condensation Nuclei Concentrations

“…In any case, PMA CCN is found to have little seasonal variability relative to biogenic CCN (Vallina et al, 2006), likely driven by small seasonal differences in wind speed (Saliba et al, 2019). Organic enrichment of PMA in biologically productive waters may further reduce their hygroscopicity (Burrows et al, 2018;Cravigan et al, 2020;Law et al, 2017;Meskhidze and Nenes, 2010).…”

“…Increased CDNC is also shown to correlate with enhanced cloud fraction, significantly increasing overall cloud shortwave forcing (Rosenfeld et al, 2019). If cloud droplets precipitate, CN and CCN concentrations are reduced through precipitation scavenging (Croft et al, 2010;Stevens and Feingold, 2009). In this study, we discuss airborne aerosol measurements in the SO region south of Australia (Fig.…”

Insights into the molecular composition of semi-volatile aerosols in the summertime central Arctic Ocean using FIGAERO-CIMS

“…shown sea spray aerosol (SSA) to be an important source for primary Arctic aerosol particles. SSA is a complex mixture of sea salt and organic matter 28,29 , and can also contain a fraction of semi-volatile organic compounds (SVOCs) 30 . SSA are produced by bubble bursting following wave breaking in the ocean.…”

“…Further, a low O : C ratio of bulk organic aerosol indicates a large fraction of aliphatic or lipid-like compounds with low solubility and hygroscopicity, whereas a high ratio corresponds to more oxidised and carbohydrate-like compounds with higher solubility and hygroscopicity, e.g. polysaccharides 29 . Molecular-level composition data can thus enable calculations of e.g.…”

Marine productivity and synoptic meteorology drive summer-time variability in Southern Ocean aerosols

“…SSA is composed of an internal mixture of inorganic salts and biogenic organic material (Cravigan et al, 2020;Ault et al, 2013). Typically it forms coarse-mode aerosol with mean diameters in the range of approximately 200-500 nm (Quinn et al, 2017;Lewis et al, 2004), although substantial number concentrations have been observed at smaller sizes, containing enhanced organic fractions (Andreae and Rosenfeld, 2008).…”