Sea spray aerosol organic enrichment, water uptake and surface tension effects
Authors: Cravigan Luke T. , Mallet Marc D. , Vaattovaara Petri , Harvey Mike J. , Law Cliff S. , Modini Robin L. , Russell Lynn M. , Stelcer Ed , Cohen David D. , Olsen Greg , Safi Karl , Burrell Timothy J. , Ristovski Zoran 
Published: September 2019

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 are therefore the region is under-represented in existing parameterisations. The Surface Ocean Aerosol Production (SOAP) voyage was a 23 day 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 87 % of the particle volume in the Aitken mode. The composition of the organic fraction was consistent throughout the voyage and was largely comprised 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 models. OCEANFILMS improved on the representation of the organic fraction predicted using chlorophyll-a, in particular when the co-adsoprtion of polysaccharides was included, however the model still under predicted the proportion of polysaccharides by an average of 33 %. Nascent sea spray aerosol hygroscopic growth factors 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 was attributed to the surface partitioning of the organic components which leads to a decrease in particle surface tension and an increase in hygroscopicity. The compressed film model was used to estimate the influence of surface partitioning and the error in the modelled hygroscopicity was low only when the entire organic fraction was available to partition to the particle surface. The modelled sea spray aerosol hygroscopicity at high organic fractions was underestimated when only a portion of the organic component was available to be partitioned to the surface. 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.