Search for: All records

Field measurements have shown that sub-micrometer sea spray aerosol (SSA) is significantly enriched in organic material, of which a large fraction has been attributed to soluble saccharides. Existing mechanistic models of SSA production struggle to replicate the observed enhancement of soluble organic material. Here, we assess the role for divalent cation mediated co-adsorption of charged surfactants and saccharides in the enrichment of soluble organic material in SSA. Using measurements of particle supersaturated hygroscopicity, we calculate organic volume fractions for molecular mimics of SSA generated from a Marine Aerosol Reference Tank. Large enhancements in SSA organic volume fractions (Xorg > 0.2) were observed for 50 nm dry diameter (dp) particles in experiments where cooperative ionic interactions were favorable (e.g., palmitic acid, Mg2+, and glucuronic acid) at seawater total organic carbon concentrations (<1.15 mM C) and ocean pH. Significantly smaller SSA organic volume fractions (Xorg < 1.5 × 10−3) were derived from direct measurements of soluble saccharide concentrations in collected SSA with dry diameters <250 nm, suggesting that organic enrichment is strongly size dependent. The results presented here indicate that divalent cation mediated co-adsorption of soluble organics to insoluble surfactants at the ocean surface may contribute to the enrichment of soluble saccharides in SSA. The extent to which this mechanism explains the observed enhancement of saccharides in nascent SSA depends strongly on the concentration, speciation, and charge of surfactants and saccharides in the sea surface microlayer. 

Abstract. Understanding the impact of sea spray aerosol (SSA) on theclimate and atmosphere requires quantitative knowledge of their chemicalcomposition and mixing states. Furthermore, single-particle measurements areneeded to accurately represent large particle-to-particle variability. Toquantify the mixing state, the organic volume fraction (OVF), defined as therelative organic volume with respect to the total particle volume, ismeasured after generating and collecting aerosol particles, often usingdeposition impactors. In this process, the aerosol streams are either driedor kept wet prior to impacting on solid substrates. However, the atmosphericcommunity has yet to establish how dry versus wet aerosol depositioninfluences the impacted particle morphologies and mixing states. Here, weapply complementary offline single-particle atomic force microscopy (AFM)and bulk ensemble high-performance liquid chromatography (HPLC) techniquesto assess the effects of dry and wet deposition modes on thesubstrate-deposited aerosol particles' mixing states. Glucose and NaClbinary mixtures that form core–shell particle morphologies were studied asmodel systems, and the mixing states were quantified by measuring the OVF ofindividual particles using AFM and compared to the ensemble measured byHPLC. Dry-deposited single-particle OVF data positively deviated from thebulk HPLC data by up to 60&thinsp;%, which was attributed to significantspreading of the NaCl core upon impaction with the solid substrate. This ledto underestimation of the core volume. This problem was circumvented by (a) performing wet deposition and thus bypassing the effects of the solid corespreading upon impaction and (b) performing a hydration–dehydration cycle ondry-deposited particles to restructure the deformed NaCl core. Bothapproaches produced single-particle OVF values that converge well with thebulk and expected OVF values, validating the methodology. These findingsillustrate the importance of awareness in how conventional particledeposition methods may significantly alter the impacted particlemorphologies and their mixing states.