Organic composition of three different size ranges of aerosol particles over the Southern Ocean

Very few measurements exist of the composition and contribution to cloud condensation nuclei (CCN) of organic mass (OM) at different sizes for particles over the Southern Ocean. Airborne and shipboard measurements of organic composition, aerosol number and mass size distributions, and CCN spectra during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) provide a comparison of organic contributions at three different size ranges: (1) by scanning transmission X-ray microscope (STXM) with near edge X-ray absorption fine structure (NEXAFS) for particles 0.25–1.75 µm diameter, (2) by scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS) for particles 0.1–0.5 and 0.5–1µm diameter, and (3) indirectly by comparing CCN spectra to particle size distributions for particles <0.15 µm diameter. Organic functional groups from STXM-NEXAFS microscopy indicate that the majority of particles between 0.25 and 1.75 µm diameter with detectable organic components were consistent with marine organic signatures associated with sea spray particles, which was also true for ice nucleating particles sampled by immersion freezing <−27 °C. Clustering of the 96 NEXAFS spectra resulted in 5 distinct types of sea spray related organic mixtures (four salt-containing sea spray particle types and one organic-rich particle type without a salt core) that accounted for 76% of the particles, including two that appear primary and three that include acid groups that are likely from secondary sources. STEM analysis indicates that particles 0.1–0.5 µm diameter were mostly sulfate and particles 0.5–1 µm diameter were mostly sodium-based sea spray. The hygroscopicity of most particles >0.1 µm diameter were consistent with sodium-based sea spray and sulfate particles, providing closure between STEM and CCN measurements. Particles <0.15 µm diameter had kappa values of 0.2–0.5, indicating the substantial presence of OM at these sizes. The organic component fraction estimated from comparing the size distribution and supersaturation spectra varied from <10% to 70% for particles <0.15 µm diameter. This result shows the impact of the organic composition of particles <0.15 µm diameter on CCN concentrations at typical supersaturations for the SO. This variable OM fraction accounted for <0.1 µg/m³ mass concentrations of particles smaller than 0.15 µm diameter but may play a role at cloud supersaturations greater than 0.3%.

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