Composition, Mixing State, and Cloud Nucleation Ability of Airborne Marine Particles

Here we use a suite of in-situ measurements to address the possible pathways through which contributions from the sea surface microlayer contribute to marine aerosol concentration, chemical composition, and cloud nucleation ability. Continuous measurements of aerosol concentration and cloud condensation nuclei (CCN) concentration were performed during the Halocarbon Air Sea Transect – Pacific (HaloCAST) campaign in 2010. To determine aerosol chemical composition, a PIXE Streaker was used to collect time- and location- resolved aerosol samples along the approximate 7000 mile transect from Punta Arenas, Chile to Seattle, WA. Raman microspectrometry (RMS) was used to identify the chemical composition and mixing state of individual particles in the Streaker samples. Chlorophyll a concentrations along the cruise track were retrieved from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite data. Sampled marine aerosols were categorized into four main groups according to the dominant chemical component in their Raman spectra: long chain organic compounds, humic-like substances (HULIS) or soot dominated species, water soluble inorganic salts, and insoluble inorganic minerals. The results of the measurement showed several important findings. First, long chain organic species are prevalent in the marine aerosol samples throughout the cruise. Long chain organics tended to be present in internally mixtures with other organic and inorganic components. In addition, a double peak at 1050-1620 cm-1 in Raman spectra representing either humic-like substances (HULIS) or soot was frequently observed, even in samples collected in the pristine southern Pacific Ocean. According to our measurements, Pacific marine aerosol may be generally characterized as multicomponent aerosol dominated by a high organic fraction. In addition, measured critical supersaturations required for ambient aerosols to activate as CCN are anticorrelated with chlorophyll a concentrations along the cruise track. This suggests that regions of high biological activity in the ocean produce aerosol which are more efficient and can act a CCN at lower supersaturations than background aerosols. The presence of long chain organic compounds may improve the nucleating ability of aerosols, since such surfactant molecules reduce droplet surface tension facilitating activation. However, the correlation between chlorophyll a in the surface waters and long chain organics in the aerosol is weak. At times throughout the cruise, additional factors clearly contribute to the aerosol's nucleating ability. Potential marine contributions to cloud formation will be summarized and atmospheric implications discussed.