Airborne particles play an important role in climate by both directly scattering and absorbing solar radiation and indirectly by serving as cloud condensation nuclei (CCN) on which clouds form. The combined effects of aerosols on climate have been identified by Intergovernmental Panel on Climate Change (IPCC) as the greatest uncertainty in evaluating overall radiative forcing. Since ocean surfaces cover 70% of the Earth’s surface, marine aerosol is one of the most important and least understood sources of aerosols. In this study, Raman microspectrometry (RMS) was used to determine the chemical composition and mixing states of marine aerosols, because both properties play a role in the particles' ability to act as CCN. Continuous measurements of aerosol concentrations and size, and CCN concentrations were performed during the Halocarbon Air Sea Transect – Pacific (HaloCAST) campaign in 2010.  Along the approximate 7000 miles transect from Punta Arenas, Chile to Seattle, WA, a PIXE Streaker was used to collect time and location resolved aerosol samples for offline chemical analysis by RMS. Sampled marine aerosols were categorized into four main groups according to their dominant chemical components: long chain organic compounds, humic-like substances (HULIS) or soot, water soluble inorganic salts, and insoluble inorganic minerals.  The results of the measurements showed several important findings. First, long chain organic surfactant molecules were prevalent in the marine aerosol samples throughout the cruise. These surfactant species tended to be present in internal mixtures with other organic and inorganic components. In addition, a double peak at 1350/1580 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 observations elevated levels of organics along observed the cruise track were not well correlated occur with elevated concentrations of chlorophyll a. This lack of correlation may be due to the fact that variations in chlorophyll A were fairly subtle, and overall aerosol composition may only be sensitive to major changes in chlorophyll A.  Alternatively, the long chain organics observed may come from another aerosol source. According to our measurements, Pacific marine aerosol may be generally characterized as multicomponent aerosol containing and dominated by a high organic fraction.

Overall, our observations suggest that 60% of the particles are internally mixed multicomponent aerosols.  Given that sea salt may be present but undetected by the RMS, 60% should be considered the lower limit to internal mixing.  Marine aerosol composition and their mixing states vary spatially and temporally. It has been reported that degree of mixing can either increase or decrease an aerosol's ability to act as a cloud condensation nucleus, with the largest enhancements in cloud condensation nuclei concentrations predicted over areas of marine biological activity. Based on our results, incorporation of the variable mixing state must be included in accurate climate modeling.

Soluble inorganic species (WSIO) is observed more frequently in the northern part of the open ocean cruise track, suggesting an influence from the heavier anthropogenic activity in northern hemisphere. Long chain organic species and HULIS are the important organic components identified in the research, while other researches usually investigated the importance of marine primary organic aerosols and its mixing states with sea salt particles. Particles collect in this Pacific cruise exist more in internally mixed state than in externally mixed phase without considering sea salt particles. The mixing state within the organic species, i.e. long chain organic species and HULIS, tends to be internally mixed as...

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