Abstract
Abstract. The composition of organic compounds in marine aerosols and the relative contributions of primary and secondary organic compounds remain uncertain. We report results from a novel approach to characterize and quantify organic components of the marine aerosol. Size-segregated discrete aerosol filter samples were collected at sea in the North Atlantic from both ambient aerosol and artificially generated primary sea spray over four cruises timed to capture the seasonal phytoplankton bloom dynamics. Samples were analyzed by Fourier transform infrared spectroscopy (FTIR), extracted into water, and analyzed by offline thermal desorption chemical ionization mass spectrometry (TDCIMS) and ion chromatography (IC). A positive matrix factorization (PMF) analysis identified several characteristic aerosol components in the TDCIMS mass spectra. Among these is a polysaccharide factor representing about 10 %–30 % of the submicron organic aerosol mass. Aerosol polysaccharide : sodium mass ratios were consistently higher in ambient air than in the artificially generated sea spray, and we hypothesize that this results from more rapid wet deposition of sodium-rich aerosol. An unquantified recalcitrant factor of highly thermally stable organics showed significant correlation with FTIR-measured alcohol groups, consistently the main organic functional group associated with sea spray aerosol. We hypothesize that this factor represents recalcitrant dissolved organic matter (DOM) in seawater and that by extension alcohol functional groups identified in marine aerosol may more typically represent recalcitrant DOM rather than biogenic saccharide-like material, contrary to inferences made in previous studies. The recalcitrant factor showed little seasonal variability in its contribution to primary marine aerosol. The relative contribution of polysaccharides was highest in late spring and summer in the smallest particle size fraction characterized (<180 nm).
Highlights
Marine aerosols are important to the climate system because of their direct interactions with incoming solar radiation and their indirect effect on cloud radiative properties
Two components did not correlate with Na+: (1) the carbonyl group, which was detected in only one of the analyzed Sea Sweep samples; and (2) the fatty acid positive matrix factorization (PMF) factor, which showed indications of volatilization as described above. We find that this correlation analysis supports the idea that the thermal desorption chemical ionization mass spectrometry (TDCIMS) recalcitrant factor comes from primary sea spray, along with Fourier transform infrared spectroscopy (FTIR) alcohol and amine groups, but beyond that we cannot infer much from the many modest relationships observed among organic signals in the Sea Sweep aerosol
A key finding is that the organic material that closely resembled polysaccharide standards was not correlated with the FTIR alcohol functional group
Summary
Marine aerosols are important to the climate system because of their direct interactions with incoming solar radiation and their indirect effect on cloud radiative properties. Marine aerosols are a complex and variable mixture of locally generated material (emissions of particles and gases from the sea surface) and transported material from the continents (pollutants, biomass burning, desert dust). Lawler et al.: Offline TDCIMS of marine aerosol rine aerosol, but the chemical composition and origin of this material is not well characterized. These organics may arise from bubble bursting, from emissions of biogenic aerosol precursors like isoprene and monoterpenes, or from oxygenated aerosol-forming volatile organic compounds perhaps arising from photochemistry occurring on the sea surface (Bernard et al, 2016; Mungall et al, 2017)
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