Abstract
It is critical to understand how variations in chemical composition in surface seawater (SSW) affect the chemistry of marine atmospheric aerosols. We investigated the sea-to-air transfer of dissolved organic carbon (DOC) via cruise measurements of both ambient aerosols and SSW in the Oyashio and its coastal regions, the western subarctic Pacific during early spring. Sea spray aerosols (SSAs) were selected based on the stable carbon isotope ratio of water-soluble organic carbon (WSOC) (δ13CWSOC) and concentrations of glucose as a molecular tracer in marine aerosols together with local surface wind speed data. For both SSA and SSW samples, excitation-emission matrices were obtained to examine the transfer of fluorescent organic material. We found that the ratios of fluorescence intensity of humic-like and protein-like substances in the submicrometer SSAs were significantly larger than those in the bulk SSW (~63%). This ratio was also larger for the supermicrometer SSAs than for the SSW. The results suggest significant decomposition of protein-like DOC on a timescale of <12–24 h and/or preferential production of humic-like substances in the atmospheric aerosols regardless of the particle size. This study provides unique insights into the complex transfer of DOC from the ocean surface to the atmosphere.
Highlights
Marine atmospheric aerosols play a key role in the climate system, as they act as cloud condensation nuclei (CCN) and ice nuclei (IN) and can control the atmospheric radiative budget through cloud formation[1,2]
Whereas a large fraction of marine organic aerosols (OAs) remains chemically unresolved, a significant fraction of water-soluble organic carbon (WSOC) has been attributed to humic-like substances (HULIS), which are operationally defined as a complex mixture of heterogeneous compounds[24]
The median number concentrations of aerosol particles at dry diameters of 14–710 nm broadly ranged from 497 to 4,947 cm−3 for respective periods of the filter samplings with the number of aerosol samples of 22. While these values are similar to those of Sea spray aerosols (SSAs) produced from low temperature (~0–10 °C) seawater using a plunging jet SSA chamber[30], the result suggests that SSAs could have co-existed with other types of aerosols, on the assumption that the number concentrations of SSAs in pristine marine air are typically
Summary
Marine atmospheric aerosols play a key role in the climate system, as they act as cloud condensation nuclei (CCN) and ice nuclei (IN) and can control the atmospheric radiative budget through cloud formation[1,2]. Classifications of organic matter with EEM spectra in SSA need to be employed in comparison with that in SSW, because their sources, chemical compositions, and transformation pathways are expected to be different from those in the marine aqueous environment In this context, direct comparison of EEMs between SSAs and SSW in the field remains rarely carried out. The major pathways for HULIS production in aerosols include direct emissions (e.g., sea spray, soil resuspension) and secondary formation from both biogenic and anthropogenic precursors (e.g., oxidation, oligomerization, and polymerization) Their formation mechanism and alternative components should be considered, as they may explain the large missing source of marine OA in global models[26]
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