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Accelerated warming and a decline in sea ice coverage in the summertime Arctic Ocean can significantly affect the emissions of marine organic aerosols and biogenic volatile organic compounds. However, how these changes affect the characteristics of atmospheric water-soluble organic carbon (WSOC), which plays an important role in the climate system, remains unclear. Thus, to improve our understanding of WSOC characteristics, including its summertime fluorescence characteristics, in the rapidly changing Arctic Ocean, we simultaneously measured atmospheric concentrations of ionic species and WSOC, fluorescence excitation–emission matrix coupled with parallel factor (EEM–PARAFAC) analysis of WSOC, and marine biological parameters in surface seawaters of the western Arctic Ocean during the summer of 2016. WSOC was predominantly present as fine-mode aerosols (diameter < 2.5 μm) (median = 92 %) with the mean concentration being higher in the coastal water areas (462 ± 130 ngC m−3) than in the sea ice-covered areas (242 ± 88.4 ngC m−3). Moreover, the WSOC in the fine-mode aerosols was positively correlated with the methanesulfonic acid in the fine-mode aerosol samples collected over the sea ice-covered areas (r = 0.88, p < 0.01, n = 10), suggesting high sea–air gas exchange and emissions of aerosol precursor gases due to sea ice retreat and increasing available solar radiation during the Arctic summer. Two fluorescent components, humic-like C1 and protein-like C2, were identified by the PARAFAC modeling of fine-mode atmospheric WSOC. The two components varied regionally between coastal and sea ice-covered areas, with low and high fluorescence intensities observed over the coastal areas and sea ice-covered areas, respectively. Further, the humification index of WSOC was correlated with the fluorescence intensity ratio of the humic-like C1/protein-like C2 (r = 0.89, p < 0.01) and the WSOC concentration in the fine-mode aerosols (r = 0.66, p < 0.05), with the highest values observed in the coastal areas. Additionally, the WSOC concentration in the fine-mode aerosols was positively correlated with the fluorescence intensity ratio of the humic-like C1/protein-like C2 (r = 0.77, p < 0.01), but was negatively correlated to the biological index (r = −0.69, p < 0.01). Overall, these results suggested that the WSOC in the fine-mode aerosols in the coastal areas showed a higher degree of polycondensation and higher aromaticity compared that in the sea ice-covered areas, where WSOC in the fine-mode aerosols was associated with relatively new, less oxygenated, and biologically-derived secondary organic components. The findings can improve our understanding of the chemical and biological linkages of WSOC at the ocean–sea ice–atmosphere interface.

The Amundsen Sea is the most rapidly melting part of the West Antarctic Ice Sheet, due to increased heat transport by Circumpolar Deep Water (CDW). Tracing CDW and resulting glacial meltwater is important since glacial meltwater may change the water mass properties, leading to the change of the biogeochemical cycles. In this study, in order to investigate the potential for using the humic-like component of fluorescent dissolved organic matter (FDOM) as a tracer for CDW and glacial meltwater in the Amundsen Sea, a hydrographic survey was conducted during the austral summer of 2018 aboard the Korean icebreaker IBR/V Araon. The meteoric water and CDW fractions calculated using the humic-like component (fmw_humic and fcdw_humic) were compared to those using oxygen isotope (δ18O) (fmw) and optimum multiparameter analysis (OMP) (fcdw_OMP), respectively. The fluorescence intensity of the humic-like component varied from 0.007 to 0.021, with higher values in the deeper layer and lower intensities in the surface waters. The range of fcdw_humic (0.5–1.0) was narrower than that of fcdw_OMP (0.1–1.0), indicating that the fcdw_humic values were overestimated due to the remained humic-like C1. To minimize the effect of the remained humic-like C1 on the calculation of CDW fraction, we used newly derived empirical equations (i.e., fcdw_OMP = 105.17 × C1–1.14 for transect 1 and fcdw_OMP = 126.04 × C1–1.41 for transect 2). The CDW fraction calculated using the empirical equations (fcdw_humic_empirical) was in good agreement with the fcdw_OMP. We also found a significant positive relationship between fmw and fmw_humic, indicating that a reasonable method can be applied with a high percentage of explained variance and that fmw can be largely explained by fmw_humic. Our results show that the humic-like component can be a useful tracer for identifying CDW and glacial meltwater in the Amundsen Sea.