Measurement report: Summertime fluorescence characteristics of atmospheric water-soluble organic carbon in the marine boundary layer of the western Arctic Ocean
Authors: Jung Jinyoung , Miyazaki Yuzo , Hur Jin , Lee Yun Kyung , Jeon Mi Hae , Lee Youngju , Cho Kyoung-Ho , Chung Hyun Young , Kim Kitae , Choi Jung-Ok , Lalande Catherine , Kim Joo-Hong , Choi Taejin , Yoon Young Jun , Yang Eun Jin , Kang Sung-Ho 
Published: April 2023

Abstract. 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 in the
rapidly changing Arctic Ocean, including its summertime fluorescence
characteristics, we simultaneously measured atmospheric concentrations of
ionic species and WSOC, a 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 rates of sea–air gas exchange and emissions of aerosol precursor gases
due to sea ice retreat and increasingly 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 the 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 with 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 to 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. These findings can improve our understanding of the
chemical and biological linkages of WSOC at the ocean–sea-ice–atmosphere
interface.