Air Mass Transport and Marine Conditions Regulate the Optical Properties and Composition of Particulate Water‐Soluble Organic Carbon Over the East China Sea

Abstract Affected by the interaction and mixing of oceanic and continental air masses, water‐soluble organic carbon (WSOC) in offshore atmospheric aerosols exhibits unique complexity and characteristics. This study examines the light‐absorption properties and chromophores of WSOC over the East China Sea (ECS). The light‐absorption properties of WSOC, termed as water‐soluble brown carbon (WS‐BrC), reached its highest level when continental outflows were dominant. An absolute high proportion of secondary BrC likely formed in high‐abundance precursor regions and was bleached during long‐range transport. The low levels of WS‐BrC occurred under the prevalence of marine air masses, and the secondary formation exceeded the bleaching effect. Combining the spectral absorption ratio (E 2 /E 3 , 250 nm/365 nm) with a spectra‐constrained positive matrix factorization model, light‐absorbing components of WS‐BrC were attributed to humic‐like substances (HULIS) and aromatics. Likewise, excitation—emission matrix spectroscopy (EEMs) results indicated that HULIS and aromatics collectively accounted for approximately 90% of WSOC when continental outflows were dominant. Protein‐like compounds constitute the primary component of WSOC (∼80%) in marine air masses, the synergistic regulation of marine dissolved oxygen and ambient temperature explained over 80% of the variability in protein‐like compounds. This study demonstrates that air mass transport drives differences in sources and secondary processes, and underscores the importance of environmental variability in regulating the physicochemical properties of marine atmospheric organic aerosols.