Pyrogenic organic matter (PyOM) is a substance produced by the incomplete combustion of fossil fuels and biomass, and the major fraction is biologically recalcitrant. A substantial fraction of PyOM produced on the land reaches the ocean via rivers and/or the atmosphere. Water-soluble PyOM (WSPyOM) is operationally defined as the filtrate of water-extracted PyOM in environmental water. It is ubiquitous in the ocean and has extremely old radiocarbon ages (∼23,000 years), suggesting that it has been stored in the ocean for a long time.
The stable carbon isotope ratios of marine WSPyOM differ significantly from those of riverine WSPyOM but are similar to those of atmospheric soot, another pyrogenic substance in atmospheric aerosols. Thus, the atmospheric deposition of WSPyOM and/or soot can be a major source of marine WSPyOM. However, as observations of WSPyOM in atmospheric aerosols are scarce and mostly conducted over land, the flux of WSPyOM to the ocean via atmospheric deposition remains poorly understood. Therefore, this study aims to characterize WSPyOM in marine atmospheric aerosols quantitatively and qualitatively. The observation was carried out in the winter Kuroshio region, an area strongly influenced by air masses from East Asia, a region with huge anthropogenic emissions of aerosols.
Aerosol samples were collected at two-day intervals on board the R/V Hakuho Maru between 9 February and 4 March 2025 along the path of the Kuroshio Current from 129°E (East China Sea) to 140°E (Izu Islands). PM_2.5 were collected on quartz fiber filters using a high-volume air sampler (Model 120SL, Kimoto Electric) attached with a single-stage slotted cascade impactor (TISCH). To avoid contamination from ship exhaust, the aerosol sampler was controlled by a wind sector. The aerosol sampler was operated only when the relative wind direction was between −90° and 90° from the bow and the relative wind speed was between 1 and 20 m s^-1. The water-soluble (WS) fraction of the aerosols was obtained by adding ultrapure water to the punched filter samples, followed by ultrasonic extraction and filtration through a 0.2 µm PTFE filter. The WSPyOM concentration in each fraction was determined using the benzene polycarboxylic acid (BPCA) method. The concentration of WS organic carbon (WSOC) was determined using the combustion catalytic oxidation method.
The WSPyOM concentration ranged from 12.5 to 34.8 ngC m^⁻³, with the highest concentration observed near the Izu Islands. The ratio of WSPyOM to WSOC (WSPyOM/WSOC) ranged from 0.7% to 4.7%, showing an increasing trend toward the west and reaching its maximum in the East China Sea. These values were comparable to those reported in previous spring observations conducted between the Chinese coast and the western North Pacific. Backward trajectory analysis indicated that all the aerosol samples were influenced by continental sources. Near the Izu Islands, on the one hand, the relatively high WSPyOM concentrations suggest a large influence from combustion sources. On the other hand, the relatively low WSPyOM/WSOC ratio indicates that non-pyrogenic aerosols, which do not contain WSPyOM, were substantially mixed with the combustion-derived aerosols. Conversely, WSPyOM concentrations were moderate in the western areas, such as the East China Sea, likely due to the loss of WSPyOM during long-range transport from continental combustion sources, including biomass and coal burning. The relatively high WSPyOM/WSOC ratio also suggests that non-pyrogenic sources make a relatively low contribution to this area.

Pyrogenic Organic Matter (PyOM),aerosol