Factors controlling the concentration of water-soluble pyrogenic carbon in aerosols in Hokkaido, Japan

Abstract. Pyrogenic carbon (PyC) is a continuum of compounds generated during the incomplete combustion of biomass or fossil fuels. Water-soluble PyC (WSPyC), a PyC smaller than 0.7 µm, is transported to the ocean via rivers or atmospheric deposition and is considered a key component in the long-term regulation of the global carbon cycle. Compared to the riverine input flux, the atmospheric deposition flux of WSPyC may have large uncertainties due to limited observations. This study examines the factors influencing WSPyC concentration in atmospheric aerosols in Sapporo, Hokkaido, based on a year-round observation. The WSPyC concentration in the aerosols, determined with the benzene polycarboxylic acid method, ranged from 1.41 to 46.5 ngC m⁻³, with an average value of 13.7±10.6 ngC m⁻³. The average concentration was lower than previously observed near combustion sources. Positive Matrix Factorization (PMF) analysis revealed that 60 % of the WSPyC concentration was attributed to a K⁺-dominated combustion, which included the burning of biomass and biofuels. Meanwhile, 40 % of WSPyC was associated with factors related to the atmospheric aging of aerosols derived from fossil fuel combustion. The global WSPyC deposition flux to the ocean has previously been estimated based on the concentration ratio of WSPyC to elemental carbon (EC) or water-soluble organic carbon (WSOC). The results of this study suggested that these ratios vary due to the atmospheric aging processes, including photochemical production of WSPyC from soot, indicating the need for a reassessment of the global WSPyC deposition flux.