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.
General Comments
This manuscript presents a comprehensive year-long observational dataset of water-soluble pyrogenic carbon (WSPyC) in atmospheric aerosols over Sapporo, Japan. The authors combine detailed chemical analyses with source apportionment (via PMF), seasonal air mass trajectory analyses, and statistical relationships with other aerosol properties to investigate the variability and sources of WSPyC. Given the scarcity of observational data on atmospheric WSPyC, particularly outside regions near primary combustion sources, this work contributes valuable information to an underrepresented area of the global carbon cycle.
The authors also engage with broader questions related to global carbon flux estimates, such as the reliability of using fixed WSPyC:EC or WSPyC:WSOC ratios across locations. This is a useful contribution, but certain key aspects of context, clarity, and presentation need refinement to fully support the manuscript’s conclusions.
Some of the methods and reasoning steps are underexplained—for example, the motivation for selecting Sapporo as a study location, how local sources were accounted for, or why specific factor names were used in the PMF. Similarly, while the figures and analyses are generally strong, a few inconsistencies and assumptions (e.g., in how seasonal backtrajectories were presented or in the interpretation of ratios) reduce the interpretability of the results. Addressing these points would enhance both transparency and the broader relevance of the findings.
I recommend publication after minor revisions, which are detailed below. These revisions primarily involve clarifications in text, improved figure presentation, and deeper discussion of certain assumptions or methodological choices.
Specific Comments