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Long-term (2009–2015) observations of atmospheric black carbon (BC) massconcentrations were performed using a continuous soot-monitoring system(COSMOS) at Fukue Island, western Japan, to provide information on wetremoval rate constraints and the emission strengths of important sourceregions in East Asia (China and others). The annual average massconcentration was 0.36 µg m−3, with distinct seasonality;high concentrations were recorded during autumn, winter, and spring and werecaused by Asian continental outflows, which reached Fukue Island in 6–46 h.The observed data were categorized into two classes, i.e., with and without awet removal effect, using the accumulated precipitation along a backwardtrajectory (APT) for the last 3 days as an index. Statistical analysis of theobserved ΔBC ∕ ΔCO ratios was performed to obtaininformation on the emission ratios (from data with zero APT only) and wetremoval rates (including data with nonzero APTs). The estimated emissionratios (5.2–6.9 ng m−3 ppb−1) varied over the six air massorigin areas; the higher ratios for south-central East China(30–35° N) than for north-central East China (35–40° N)indicated the relative importance of domestic emissions and/or biomassburning sectors. The significantly higher BC ∕ CO emission ratios adoptedin the bottom-up Regional Emission inventory in Asia (REAS) version 2(8.3–23 ng m−3 ppb−1) over central East China and Korea neededto be reduced at least by factors of 1.3 and 2.8 for central East China andKorea, respectively, but the ratio for Japan was reasonable. The wintertimeenhancement of the BC emission from China, predicted by REAS2, was verifiedfor air masses from south-central East China but not for those from north-central East China. Wet removal of BC was clearly identified as a decrease inthe ΔBC ∕ ΔCO ratio against APT. The transport efficiency(TE), defined as the ratio of the ΔBC ∕ ΔCO ratio withprecipitation to that without precipitation, was fitted reasonably well by astretched exponential decay curve against APT; a single set of fittingparameters was sufficient to represent the results for air masses originatingfrom different areas. An accumulated precipitation of 25.5 ± 6.1 mmreduced the TE to 1∕e. BC-containing particles traveling to Fukue must havealready been converted from hydrophobic to hydrophilic particles, because thebehavior of TE against APT was similar to that of PM2.5, the majorcomponents of which are hydrophilic. Wet loss of BC greatly influencedinterannual variations in the ΔBC ∕ ΔCO ratios and BCmass concentrations. This long-term data set will provide a benchmark fortesting chemical transport/climate model simulations covering East Asia

atmospheric chemistry and physics

Long-term observations of black carbon mass concentrations at Fukue Island, western Japan, during 2009–2015: constraining wet removal rates and emission strengths from East Asia