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
Authors: Kanaya Yugo , Pan Xiaole , Miyakawa Takuma , Komazaki Yuichi , Taketani Fumikazu , Uno Itsushi , Kondo Yutaka 
Published: March 2016

Abstract. Long-term (2009–2015) observations of atmospheric black carbon (BC) mass concentrations were performed using a continuous soot-monitoring system (COSMOS) at Fukue Island, western Japan, to provide information on wet removal rate constraints and the emission strengths of important source regions in East Asia (China and others). The annual average mass concentration was 0.36 µg m−3, with distinct seasonality; high concentrations were recorded during autumn, winter, and spring, and were caused by Asian continental outflows, which reached Fukue Island in 6–46 hours. The observed data were categorized into two classes, i.e., with and without a wet removal effect, using the accumulated precipitation along a backward trajectory (APT) for the last 3 days as an index. Statistical analysis of the observed ΔBC/ΔCO ratios was performed to obtain information on the emission ratios (from data with zero APT only) and wet removal rates (including data with non-zero APTs). The estimated emission ratios (5.2–6.9 ng m−3 ppb−1) varied over the six air mass origin areas; the higher ratios for South Central East China (30–35° N), South China (< 30° N), and Korea than for the other areas, i.e., Northeast China (> 40° N), North Central East China (35–40° N), and Japan, indicated the relative importance of domestic emissions and/or biomass burning sectors. The significantly higher BC/CO emission ratios adopted in 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 needed to be reduced at least by factors of 1.3 and 2.8 for Central East China and Korea, respectively, but the ratio for Japan was reasonable. The wintertime enhancement of the BC emission from China, predicted by REAS2, was verified for 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 in the ΔBC/ΔCO ratio against APT. The transmission efficiency (TE), defined as the ratio of the ΔBC/ΔCO ratio with precipitation to that without precipitation, was fitted reasonably well by a stretched exponential decay curve against APT; a single set of fitting parameters was sufficient to represent the results for air masses originating from different areas. An accumulated precipitation of 15.0 ± 3.2 mm halved the BC mass concentration. BC-containing particles traveling to Fukue must have already been converted from hydrophobic to hydrophilic particles, because the behavior of TE against APT was similar to that of PM2.5, the major components of which are hydrophilic. After correcting for the wet removal effect, trends in the BC mass concentrations were almost flat for the air masses from CEC and Korea and decreasing for those from Japan during 2009–2015. This long-term data set will provide a benchmark for testing chemical transport/climate model simulations covering East Asia.