Long-term observations of black carbon at Fukue Island, western Japan, since 2009: constraining emission strengths from East Asia and wet removal rates relevant to long-range transport

Both emission strengths and wet removal rates of black carbon (BC) particles need to be well constrained to test if particles transported from Asia dominate the Arctic burden. To study these key factors, we analyzed BC mass concentration data at Fukue Island (32.75°N, 128.68°E), western Japan since 2009 based on a continuous soot-monitoring system (COSMOS). 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 nonzero APTs). Here the reduction in the ratios with APT was quantified as the wet deposition term. Results suggested that BC/CO emission ratios in the bottom up inventories (e.g., MEIC, REAS_V2, and HTAP_V2) need to be lowered by factors of >1.3 and 2.8 for Central East China and Korea, respectively. The wet deposition term was fitted reasonably well by a stretched exponential decay curve against APT. An accumulated precipitation of 25.5 ± 6.1 mm reduced the atmospheric BC amount to 1/e. BC-containing particles traveling to Fukue must have already been converted from hydrophobic to hydrophilic particles, because the behavior of PM2.5 against APT was similar, major components of which are hydrophilic. The dominance of coated BC particles was verified from observations with a Single Particle Soot Photometer (SP2) at Fukue during spring 2015. The observed relationship between the wet deposition term and APT was used as a benchmark to test the performances of a number of chemical transport models.