High concentrations of black carbon over middle latitudes in the North Pacific Ocean

During 4 years of research cruises over the central Pacific Ocean from 1993 through 1996, a region of extremely high concentration of black carbon aerosol (>150 ng C m(-3)) was routinely observed in the middle-latitude zone (27 degrees-48 degrees N) along the 175 degrees E line of longitude (near the International Date Line). In the cruises along the latitude lines of 30 degrees N and 35 degrees N, a strong west-to-east gradient of black carbon was also observed. Backward trajectory analysis shows that the air parcels containing high concentration of black carbon had passed over the Asian continent during the previous 10 days. The concentration of black carbon was relatively well correlated with non-sea-salt (nss.) SO42- in the aerosols. This indicates that the high concentration of black carbon observed originated from the combustion of sulfur-containing materials, presumably coal, on the Asian continent. Mass size distributions of the black carbon exhibit a mode at 0.5-0.7 mu m in aerodynamic diameter, which is unexpectedly large and coincides with the mode of SO42- aerosol. This suggests that black carbon and SO42- were internally mixed, and additional growth in particle size occurred through cloud processing. This study shows that anthropogenic aerosols are transported over greater distances than previously indicated by global dispersion models. The trajectory analysis indicates that this is due to the high-altitude transport of materials which reduces the possibility of encountering precipitating clouds; the aerosols are lofted over the continent initially, transported at relatively high altitudes over the ocean, and then brought to the surface in the central part of the North Pacific Ocean by mesoscale weather systems. The measured mixing ratio of black carbon/nss.SO42- and the state of the mixture (internal) suggest that the aerosols transported from Asia over the North Pacific Ocean have a low single-scattering albedo.