Mixing State of Black Carbon Particles in Asian Outflow Observed at a Remote Site in Taiwan in the Spring of 2017

Anthropogenic emissions of aerosols and trace gases from East Asia have significant impacts on air quality and climate on regional and global scales. We conducted intensive measurements of aerosols at a remote site (Cape Fuguei; 25.30 degrees N, 121.54 degrees E) in Taiwan in the spring of 2017, focusing on the chemical composition and mixing state of aerosols in Asian outflow. A laser-induced incandescence-mass spectrometric analyzer (LII-MS) was used to measure the chemical composition of aerosols internally or externally mixed with black carbon (BC). Offline analysis by transmission electron microscopy (TEM) was used to characterize the mixing state of aerosols. The observation period was classified into long-range transport (LRT) periods and local pollution periods on the basis of cluster analysis of 3-day backward trajectories. LII-MS data showed that the average mass fractions of sulfate and nitrate internally mixed with BC were 7% +/- 13% and 11% +/- 15%, respectively, for high-pollution episodes during LRT periods. By TEM analysis of selected samples from LRT periods, the number fraction of aerosol particles internally mixed with BC was estimated to be 5%-36% for core-shell types and 5%-23% for attached types. Factors affecting the differences between LII-MS and TEM measurements are discussed. TEM data indicated the presence of a variety of aerosol mixing states in widespread regional pollution transported 500-1,000 km downwind of emission sources. Our results provide new constraints for three-dimensional models that might resolve the mixing state of aerosol particles, which are critical for accurately estimating the radiative impact of BC.