Analysis and Optical Modeling of Individual Heterogeneous Asian Dust Particles Collected at Mauna Loa Observatory

We have determined optical properties of heterogeneous particles from aerosol samples collected at Hawaii's Mauna Loa Observatory. Back trajectories, satellite imagery, and composition differences among particles from scanning electron microscopy revealed a subset of particles with dolomite or calcite that likely came from Asia. Using focused ion-beam tomography and the discrete dipole approximation, we show how small amounts of an iron phase (oxide or carbonate), or in one case soot, affected extinction and scattering compared with particles of neat dolomite or calcite. We show how particles exhibit a range scattering values due to varying orientations of the inclusion phases. Extinction efficiencies for the heterogeneous particles with dolomite (3.47) and calcite (3.36) were 19% to 21% lower than extinction for marine background air particles (3.72). Extinction for the Asian dust was, however, generally higher than for the neat particles. Compared to iron carbonate, the presence of an absorbing iron oxide affected scattering in Asian dust particles even at the low oxide concentrations studied here (0.6% to 8.1%). Scattering efficiency decreased by <1% with a 1% increase in hematite but by 2% to 5% with magnetite. Asian dust scattered light strongly forward, but backscattering was 56% larger than for the marine background air particles. Backscattering in the Asian dust was also larger with magnetite than hematite. Single scattering albedo for Asian dust with hematite, magnetite, or soot averaged 0.96 ± 0.06 $(\overline{x}\pm s,n=19)$ but was as low as 0.72 with a magnetite mass of 5.8%.