, Robert D. Willis,
We have determined optical properties of heterogeneous particles from aerosol samples collected at Mauna Loa Observatory (MLO) in Hawaii. Back trajectories, satellite imagery, and composition differences among particles from scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry revealed a subset of particles containing dolomite or calcite that likely originated from Asia. We show how the presence of minor phases in a particle, particularly an iron phase (hematite, magnetite, or iron carbonate) or, in one case soot, affected optical properties compared with neat dolomite or calcite particles. We created three-dimensional spatial models of each particle with focused ion beam (FIB) SEM and FIB tomography. From the spatial models, extinction, scattering, single scattering albedo (SSA), and the backscatter fraction at 589 nm were calculated with the discrete dipole approximation method. We assessed how varying spatial orientations of the phases might affect optical properties using upper and lower limits for refractive indexes. Extinction efficiencies for the heterogeneous particles with dolomite (3.47) and calcite (3.36) were 19 to 21 % lower than extinction for the marine background air particles (3.72). Extinction for the Asian dust was, however, consistently 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 0.6 % with a 1 % increase in hematite but by 2 to 5 % with magnetite. Asian dust scattered light strongly forward, but backscattering was 56 % higher than for the marine background air particles. Backscattering in the Asian dust was also larger with magnetite than hematite. SSA for Asian dust with hematite, magnetite, or soot averaged 0.96 ± 0.06 (x ̅±s,n=19), but was as low as 0.72 with magnetite at an iron concentration of 5.8 %.
Journal of Geophysical Research-Atmospheres
atmospheric aerosol, mineral dust, scanning electron microscopy, focused ion-beam tomography, FIB-SEM, discrete dipole approximation, extinction, scattering, backscattering, single scattering albedo