Optical properties of aged Asian aerosols observed over the US Pacific Northwest

A suite of gas-phase and aerosol measurements were made during spring 2008 and spring 2009 at the Mount Bachelor Observatory (2763 masl), located in Oregon. Here we focus on multiwavelength observations of low RH submicron (mu m) aerosol scattering (sigma(sp)) and absorption (sigma(ap)), made with an integrating nephelometer and a particle soot absorption photometer. Using a combination of in situ observations, trajectory calculations and satellite observations, we identified seven plumes of Asian origin. These plumes included many of the highest sigma(sp) (34.8 Mm(-1)) and sigma(ap) (5.7 Mm(-1)) hourly average values observed at MBO over the 2008 and 2009 campaigns. Of interest in this analysis is (1) whether the intensive optical properties differ between these seven plumes, (2) whether these differences can be linked to differences in composition, and (3) whether the intensive optical properties change during transpacific transport. Results show that the plumes clustered in terms of their optical properties; plumes hypothesized to contain a large fraction of mineral dust were the most distinct. We observed variability between plumes in the scattering Angstrom (angstrom(s)) exponent. The average submicrometer angstrom(s) for all seven plumes was significantly larger than the same parameter observed closer to Asia. Therefore, we hypothesize that the aerosol size distribution shifts toward smaller particles during transpacific transport. The average submicrometer low-RH aerosol single scatter albedo (omega) observed at MBO (0.88) was slightly larger than previous observations closer to the Asian coast, and the average backscatter fraction (b) was smaller, on the order of 20%.