Optical and chemical properties of aerosols transported to Mount Bachelor during spring 2010

Abstract

[1] We report on springtime 2010 observations of aerosol optical properties and size-resolved elemental composition from Mount Bachelor Observatory (MBO; 2763 meters above sea level). Observations included multiwavelength aerosol scattering and absorption, made with a nephelometer and a particle soot absorption photometer, and size-resolved composition, made using a rotating DRUM impactor with substrates analyzed by synchrotron X-ray fluorescence. Our main tool for investigating variability in composition was empirical orthogonal function (EOF) analysis. In April, dust and sulfate explained 96% of the variance in the observed fine composition and accounted for the majority of the fine mode scattering. Three coincident Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation overpasses also identified aerosol layers classified as dust or polluted dust over MBO. Later in the spring, we deduce that organics and nitrate comprised more than 50% of the submicrometer aerosol mass. We used the EOF analysis to identify systematic relationships between composition and optical properties. We observed dust accompanied by anthropogenic pollutants including sulfate. When present, dust aerosol controlled ∼30% of the variability in the wavelength dependence of fine mode scattering. Many of the samples containing sulfate had absorption Angstrom exponents near 1, suggesting black carbon was also present. Most of the sulfate was in the fine mode, but sulfate was also observed on coarse aerosols, and we inferred that much of the coarse sulfur was coated on the dust or had formed CaSO4 during transport. The relationships between Fe, Ca, Al, and Si observed at MBO were consistent with previous observations of Asian dust transported to North America.