The effects of sulfate and non-sulfate particles on light scattering at the Mauna Loa Observatory

The results of a 1 yr sampling program designed to quantify the effects of sulfate and non-sulfate particles on light scattering measurements routinely made at the Mauna Loa Observatory in Hawaii are described. Aerosol sampling with a cascade impactor was conducted at an altitude of 3400 m above sea level near the summit of a large, gently sloping volcano during nocturnal downslope flow, when particles are believed representative of clean, mid-tropospheric background conditions. A microchemical spot test using a transmission electron microscope was applied to quantitatively identify individual sulfuric acid, ammonium sulfate, and non-sulfate particles. Simultaneous measurements of integrated light scattering with a four-wavelength nephelometer were also made. The results of the study indicate that significant variations in aerosol chemistry and morphology occur at Mauna Loa Observatory over an annual period. Episodes of high particle scattering caused by large non-sulfate particles, presumably soil dust, occur mainly in the spring and are generally associated with strong northwesterly large scale flow. Evidence has been gathered suggesting that dust storms over the Asian continent largely account for the springtime turbidity peak in Hawaii. During other times of the year, the effects of sulfate and non-sulfate particles on light scattering are approximately equal.