Ambient airborne solids concentrations including volcanic ash at Hanford, Washington sampling sites subsequent to the Mount St. Helens eruption

Abstract

A major eruption of Mount St. Helens occurred on May 18, 1980. Subsequently, airborne solid concentrations were measured as a function of time at two sites within the southern edge of the fallout plume about 211 km east of Mount St. Helens. This ash was a source for investigating area‐wide resuspension. Rain had a variable effect on decreasing airborne concentrations from resuspension. From 0.5 to 1.5 cm of rain were required to significantly reduce airborne solid concentrations through July. For a more aged resuspension source in September, a rain of 2.0 cm had a negligible effect. A monthly average threshold‐wind speed for resuspension was defined as 3.6 m/s. For monthly‐average wind speeds less than the threshold wind speed, monthly‐average airborne concentrations tended to decrease with time. A decrease was recorded between September and October. For this 4‐month time period, the half‐life was on the order of 50 days, corresponding to a weathering rate of 5.1 year−1.