Rain Scavenging of Tephra Aerosols from Mount St. Helens 1980 Eruptions

Abstract

Mount St. Helens had three major eruptions in 1980: 18 May, 25 May, and 12 June. Tephra in the atmosphere from these eruptions was traced by analyzing the particulate matter from weekly rain collections at 62 NADP (National Atmospheric Deposition Program) sites across the United States. The particulate matter was on 0.45 micron pore-sized filters (47 mm diameter) in amounts of 0-214 mg. Identification of tephra in the particulate matter was made by x-ray diffraction identification of feldspar (70% of the tephra) and chemical analyses compared to a standard tephra. Tephra was identified at sites in the path defined by observable amounts on the ground and outside and beyond this path. Dilution of the tephra by other aerosols varied from 20% new Mount St. Helens to about 300% at downwind sites, which were two days and 3000 km from Mount St. Helens. Atmospheric cleanup of the tephra at a given site occurred in 1–2 weeks, sometimes in a few days, as measured by the particulate matter in rain. Particulate matter captured in rains before the eruptions was mainly alpha quartz, feldspar, illite, kaolinite and organics with an average flux for the United States of 112 kg ha−1 per week.