PHYSICAL AND OPTICAL-PROPERTIES OF THE PINATUBO VOLCANIC AEROSOL - AIRCRAFT OBSERVATIONS WITH IMPACTORS AND A SUN-TRACKING PHOTOMETER

As determined in situ by impactor samplers flown on an ER-2 at 16.5- t o 20.7-km pressure altitude and on a DC-8 at 9.5- to 12.6-km pressure altitudes, the 1991 Pinatubo volcanic eruption increased the particle surface area of stratospheric aerosols up to 50-fold and the particle volume up to 2 orders of magnitude. Particle composition was typical o f a sulfuric acid-water mixture at ER-2 altitudes. Ash particles coate d with sulfuric acid comprised a significant fraction of aerosol at DC -8 altitudes. Mie-computed light extinction increased up to 20-fold at midvisible and > 100-fold at near-IR wavelengths. The optical thickne ss measured through the aerosol layer by an autotracking Sun photomete r aboard a DC-8 aircraft at 10.7- to 11.3-km pressure altitudes shows a spectral shape that is similar to the Mie-calculated spectral extinc tion at ER-2 altitudes. Surface area distributions calculated by inver sion of spectral optical depth measurements show characteristics that are similar to the mean surface area distribution resulting from 35 in situ measurements.