Global to microscale evolution of the Pinatubo volcanic aerosol derived from diverse measurements and analyses

Abstract

We assemble data on the Pinatubo aerosol from space, air, and ground measurements, develop a composite picture, and assess the consistency and uncertainties of measurement and retrieval techniques. Satellite infrared spectroscopy, particle morphology, and evaporation temperature measurements agree with theoretical calculations in showing a dominant composition of H2SO4‐H2O mixture, with H2SO4 weight fraction of 65–80% for most stratospheric temperatures and humidities. Important exceptions are (1) volcanic ash, present at all heights initially and just above the tropopause until at least March 1992, and (2) much smaller H2SO4 fractions at the low temperatures of high‐latitude winters and the tropical tropopause. Laboratory spectroscopy and calculations yield wavelength‐ and temperature‐dependent refractive indices for the H2SO4‐H2O droplets. These permit derivation of particle size information from measured optical depth spectra, for comparison to impactor and optical‐counter measurements. All three techniques paint a generally consistent picture of the evolution of Reff, the effective radius. In the first month after the eruption, although particle numbers increased greatly, Reff outside the tropical core was similar to preeruption values of ∼0.1 to 0.2 μm, because numbers of both small (r < 0.2 μm) and large (r > 0.6 μm) particles increased. In the next 3–6 months, extracore Reff increased to ∼0.5 μm, reflecting particle growth through condensation and coagulation. Most data show that Reff continued to increase for ∼1 year after the eruption. Reff values up to 0.6–0.8 μm or more are consistent with 0.38–1 μm optical depth spectra in middle to late 1992 and even later. However, in this period, values from in situ measurements are somewhat less. The difference might reflect in situ undersampling of the very few largest particles, insensitivity of optical depth spectra to the smallest particles, or the inability of flat spectra to place an upper limit on particle size. Optical depth spectra extending to wavelengths λ > 1 μm are required to better constrain Reff, especially for Reff > 0.4 μm. Extinction spectra computed from in situ size distributions are consistent with optical depth measurements; both show initial spectra with λmax ≤ 0.42 μm, thereafter increasing to 0.78 ≤ λmax ≤ 1 μm. Not until 1993 do spectra begin to show a clear return to the preemption signature of λmax ≤ 0.42 μm. The twin signatures of large Reff (>0.3 μm) and relatively flat extinction spectra (0.4–1 μm) are among the longest‐lived indicators of Pinatubo volcanic influence. They persist for years after the peaks in number, mass, surface area, and optical depth at all wavelengths ≤1 μm. This coupled evolution in particle size distribution and optical depth spectra helps explain the relationship between global maps of 0.5‐ and 1.0‐μm optical depth derived from the Advanced Very High Resolution Radiometer (AVHRR) and Stratospheric Aerosol and Gas Experiment (SAGE) satellite sensors. However, there are important differences between the AVHRR ...

Critical surface albedo and its implications to aerosol remote sensing

“…The continental, urban, and maritime aerosol models are composed 165 from specific mixtures of basic components (water-soluble, soot, dust, and oceanic) described by d 'Almeida et al (1991). The background desert aerosol model was adopted from d 'Almeida et al (1991), the stratospheric volcanic aerosol model from Russell et al (1996) and the biomass burn-170 ing aerosol model from Dubovik et al (2002). The herein used aerosol models span a large range of SSA from highly absorbing (e.g.…”

“…The continental, urban and maritime aerosol models are composed of a mixture of basic aerosol properties (water-soluble, soot, dust, and oceanic) from d 'Almeida et al (1991). The background desert aerosol model was adopted from d 'Almeida et al (1991), the biomass burning model from Dubovik et al (2002) and the stratospheric model from Russell et al (1996). study.…”

Optical extinction properties of volcanic stratospheric aerosol derived from ground‐based lidar and Sun photometer measurements

“…measurements by a number of different sensors were found to be consistent with a sulfuric component percentage ranging from 60 to 80% [Russell et al, 1996]. For our purposes the variation in the optical properties due to changes in composition and in the related refractive indices is believed to be negligible and therefore ignored.…”

Ultraviolet optical model of volcanic clouds for remote sensing of ash and sulfur dioxide

“…The ash usually consists of glassy and crystalline material, reflecting its origination from volcanic magma and minerals [Patterson, 1981;Patterson et al, 1983]. The sulfate aerosol consists of sulfuric acid droplets which form after an eruption with a timescale of about 1 month and substantially increase the aerosol optical thickness of the stratosphere after large explosive eruptions [Hofmann and Rosen, 1983;Deshler et al, 1992;Bhartia et al, 1993;Russell et al, 1996]. The ash may be dry or covered with sulfuric acid [Farlow et al, 1981].…”

“…from near the surface to greater than 30 km in the most vigorous eruptions [Pollack et al, 1976;Russell et al, 1996]. The initial eruption consists of a basal gas thrust phase which propels the liquid and gaseous magma hundreds of meters above the volcano, followed by a convective phase in which the thermal energy of the magma heats the air, producing in minutes a tall, cumulonimbus-like cloud which quickly stabilizes hydrostatically [Sparks, 1986].…”