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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 H 2 SO 4 ‐H 2 O mixture, with H 2 SO 4 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 H 2 SO 4 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 H 2 SO 4 ‐H 2 O 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 R eff , the effective radius. In the first month after the eruption, although particle numbers increased greatly, R eff outside the tropical core was similar to preeruption values of ∼0.1 to 0.2 μm, because numbers of both small ( r 0.6 μm) particles increased. In the next 3–6 months, extracore R eff increased to ∼0.5 μm, reflecting particle growth through condensation and coagulation. Most data show that R eff continued to increase for ∼1 year after the eruption. R eff 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 R eff , especially for R eff > 0.4 μm. Extinction spectra computed from in situ size distributions are consistent with optical depth measurements; both show initial spectra with λ max ≤ 0.42 μm, thereafter increasing to 0.78 ≤ λ max ≤ 1 μm. Not until 1993 do spectra begin to show a clear return to the preemption signature of λ max ≤ 0.42 μm. The twin signatures of large R eff (>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 and SAGE midvisible optical thickness products. We discuss possible reasons for these differences and how they might be resolved.
Russell et al. (Thu,) studied this question.