A Unified Picture of Pinatubo Aerosol Global-to Micro-Scale Evolution, From Space, Air, and Ground Measurements
We combine a variety of measurements to develop a composite picture of the post-Pinatubo aerosol and assess the consistency and uncertainties of the measurement and retrieval techniques. Satellite infrared spectroscopy, particle morphology, and evaporation temperature measurements are in accord with theoretical calculations in showing a dominant particle composition of H2SO4-H2O mixture, with H2SO4 weight fraction of 65-80% for most stratospheric temperatures and humidities. Important exceptions are: (1) the presence of volcanic ash at all altitudes initially and in a layer just above the tropopause until at least March 1992, and (2) much smaller H2SO4 weight fractions at the low temperatures attained in high latitude winters and at the tropical tropopause, Laboratory spectroscopy and theoretical calculations yield wavelength- and temperature-dependent refractive indices for the dominant H2SO4-H2O droplets. These in turn permit derivation of particle size spectra from measured optical depth spectra, for comparison to direct measurements by impactors and optical counters. All three techniques paint a generally consistent picture of the evolution of R(sub eff), the effective, or area-weighted, particle radius. In the first month after the eruption, although particle numbers increased by orders of magnitude, R(sub eff) was similar to the preemption value of 0.1 to 0.2 microns, because both small (r less than 0.2 microns) and large (r greater than 0.6 micron particles increased in number. Over the next 3-6 months, R(sub eff) increased to about 0.5 microns reflecting particle growth through condensation and coagulation. In general, R(sub eff) continued to increase for about a year after the eruption. Extinction spectra computed from in situ size distribution measurements are consistent with optical depth measurements, which show spectra with maxima initially at wavelengths of 0.42 microns or less, and thereafter progressively increasing to between 0.78 and 1 micron. Not until 1993 do optical depth spectra begin to show a clear return to the preemption signature of maximizing at the shortest visible wavelengths or in the near UV. This coupled evolution in particle size distribution and optical depth spectra helps explain the relationship between the global maps of 0.5- 1.0- micron optical depth derived from the AVHRR and SAGE satellite measurements.