A stratospheric aerosol climatology from SAGE II and CLAES measurements: 2. Results and comparisons, 1984-1999

Abstract

This paper presents a climatology of the stratospheric aerosol produced with our look-up table (LUT) technique using data from the Stratospheric Aerosol and Gas Experiment (SAGE II) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) instruments. The LUT climatology spans the period from December 1984 to August 1999. It includes values and uncertainties of measured extinction and optical depth at four SAGE II wavelengths (0.385-1.02 μm) and supplements these with results from the CLAES 12.82 μm wavelength during the key period January 1992 through May 1993, when the large particle sizes from the Pinatubo volcanic injection made many SAGE II extinction and optical depth spectra wavelength independent. Also included are retrieved values of effective radius Reff, distribution width σg, surface area S, and volume V. Aerosol retrievals show notable increases in all these parameters after major volcanic eruptions, with increases in Reff lagging increases in the others. Postvolcanic increases in σg, indicative of broader size distributions, are consistent with sudden increases in numbers of both small and large particles that exceed increases in intermediate-size particles. After Pinatubo, retrieved Reff and σg took nearly 5 years to return to pre-eruption values, while slightly shorter recovery times were obtained for S and V. During low-aerosol-loading periods, size distributions narrow in going from the tropical core to higher latitudes at altitudes between 20 and 22 km. Seasonal variations in S and V are observed at high latitudes for several altitude bands, but are less obvious in Reff. With some exceptions, LUT retrievals agree well with most previous climatologies. For example, agreement is good between LUT retrieved surface area and results from balloon-borne measurements, where available. However, there are a few noteworthy discrepancies. For example, values of surface area from principal component analysis of SAGE II data are less than LUT retrievals during near-background periods (e.g., 1989 to mid-1991, and after 1996) and greater than LUT retrievals in the peak of the Pinatubo plume. The smaller LUT-derived surface areas during the Pinatubo peak result from the constraint provided by the CLAES 12.82 μm extinction measurements.