HR: 1340h
AN: A23B-0953    [Abstracts]
TI: The Truth about Stratospheric Aerosols: Key Results from SPARC`s Assessment of Stratospheric Aerosol Properties
AU: * Thomason, L W
EM: l.w.thomason@nasa.gov
AF: NASA Langely Research Center, Mail Stop 475, Hampton, VA 23681 United States
AU: Peter, T
EM: Thomas.Peter@ethz.ch
AF: Institute for Atmospheric and Climate Science, Hoenggerberg HPP L8.2, Zuerich, CH-8093 Switzerland
AB: Given the critical role it plays in ozone chemistry, the Assessment of Stratospheric Aerosol Properties (ASAP) has been carried out by the WCRP project on Stratospheric Process and their Role in Climate (SPARC). The objective of this report was to present a systematic analysis of the state of knowledge of stratospheric aerosols including their precursors. It includes an examination of precursor concentrations and trends, measurements of stratospheric aerosol properties, trends in those properties, and modeling their formation, transport, and distribution in both background and volcanic conditions. The assessment found that the dominant nonvolcanic stratospheric aerosol precursor gases are OCS, SO2, and tropospheric aerosol. Therefore, though SO2, human-related activities play a significant role in the observed background stratospheric aerosol. There is general agreement between measured OCS and modeling of its transformation to sulfate aerosol, and observed aerosols. However, there is a significant dearth of SO2 measurements, and the role of tropospheric SO2 in the stratospheric aerosol budget - while significant - remains a matter of some guesswork. The assessment also found that there is basic agreement between the various data sets and models particularly during periods of elevated loading. However, at background levels significant differences were found that indicate that substantial questions remain regarding the nature of stratospheric aerosol during these periods particularly in the lower stratosphere. For instance, during periods of very low aerosol loading significant differences exist between systems for key parameters including aerosol surface area density and extinction. At the same time, comparisons of models and satellite observations of aerosol extinction found good agreement at visible wavelengths above 20-25 km altitude region but are less satisfactory for infrared wavelengths. While there are some model short-comings relative to observations in the lower stratosphere, it seems likely that the space-based data sets underestimate, perhaps significantly, aerosol surface area density in the lower stratosphere in low loading periods. Since the beginning of systematic stratospheric aerosol measurements there have been three periods with little or no volcanic perturbation, though only the period from 1999 onwards can be confidently identified as free of volcanic aerosols. The other periods (late 1970's and late 1980's) are too short in duration to evaluate, given the complex variability observed and the period in the late 1980's likely did not reached a stable non-volcanic level. Trends with their uncertainties computed from the late 1970's to the current period completely encompass a value of zero.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0340 Middle atmosphere: composition and chemistry
DE: 0370 Volcanic effects (8409)
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005