A method of removing aerosol extinction from HALOE measurements.

Abstract

After the eruption of Mt. Pinatubo in June, 1991, a large number of volcanic aerosols were formed in the lower and middle stratosphere. At these altitudes, the aerosol presence has adversely affected the gas retrievals in the radiometer channels of the Halogen Occultation Experiment (HALOE) because of the excessive extinction of radiation. A radiative transfer model has been developed along with a retrieval algorithm that can correct for these aerosol effects, using information derived from the HALOE gas correlation channels. Because the gas correlation modulation signal is virtually unaffected by the aerosols, accurate aerosol optical depth measurements can be retrieved. It is shown that by utilizing this optical depth information, an "effective" size distribution may be obtained at each retrieval altitude. With knowledge of temperature and water vapor concentration profiles, and NO and $CH\sb4$ channel aerosol optical depth data, it is possible to calculate the average mass fraction of sulfuric acid through an iterative scheme. This information, in conjunction with the optical constants, allows aerosol scattering and absorption to be calculated for any desired wavelength. Because of the nature of the correction sensitivity to the retrieved aerosol parameters, this correction can be made to within ten percent in the water vapor and ozone channels. In addition to providing a correction, the techniques presented provide a useful retrieval of line of sight aerosol optical depth and aerosol composition. It is only possible to determine a size distribution if HALOE information is combined with other measurements. Because larger particles play a more significant role when aerosol volume and surface area are desired, these quantities should be able to be obtained with more accuracy. However, further study is required in this area. The study of all these parameters should allow for the observation of spatial and temporal changes in aerosol characteristics from October, 1991, until HALOE is no longer operable.