M. P. McCormick

Stratospheric Aerosol and Gas Experiment II instrument: a functional description

Design and performance data are presented for the Strato-spheric Aerosol and Gas Experiment II (SAGE II) instrument, which has been developed for the Earth Radiation Budget Satellite (ERBS). SAGE II is designed to monitor globally the vertical distribution of stratospheric aerosols, ozone, water vapor, and nitrogen dioxide by measuring the ex-tinction of solar radiation through the earths atmosphere during the ERBS observatory solar occultations. Solar radiation is reflected from a flat scanning mirror into a Cassegrain telescope, which forms a solar image on the entrance slit of a grating spectrometer. The SAGE II instan-taneous field of view is scanned along the vertical solar diameter by the elevation scan mirror. The entire optical system is contained within an azimuth gimbal that tracks the solar radiometric centroid during the data event. This spectrometer, with help from three interference filters, iso-lates seven spectral wavelengths ranging from 0.385 um to 1.02 um. All seven channels use silicon photodiode detectors operated in the photo-voltaic mode. Detector outputs are multiplexed into a serial data stream for readout by the ERBS telemetry system. Each output is sampled 64 times per second and digitized to 12 bit resolution. SAGE II is a third generation instrument, following the highly successful Stratospheric Aerosol Measurement II (SAM II) and SAGE programs.

Sage II: An overview

Abstract The Stratospheric Aerosol and Gas Experiment II (SAGE II) aboard the Earth Radiation Budget Satellite was launched from shuttle in October 1984. SAGE II is a seven-channel sunphotometer measuring stratospheric aerosols, ozone, water vapor, and nitrogen dioxide during each spacecraft sunrise and sunset. In addition to stratospheric information, mid-tropospheric and higher water vapor, ozone, and aerosol data are being produced in cloud-free regions, and cloud data everywhere else. Aerosol information is being produced at three wavelengths and, together with water vapor data, is providing a global microphysical description of the aerosol.

Multiyear Stratospheric Aerosol and Gas Experiment II measurements of upper tropospheric aerosol characteristics

Measurements of aerosol extinction at wavelengths of 0.525 and 1.02 μm, made by the Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation satellite experiment, have been used to study the global-scale characteristics of the upper tropospheric aerosol. Extinction measurements, in which only aerosols occurred along the optical path, have been separated from those that included high-altitude cloud by examining the wavelength variation of the extinction. Data for the time period October 1984 to May 1991 show that the two main influences on the upper tropospheric aerosol were seasonal lifting of material from below and downward transfer of volcanic aerosol from the stratosphere. Maximum lifting of surface material occurs in local spring in both hemispheres and is observed at all latitudes between 20°N and 80°N and 20°S and 60° S ; the data also show a strong hemispheric asymmetry with more aerosol in the northern hemisphere. Downward transfer of volcanic aerosol is particularly observed poleward of 40° latitude, where a substantial enhancement of material occurs down to altitudes 2-3 km below the tropopause. By comparing tropospheric aerosol concentrations at different times during the period of observation, it has been possible to differentiate the effects of volcanic aerosols from those of the background, or baseline, aerosols. A simple model, based on the ratio of the extinctions at the two measurement wavelengths, has been used to calculate the aerosol mass density and effective radius. It was found that in 1984-1985, approximately 15% of the volcanic aerosol still present from the eruption of El Chichon in 1982, resided in the upper troposphere. Particle sizes for the volcanic aerosol in the lower stratosphere and upper troposphere were of the order of 0.5 μm, while those for the baseline aerosol were about 0.15 μm. Slightly larger aerosol sizes, of the order of 0.25 μm, were observed at altitudes 6-8 km during the springtime enhancements. The low-latitude aerosol enhancements in both hemispheres appear to have the characteristics of material derived from arid surface regions, while the higher-latitude aerosol in the northern hemisphere appears more likely to be derived from anthropogenic sources.

Stratospheric aerosol optical depth observed by the Stratospheric Aerosol and Gas Experiment II: Decay of the El Chichon and Ruiz volcanic perturbations

The decay of the El Chichon perturbation to the optical depth of stratospheric aerosols at 1.02 μm, 0.525 μm, and 0.453 μm is calculated from the Stratospheric Aerosol and Gas Experiment II (SAGE II) data set for the period December 1984 to December 1988. It is found that the perturbed optical depths at middle and higher latitudes of both hemispheres exhibited an exponential decay superimposed by a seasonal oscillation with maximum and minimum occurring in local winter and local summer, respectively. Microphysical processes and variation of the tropopause height alone cannot explain this seasonal change of optical depth. The magnitudes of the exponential component at higher latitudes were, in general, larger than those at lower latitudes. For optical depths in tropical regions, the seasonal oscillations were small and were disturbed by the eruption of Nevado del Ruiz on November 13, 1985. The increase in ratio of optical depth at 0.525 μm to that at 1.02 μm from about 2.0 at the beginning of 1985 to about 3.5 at the end of 1988 indicates the average size of aerosol particles in the stratosphere is diminishing since the eruption of El Chichon. The l/e folding time for El Chichon decay derived from the SAGE II data set is in reasonably good agreement with those derived by other methods.

Background stratospheric aerosol and polar stratospheric cloud reference models

Abstract A global aerosol climatology is evolving from the NASA satellite experiments SAM II, SAGE I, and SAGE II. In addition, polar stratospheric cloud (PSC) data have been obtained from these experiments over the last decade. This paper will describe an updated reference model of the optical characteristics of the background aerosol and propose a new aerosol reference model derived from the latest available data. The aerosol models are referenced to the height above the tropopause. The impact of a number of volcanic eruptions will be described. In addition, a model describing the seasonal, longitudinal, and interannual variations in PSCs will be presented.

Characterization of aerosols from simulated SAGE III measurements applying two retrieval techniques

We investigated the retrieval of aerosol properties and the extinction due to aerosols at the ozone and water vapor channels from simulated measurements at variations of the planned Stratospheric Aerosol and Gas Experiment (SAGE) III aerosol channels. The aerosol quantities surface area, volume, and effective radius are retrieved through the application of two distinct algorithms in the form of the randomized-minimization-search technique (RMST) and the constrained linear inversion (CLI) method. These aerosol quantities are important as inputs in climate, photochemical, and radiative forcing models and are useful in comparing diverse measurements. Ten analytical size distributions fitted to aerosol populations measured in situ are used with a Mie scattering code in conjunction with a Monte Carlo technique to simulate SAGE III measurements. These models consist of variations of prevolcanic and postvolcanic size distributions that exhibit various spectral shapes. Neither the complex components nor the uncertainties of the refractive indices are considered. We developed an objective scheme to estimate the systematic, random, and total uncertainties of each retrieved quantity that considers the contribution of the particles that lie outside the retrieved size range. Results, based on the 10 selected aerosol models, indicate that in the seven-eight SAGE III channel retrievals, both algorithms obtain estimated total errors in the range 8-50% for the surface area with an average total error (R*) of ∼25%; for the volume the range is 5-25% with an R* of ∼12%, and for the effective radius, the range is 6-36% with an R* of 20% though both inversion techniques are applied in different size ranges. The inversion of the six longest channels to study aerosol properties in both the lower stratosphere and the upper troposphere leads to RMST R* values of ∼32, ∼15, and ∼20% and CLI R* values of ∼48, ∼22, and ∼31% for the surface area, volume, and effective radius, respectively. In the seven wavelength retrievals, both algorithms retrieved the extinction coefficients at the unused channel to within their measurement uncertainties except at the 0.385 and 1.550 μm channels located at the tail ends of the SAGE III aerosol extinction spectrum. The calculated extinction due to aerosols at the water vapor channel at 0.940 μm and the ozone channel at 0.600 μm produced R* values of <10 and <15% for both techniques. We have shown that the application of either technique, when properly tailored to the SAGE III system, not only can obtain useful aerosol information in most cases but also can estimate reasonably the extinction due to aerosols at other wavelengths within the SAGE III wavelength range.

Lidar Measurements Of Mount St. Helens Effluents

Lidar measurements of the worldwide movement of stratospheric aerosols produced by the 18 May 1980 eruption of Mount St. Helens are described. Ground-based and airborne measurements show that the layers below 20 km produced by this eruption moved in an easterly direction while those above 20 km moved in a westerly direction. The effluent at jet stream altitudes of 10 to 12 km circled the globe in about 16 days and the effluent at 23 km (the highest altitude recorded) circled the globe in about 56 days. Mass calculations, using backscatter-to-mass conversion models, indicate that approximately 0.5 X 106 metric tons of new stratospheric material were produced by this eruption. Even though this represents a 200% increase in North-ern Hemispheric aerosol, no significant long-term atmospheric temperature change should occur.

Design and performance of the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument

Design and performance data are presented for the Stratospheric Aerosol and Gas Experi-ment II (SAGE II) instrument, which has been developed for the Earth Radiation Budget Satellite (ERBS). SAGE II is designed to monitor globally the vertical distribution of strato-spheric aerosols, ozone, water vapor and nitrogen dioxide by measuring the extinction of solar radiation through the earths atmosphere during the ERBS observatory solar occultations. Solar radiation is reflected from a flat scanning mirror into a Cassegrain type telescope, which forms a solar image on the entrance slit of a grating spectrometer. The SAGE II instantaneous-field-of-view (IFOV) is scanned along the vertical solar diameter by the elevation scan mirror. The entire optical system is contained within an azimuth gimbal which tracks the solar radiometric centroid during the data event. This spectrometer, with help from three interference filters, isolates seven spectral wavelengths ranging from 0.385 micrometers to 1.02 micrometers. All seven channels use silicon photodiode detectors oper-ated in the photovoltaic mode. Detector outputs are multiplexed into a serial data stream for readout by the ERBS telemetry system. Each output is sampled 64 times per second and digitized to 12 bit resolution. SAGE II is a third generation instrument following the highly successful SAM II and SAGE programs.