[1] The six longest records of stratospheric aerosol (in situ measurements at Laramie, Wyoming, lidar records at: Garmisch-Partenkirchen, Germany; Hampton, Virginia; Mauna Loa, Hawaii; São José dos Campos, Brazil, and SAGE II measurements) were investigated for trend by (1) comparing measurements in the 3 volcanically quiescent periods since 1970 using standard analysis of variance techniques, and (2) analyzing residuals from a time/volcano dependent empirical model applied to entire data sets. A standard squared-error residual minimization technique was used to estimate optimum parameters for each measurement set, allowing for first order autocorrelation, which increases standard errors of trends but does not change magnitude. Analysis of variance over the 3 volcanically quiescent periods is controlled by the end points (pre-El Chichón and post-Pinatubo), and indicates either no change (Garmisch, Hampton, São José dos Campos, Laramie-0.15 mm) or a slight, statistically insignificant, decrease (Mauna Loa, Laramie-0.25 mm), À1 ± 0.5% yr À1 . The empirical model was applied to the same records plus 1020 nm SAGE II data separated into 33 latitude/altitude bins. No trend in stratospheric aerosol was apparent for 31 of 33 SAGE II data sets, 3 of 4 lidar records, and in situ measurements at 0.15 mm. For Hampton and Laramie-0.25 mm, the results suggest a weak negative trend, À2 ± 0.5% yr À1, while 2 SAGE II data sets (30-35 km, 30°and 40°N) suggest a positive trend of similar magnitude. Overall we conclude that no long-term change in background stratospheric aerosol has occurred over the period 1970-2004.
The Stratospheric Aerosol and Gas Experiment (SAGE) II satellite experiment measures the extinction due to aerosols and thin cloud, at wavelengths of 0.525 and 1.02/am, down to an altitude of 6 km. The wavelength dependence of the extinction due to aerosols differs from that of the extinction due to cloud and is used as the basis of a model for separating these two components. The model is presented and its validation using airborne lidar data, obtained coincident with SAGE II observations, is described. This comparison shows that smaller SAGE II cloud extinction values correspond to the presence of subvisible cirrus cloud in the lidar record. Examples of aerosol and cloud data products obtained using this model to interpret SAGE II upper tropospheric and lower stratospheric data are also shown. INTRODUCTIONThe Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation experiment was designed for the study of stratospheric aerosols and gases [Mauldin et al., 1985]. In the absence of high-altitude cloud, measurements are possible down into the troposphere, with a vertical resolution of 1 km [Kent et al., 1988, 1991]. The principal SAGE II aerosol wavelength is 1.02 /am, and if the atmosphere is clear of clouds, measurements are possible down to the Earth's surface. SAGE II has additional aerosol channels at shorter wavelengths. Use of these wavelengths is limited by increased atmospheric attenuation, but in the absence of clouds, data are obtainable at a wavelength of 0.525 /am, down to an altitude of 6 km. At tropospheric altitudes the extinction may be due to aerosol alone or to a combination of aerosol and thin cloud lying along the optical path from the Sun to the satellite. The effective length of this path in the atmosphere is of the order of 200 km, and the optical properties of the atmosphere along it may be quite inhomogeneous. Several publications have described the statistical properties of the 1-/am extinction values [Kent et al., 1988, 1991], and on the assumption that the higher extinction values are due to the presence of cloud, climatologies of upper tropospheric cloud have been developed [Woodbury and McCormick, 1983, 1986; Chiou et al., 1990]. These climatologies have used an arbitrary extinction level to separate aerosol from cloud, and although statistical comparisons with other cloud databases have been made, direct validation has not been possible. Recently, Kent and McCormick [1991] have discussed the functional relationship between the measured extinction at 1.02/am and at 0.525 /am and interpreted this in terms of a simple extinction model. In this model there is assumed to be little or no wavelength dependence of the extinction due to cloud, whereas that due to aerosol shows a strong wave-1Science and Technology Corporation, Paper number 93JD00340. 0148-0227/93/93 JD-00340505.00 length variation. The purpose of this paper is to present the development of the model for systematic application to the SAGE II data set. A series of airborne measurements made in April 1991 near Califo...
A dual‐polarization lidar aboard the NASA Electra aircraft was used in July 1991 to survey the stratospheric plume from the recent Mt. Pinatubo eruption. Both depolarizing and non‐depolarizing volcanic layers were observed, ranging from 17 to 26 km in altitude. Differences in the depolarization signatures of the layers indicates differences in the composition or physical state of the particles in the layers.
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