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P. B. Russell, J. M. Livingston, T. J. Swissler, M. P. McCormick, W. P. Chu, and T. J. Pepin

Abstract

We present a model of stratospheric aerosol optical properties (refractive index and relative size distribution) and their variability. The model's purposes are 1) providing flexible, efficient means for converting between different aerosol macroproperties (e.g., number or mass concentration, extinction or backscatter coefficient), and 2) quantifying the uncertainties in the conversion process. The latter purpose is achieved by including the results of a sensitivity analysis in the model output products.

The model has three layers, the boundaries of which are defined by tropopause height. Each layer includes a set of empirically based refractive indices and relative size distribution types. In contrast to previous models, this model allows for a range of sulfuric acid and ammonium sulfate refractive indices within the “inner stratospheric” layer (∼2 to 20 km above the tropopause, where the major peak in aerosol mixing ratio occurs). We show that nine different analytical types of size distribution previously recommended for this layer can be parameterized in terms of channel ratio—i.e., the relative size distribution indicator that has been extensively measured by dustsondes.

When so parameterized, all nine inner stratospheric function types give very similar results for the several conversion ratios of interest. This parameterization allows considerable saving of computer time while preserving the flexibility to handle certain types of size distribution change. We show that the inner stratospheric parameterization works because all nine inner stratospheric size distribution types are relatively narrow, and their optical integrals of interest are determined primarily by a size range that is well characterized by channel ratio.

Data from previous measurements made near the tropopause are used to demonstrate that, in that region, size distributions are broader than any of the inner stratospheric types, and that their optical integrals are strongly influenced by particles too large to be characterized by channel ratio. Hence, in the layer near the tropopause, conversion ratios can differ significantly from the inner stratospheric values; consequently, parameterization by channel ratios is not successful.

We develop methods for deriving vertical profiles of several conversion ratios and their uncertainties. We also demonstrate an application of the model: deriving profiles of number density and its uncertainty from satellite-measured profiles of extinction and its uncertainty. A companion paper applies the model to the task of validating satellite measurements of stratospheric aerosol extinction.

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M. P. McCormick, T. J. Swissler, W. P. Chu, and W. H. Fuller Jr.

Abstract

Liday observations of the stratosphere aerosol vertical distribution from October 1971 to July 1976 over midlatitude North America are presented. The results show the sudden increase in the stratospheric aerosol content after the eruption of the Volc´n de Fuego and its subsequent decline. The data are presented in terms of lidar scattering ratio profiles, vertically integrated aerosol backscattering, and rawinsonde temperature profiles. In the months immediately following the volcanic eruption, the lidar-derived aerosol structure is correlated with rawinsonde temperature structure showing the stratospheric temperature minimum occurring near the aerosol layer peak. Analysis of the time dependence of the integrated aerosol backscattering and the tropopause altitude indicates an approximate 0.9 correlation between aerosol loading and tropopause pressure. In addition, the integrated aerosol backscattering also showed some correlation with the minimum stratospheric temperature, i.e., a warmer stratospheric minimum is associated with a relatively higher aerosol loading.

The lidar backscatter data also show that rapid decay of the stratospheric aerosol occurred over the late winter to early spring period and that the summer to fall interval was quite stable. For both winter to summer periods of 1975 and 1976 in approximate 40% decrease in the total integrated aerosol backscattering was observed, while from January 1975 to January 1976 a 65% decrease occurred. For the 19-month period from January 1975 to July 1976 the exponential l/e decay time for the integrated aerosol backscattering was 11.6 months.

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T. J. Swissler, P. Hamill, M. Osborn, P. B. Russell, and M. P. McCormick

Abstract

We compare a series of 85 dustsonde measurements and 84 lidar measurements made in midlatitude North America during 1974–80. This period includes two major volcanic increases (Fuego in 1974 and St. Helens in 1980), as well as an unusually clean, or background, period in 1978–79. An optical modeling technique is used to relate the dustsonde-number data to the lidar-backscatter data. The model includes a range of refractive indices and of size distribution functional forms, to show its sensitivity to these factors. Moreover, two parameters of each size distribution function are adjustable, so that each distribution can be matched to any two-channel dustsonde measurement.

We show how the mean particle radius for backscatter, rB, changes in response to size distribution changes revealed by the dustsonde channel ratio, N r>0.15/N r>0.25. (N r>x is the number of particles with radius larger than x microns.) In early 1975, just after the Fuego injection, N r>0.15/N r>0.25 was ∼3, and the corresponding rB, was ∼0.5 μm; by early 1980, when N r>0.15/N r>0.25 had increased to eight or larger, rB had correspondingly decreased to ∼0.25 μm. Throughout the 1975–76 Fuego decay, rB always exceeded 0.3 μm; thus, lidar backscatter was influenced primarily by particles larger than those that contribute most to N r>0.15 and N r>0.25. This is in accord with the shorter lidar background-corrected, 1/e decay time: 7.4 months, versus 10.4 and 7.9 months for N r>0.15 and N r>0.25.

The modeling technique is used to derive a time series of dustsonde-inferred peak backscatter mixing ratio, which agrees very well with the lidar-measured series. The best overall agreement for 1974–80 is achieved with a mixture of refractive indices corresponding to aqueous sulfuric acid at about 210 K with an acid-weight fraction between 0.6 and 0.85.

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M. P. McCormick, H. M. Steele, Patrick Hamill, W. P. Chu, and T. J. Swissler

Abstract

Sightings of polar stratospheric clouds (PSC's) by the SAM II satellite system during the northern and southern winters of 1979 are reported. PSC's were observed in the Arctic stratosphere at altitudes between about 17 and 25 km during January 1979, with a single sighting in November 1978, and in the Antarctic stratosphere from June to October 1979 at altitudes from the tropopause up to about 23 km. The measured extinction coefficients at 1 μm wavelength were as much as two orders of magnitude greater than that of the background stratospheric aerosol. with peak extinctions up to 10−2 km−1. The PSC's were observed when stratospheric temperatures were very low with a high probability of observation when temperatures were colder than 190 K and a low probability when temperatures were warmer than 198 K. In the Antarctic, clouds were observed in more than 90% of the events in which the minimum temperature was 185 K or less, and were observed in fewer than 10% of the occasions when the temperature was greater than 196 K.

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H. M. Steele, Patrick Hamill, M. P. McCormick, and T. J. Swissler

Abstract

Measurements of the stratospheric aerosol by SAM II during the northern and southern winters of 1979 showed a pronounced increase in extinction on occasions when the temperature fell to a low value (below 200 K). In this paper we evaluate, from thermodynamic considerations, the correlation between extinction and temperature. As the temperature fails, the hygroscopic aerosols absorb water vapor from the atmosphere, growing as they do so. The effect of the temperature on the size distribution and composition of the aerosol is determined, and the optical extinction at 1 μm wavelength is calculated using Mie scattering theory. The theoretical predictions of the change in extinction with temperature and humidity am compared with the SAM II results at 100 mb, and the water vapor mixing ratio and aerosol number density are inferred from these results. A best fit of the theoretical curves to the SAM II data gives a water vapor content of 5–6 ppmv, and a total particle number density of 6–7 particles cm−3.

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M. P. McCormick, Patrick Hamill, T. J Pepin, W. P. Chu, T. J Swissler, and L. R. McMaster

The potential climatological and environmental importance of the stratospheric aerosol layer has prompted great interest in measuring the properties of this aerosol. In this paper we report on two recently deployed NASA satellite systems (SAM II and SAGE) that are monitoring the stratospheric aerosol. The satellite orbits are such that nearly global coverage is obtained. The instruments mounted in the spacecraft are sun photometers that measure solar intensity at specific wavelengths as it is moderated by atmospheric particulates and gases during each sunrise and sunset encountered by the satellites. The data obtained are “inverted” to yield vertical aerosol and gaseous (primarily ozone) extinction profiles with 1 km vertical resolution. Thus, latitudinal, longitudinal, and temporal variations in the aerosol layer can be evaluated. The satellite systems are being validated by a series of ground truth experiments using airborne and ground lidar, balloon-borne dustsondes, aircraft-mounted impactors, and other correlative sensors. We describe the SAM II and SAGE satellite systems, instrument characteristics, and mode of operation; outline the methodology of the experiments; and describe the ground truth experiments. We present preliminary results from these measurements.

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P. B. Russell, M. P. McCormick, T. J. Swissler, J. M. Rosen, D. J. Hofmann, and L. R. McMaster

Abstract

A large satellite validation experiment was conducted at Poker Flat, Alaska, 16–19 July 1979. Instruments included the SAM II and SAGE satellite sensors, dustsondes impactors, a fitter collector and an airborne lidar. We show that the extinction profiles that were measured independently by SAM II and SAGE agree with each other. We then use a generalized optical model (which agrees with the Poker Flat optical absorption and relative size distribution measurements) to derive extinction profiles from the other measurements. Extinction profiles thus derived from the dustsonde, fitter and lidar measurements agree with the satellite-measured extinction profiles to within the combined uncertainties. (Individual 1 σ uncertainties are, at most heights, roughly 7 to 20% each for the satellite, dustsonde and filter measurements, 30 to 60% for the lidar measurements, and 10 to 20% for the process of converting one measured parameter to another using the optical model.)

The wire impactor-derived results are also consistent with the other results, but the comparison is coarse because of the relatively large uncertainties (±35% to a factor of 4) in impactor-derived mass, extinction, N 0.15 and N 0.25 (Nx is the number of particles per unit volume with radius greater than x μm.) These uncertainties apply to background stratospheric aerosol size distributions, and result primarily from relatively small uncertainties (±8 to ±20% for confidence limits of 95%) in radii assigned to impacted particles, combined with the steepness of background size distributions in the radius range that contributes most to mass, extinction, N 0.15 and N 0.25. Polar nephelometer-measured asymmetry parameters (0.4 to 0.6) agree with a previous balloon photometer inference, but are significantly less than the value (∼0.7) obtained from Mie scattering calculations assuming either model or measured size distributions.

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P.B. Russell, M.P. McCormick, T.J. Swissler, W.P. Chu, J.M. Livingston, W.H. Fuller, J.M. Rosen, D.J. Hofmann, L.R. McMaster, D.C. Woods, and T.J. Pepin

Abstract

We show results from the first set of measurements conducted to validate extinction data from the satellite sensor SAM II. Dustsonde-measured number density profiles and lidar-measured backscattering profiles for two days are converted to extinction profiles using the optical modeling techniques described in the companion Paper I (Russell et al., 1981). At heights ∼2 km and more above the tropopause, the dustsonde data are used to restrict the range of model size distributions, thus reducing uncertainties in the conversion process. At all heights, measurement uncertainties for each sensor are evaluated, and these are combined with conversion uncertainties to yield the total uncertainty in derived data profiles.

The SAM II measured, dustsonde-inferred, and lidar-inferred extinction profiles for both days are shown to agree within their respective uncertainties at all heights above the tropopause. Near the tropopause, this agreement depends on the use of model size distributions with more relatively large particles (radius ≳0.6 μm) than are present in distributions used to model the main stratospheric aerosol peak. The presence of these relatively large particles is supported by measurements made elsewhere and is suggested by in situ size distribution measurements reported here. These relatively large particles near the tropopause are likely to have an important bearing on the radiative impact of the total stratospheric aerosol.

The agreement in this experiment supports the validity of the SAM II extinction data and the SAM II uncertainty estimates derived from an independent error analysis. Recommendations are given for reducing the uncertainties of future correlative experiments.

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