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D. J. Hofmann and J. M. Rosen

Abstract

Balloonborne aerosol particle counter measurements are used in studying the stratospheric sulfate layer at Laramie, Wyoming, during 1978 and 1979, a 2-year volcanically quiescent period in which the layer appears to have been in a near equilibrium background state. Subtracting the background aerosol concentration from data obtained during an earlier volcanically active period indicates that the actual decay rate of volcanic aerosol is over 30% faster than one would obtain without this correction. At background, the aerosol size distribution is found to remain remarkably constant between the tropopause and an altitude of ∼25 km, with a sudden transition to a distribution dominated by smaller particles above this altitude. The observations, in some respects, compare favorably with equilibrium one-dimensional stratospheric aerosol models and thus to some extent support the concept of relatively inert tropospheric sulfurous gases, such as carbonyl sulfide and carbon disulfide as the main background stratospheric aerosol sulfur source. Models which incorporate sulfur chemistry are apparently not able to predict the observed variation of particle size with altitude. The 2-year background period is not long enough in itself to establish long-term trends. The eruption of Mt. St. Helens in May 1980 has considerably disrupted the background stratospheric aerosol which will probably not recover for several years. A comparison of the 1978-79 observations with Junge's original measurements made some 20 years earlier, also during a period void of volcanic perturbations, does not preclude a long-term increase in the background stratospheric aerosol level.

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J. M. Rosen and D. J. Hofmann

Abstract

A condensation nuclei (CN) counter has been developed for balloonborne use at ambient pressures in the troposphere and stratosphere. The instrument employs a thermal gradient diffusion cloud chamber to produce particle growth and a photoelectric particle counter for detection. After extensive laboratory tests, the instrument was successfully flown on several balloon soundings over Laramie. The results show a roughly constant mixing ratio in the stratosphere with a CN concentration of about 20 cm−3 at 15 km. The vertical profile of CN in the troposphere displayed concentration fluctuations ranging between 200 and 2000 cm−3 with a definite maximum in the mixing ratio just below the tropopause.

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D. J. Hofmann, J. M. Rosen, and T. J. Pepin

Abstract

Seasonal tropopause height variations are utilized in studying the global stratospheric aerosol burden and possible ozone asymmetries and long-term variations. It is concluded that an intimate relationship between tropopause height and total stratospheric aerosol exists and that seasonal fluctuations in tropopause height may be responsible for at least a portion of the north-south hemisphere total ozone asymmetry and the recent long-term increase trend in total ozone.

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J. M. Rosen, D. J. Hofmann, and S. P. Singh

Abstract

This paper deals with the development of a one-dimensional steady-state stratospheric aerosol model and the subsequent perturbations caused by including the expected space shuttle particulate effluents in the model. Two approaches to the basic modeling effort have been made: in one, enough simplifying assumptions were introduced so that a more or less exact solution to the descriptive equations could be obtained; in the other, very few simplifications were made and a computer technique was used to solve the equations. The most complete form of the model contains the effects of sedimentation, diffusion, particle growth and coagulation. The results indicate that the model is capable of describing many aspects of the stratospheric aerosol layer, such as size distribution and the vertical profile of particles >0.3 μm diameter.

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R. G. Pinnick, J. M. Rosen, and D. J. Hofmann

Abstract

Mie single scattering, absorption, and total extinction calculations for various size distribution and composition models of the stratospheric aerosol are presented. These models are derived from global in-situ measurements made with a balloon-borne photoelectric particle counter during the period December 1971 through July 1974. The models are in agreement with simultaneous aerosol mass measurements made by aircraft filter sampling and by balloon-borne impactor over Laramie, Wyo. Nominal stratospheric aerosol optical depths at 0.53 µm wavelength are 0.005 to 0.007. The maximum stratospheric aerosol absorption cross section at this wavelength is 0.04×10−3 km−1 at 18–20 km altitude, assuming a refractive index imaginary part of 0.01. The predicted 180° backscatter lidar return at the 18–20 km altitude of maximum aerosol mixing ratio is 9% to 17% of the Rayleigh return at a wavelength of 0.6943 µm for the various aerosol models. Measured and predicted lidar returns over Laramie in September 1972 are in good agreement for several of the size distribution and composition models used here. Values of the global stratospheric aerosol albedo at 0.53 µm are 0.002 to 0.003.

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J. M. Rosen, D. J. Hofmann, and K. H. Kaselau

Abstract

Condensation nuclei measurements using a low supersaturation (∼10%) thermal gradient diffusion cloud chamber (TGDCC) and a high supersaturation (∼200%) expansion type instrument were compared on a series of three balloon flights over Laramie, Wyoming. In general the two instruments produced similar vertical profiles but some discrepancies remain unexplained. Agreement between the two would indicate that the low supersaturations used in the TGDCC were still large enough to cause the instrument to count essentially all of the particles present. The TGDCC condensation nuclei (CN) counter was flown at several sites in both the Northern and Southern Hemispheres. The results indicate the existance of a relative maximum in the CN mixing ratio associated with the upper equatorial troposphere and what appears to be a worldwide constant mixing ratio of CN above 20–25 km.

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J. M. Rosen, D. J. Hofmann, and Jean Laby

Abstract

Global surveys of stratospheric and upper tropospheric aerosols have been made using balloon-borne photoelectric particle counters. The natural variability observed at each flight station was small enough so that typical profiles could be identified. Data are presented in the form of latitude cross sections showing lines of constant aerosol mixing ratio. The stratospheric aerosol layer is clearly delineated as well as small transient layers in the troposphere and lower stratosphere. At high and low latitudes the aerosol mixing ratio profile apparently experiences a simple shift in altitude corresponding to the change in local tropopause height.

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D. J. Hofmann, J. M. Rosen, T. J. Pepin, and R. G. Pinnick

Abstract

The results of over 70 balloon soundings, by the University of Wyoming's Atmospheric Physics Group mostly during 1972 and 1973 from a number of stations, are being utilized in a study of the temporal and spatial distribution of the global stratospheric aerosol. This paper deals with the instrumentation, calibration, etc., and with the results of monthly soundings from the Laramie (41°N) station during the approximately two-year period of measurement. This period comprises an interval apparently free of major volcanic activity just prior to the extensive volcanic contributions to the stratospheric aerosol which occurred in late 1974. It thus may be compared to the pre-Agung era and is perhaps as close to the so-called “natural stratospheric background conditions,” if indeed such conditions ever exist, as will likely be attained in the near future.

A simple seasonal variation in the total stratospheric aerosol loading below about 20 km altitude dominates the temporal variation at Laramie, resulting in a maximum in winter and a minimum in summer. A high correlation with tropopause height is observed. The seasonal variation appears to be superimposed on a long-term variation, the nature of which is unknown. Above 20 km, no seasonal variation is evident, and the natural aerosol production processes appear to be nearly in equilibrium with loss processes.

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D. J. Hofmann, J. M. Rosen, J. M. Kiernan, and Jean Laby

Abstract

Balloon-borne measurements of the stratospheric sulfate aerosol from late 1971 to mid-1974, a quiescent period in terms of large volcanic eruptions at stations ranging from 85°N to 90°S, are utilized in a study of the global spatial and temporal variations and for sulfur budget and aerosol source considerations. Similarities in the aerosol loading in the two hemispheres, both spatial and temporal, are evident. An apparent long-term decay in total aerosol appears to have occurred globally during the period suggesting a transient source. SO2 budgetary considerations and model calculations favor a larger injection of SO2 than would be expected from a quasi-static natural exchange of tropospheric air.

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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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