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Peter Chýlek, R. G. Pinnick, and V. Srivastava

Abstract

The radar backscattering cross sections of spongy hailstones are considerably influenced by the topological structure of the water-ice mixture. Observational evidence suggests that such an inhomogeneous composite medium can consist of regions of different topologies. We investigate three different kinds of topologies and derive a general mixed topology mixing rule (MTMR). The mixed topology rule is used to calculate the effective dielectric constant of spongy ice as a function of its liquid-water content. This leads to predictions of backscatter cross sections that are in good agreement with measurement.

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R. G. Pinnick, S. G. Jennings, and G. Fernandez

Abstract

Volatile properties of aerosols at an isolated rural site in south-central New Mexico were measured with a light-scattering particle counter equipped with a temperature-controlled heated inlet. Intermittent measurements throughout a one-year period show that submicron particles am highly volatile and display temperature-fractionation characteristics of ammonium sulfate or bisulfate. It is estimated that 60–98% of the submicron aerosol fraction (by mass) is composed of these sulfates. Larger supermicron particles with radii r > 0.4 μm, which are composed mostly of quartz and clay minerals of soil origin, are relatively involatile.

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Peter Chýylek, Peter Damiano, Dat Ngo, and R. G. Pinnick

Abstract

We have developed a simple approximation for the absorption, extinction and scattering coefficients, infrared emittance, single-scattering albedo, and asymmetry factor of water clouds within the 8–12-μm spectral region. The aforementioned cloud-scattering characteristics are obtained as continuous functions of the wavelength λ liquid water content W, effective r eff, and effective v eff of the droplet-size distribution. The accuracy of the proposed approximation is shown to be within 6% for the most types of water clouds when compared to the exact Mie theory calculation and integration over the size distribution. At the same time the required computer time is reduced by factor of 102–103.

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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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R. G. Pinnick, D. M. Garvey, and L. D. Duncan

Abstract

Measurement of cloud drop size distributions with the Knollenberg model FSSP-100 light-scattering counter can lead to artificial bumps or knees in the distributions at ∼0.6 μm and sometimes 2–4 μm radius if the manufacturer-supplied calibration is used. These artifacts are a consequence of the instrument having multivalued or slowly changing response in these regions of particle size. A modified calibration procedure is given that removes these artifacts so that the true droplet size distribution can be obtained. Measurement of slightly nonspherical particles with refractive indexes characteristic of those of atmospheric aerosols will generally lead to undersizing if the FSSP manufacturer-calibration is used, but likely by not more than a factor 2.

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R. G. Pinnick, S. G. Jennings, Petr Chýlek, and H. J. Auvermann

Abstract

A linear relationship, independent of the form of the size-distribution, between extinction at wave-lengths around λ = 11 µm, absorption around λ = 3.8 and 9.5 µm, and liquid water content of atmospheric fogs has been verified using 341 droplet size distribution measurements made under a variety of meteorological conditions. The results suggest that integrated liquid water content along a path in fog can be determined from measurement of CO2 laser (λ = 10.6 µm) transmission along the path, and that liquid water content at a particular point in fog can be inferred from in situ measurement of fog-droplet absorption with a deuterium fluoride laser (λ = 3.8 µm) or a suitably tuned C02 laser (λ = 9.5 µm) spectrophone.

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R. G. Pinnick, D. L. Hoihjelle, G. Fernandez, E. B. Stenmark, J. D. Lindberg, G. B. Hoidale, and S. G. Jennings

Abstract

Vertical structure of the size distribution and number concentration of particulates in atmospheric fog and haze near Grafenwöhr, West Germany, were measured with a balloonborne light-scattering aerosol counter for periods spanning parts of eight days in February 1976. For haze (∼5 km visibility) conditions, little vertical variation is seen; but for low visibility (<1 km) fog conditions, significant vertical increases in concentration of droplets with radii larger than 4 μm are seen over the first 150 m altitude. For haze, the particle size distribution is approximated by a log-normal with geometric mean radius rg≈0.2 μm and geometric standard deviation σg≈1.9. For fog, a bimodal distribution is found with a relative maximum for the larger particle mode at radii of 4 to 6 μm and corresponding values rg≈5 μm and σg≈1.6; the smaller particle mode has values of rg≈0.3 μm to rg≈0.6 μm and σg≈1.8 to σg≈2.5. Liquid water content values for haze and fog range from 10−4 to 0.45 g m−3. Extinction calculated from the particle size distributions shows an approximate 1/λ wavelength dependence for haze conditions, but nearly neutral (wavelength independent) extinction for heavy fog. A correlation exists between calculated particulate extinction and calculated liquid water content, independent of particle size distribution, for the fogs and hues studied.

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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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C. W. Bruce, Y. P. Yee, B. D. Hinds, R. J. Brewer, J. Minjares, and R. G. Pinnick

Abstract

A field adapted spectrophone system employing a tuneable CO2 laser source (over wavelengths 9.2–10.8 μm) was used to measure atmospheric gaseous and particulate absorption at an isolated desert location in the southwestern United States. Measurements were made both for ambient conditions (when aerosol particulate absorption was found to be negligible compared to that of gases) and for dusty conditions resulting from vehicular traffic. For ambient conditions the gaseous absorption coefficient was found to vary with time from expected levels upward by as much as a factor of 3. Sources which could be correlated with increased absorption are discussed. For dusty conditions the spectrophone data were compared with estimates of the absorption coefficient calculated on the basis of measured particle size distributions together with estimates of particle complex indices of refraction. Temporal variation of the absorption coefficient correlated quite closely for the two methods while the calculated values were generally higher. Sampling and calculational uncertainties are suggested as likely to be responsible for this discrepancy.

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