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Austin W. Hogan

Abstract

The concentration of sulfate ion in precipitated water was examined in instances when the type of cloud and its base temperature were well defined, and the precipitating cloud isolated from continental surface air. The concentration of sulfate ion in precipitated water was inversely related to the concentration which could have been achieved if sulfur precipitation had been complete. The precipitated concentration of sulfate ion was proportional to the natural logarithm of cloud base temperature.

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Austin W. Hogan

Abstract

A series of aerosol and meteorological observations have been obtained for the trade wind region, from a meteorological tower on the north shore of Oahu. These observations have been supported by aircraft experiments, and observations at the Mauna Loa Observatory.

The mean surface concentration found in trade winds is 242 cm−3, the median 262 cm−3, but extreme variation over the range 30 to more than 400 cm−3 occurs. This extreme variation is similar in amplitude to the spatial variation in the vicinity of cumulus clouds, and in agreement with the periodicities in aerosol concentration described by Blanchard.

A similar aerosol concentration, of much smaller particles, is found above the trade inversion. This aerosol exhibits less short-term variation than that at the surface, and is the same at the Mauna Loa Observatory as at several miles seaward. This above-inversion aerosol may be representative of the world background tropospheric aerosol.

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Austin W. Hogan

Abstract

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Austin W. Hogan

Abstract

Aerosol number concentrations were measured, using a photoelectric nucleus counter, at three locations in Antarctica. This nucleus counter was of high sensitivity and equipped with a diffusion battery to permit estimation of particle size.

Concentrations of 50 to 100 cm−3 were predominant at the South Pole; concentrations of 100 to 150 cm−3 were most common at Siple Station (78°S, 84°W) where some maritime influence might be present. At both of these stations, concentrations of 500 to 1200 cm−3 very small particles were detected when the polar tropopause lowered.

Concentrations of 50 to 150 cm−3 were found at Ross Island with continental winds; concentrations of 300 cm−3 accompanied maritime winds.

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Austin W. Hogan

Abstract

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Austin W. Hogan

Abstract

Ice crystal precipitation was observed and the crystals replicated, at the South Pole during January and February 1974. The crystals were of columnar form. These columns were hollow or prismatic, and sometimes were in the form of combinations of bullets. These combinations were very fragile, disintegrating into individual bullets upon impaction. Smaller “diamond dust” crystals were observed on two occasions.

NOAA-NWS radiosonde data showed that the air was supersaturated with respect to ice at 650 to 600 mb (i.e., just above the surface) throughout the period. Ice crystal precipitation was only observed at the surface when cirrus bands were present at higher altitudes. It is likely that ice crystals, descending from the cirrus only a short distance above, grew to the larger columnar crystals while falling through the moist layer. As these layers were able to remain saturated, without precipitating or forming ice fogs or clouds at temperatures of −35°C, heterogeneous freezing nuclei were probably absent at these levels throughout the period.

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Austin W. Hogan

Abstract

A mensuration and description method is proposed that allows objective and simultaneous expression of the physical properties of snow crystals, rime drops, and aerosol particles. It is based on the statistics of Hatch and Choate, which are commonly used to describe “dusts” and supermicrometer aerosol particles with irregular shape. These statistics simplify integration of physical properties of snowflakes over their diameter distribution and facilitate expression of simultaneous formulas that can be used to calculate the contribution of riming to airborne snow mass and the contribution of rime nuclei to airborne snow chemistry.

These formulated expressions are combined with tabulations of size properties of snowflakes, cloud droplets, and aerosol to provide the skeleton of a calculation program. These calculations can transform the area, mass, or number concentration of snow fluxes into other parameters when one flux parameter and the snow-crystal habit are specified. Other size-dependent parameters are easily inserted in this skeleton to facilitate estimation of physical properties of suspended snow.

The cross-sectional area-to-mass ratio of airborne snow, with respect to the distribution of particle diameters, is relatively large when compared to liquid precipitation particles. The cross-sectional area-to-mass ratio provides an objective parameter for comparison of snowflake properties with respect to crystal type. Several figures are presented that show the variation of airborne snow area, mass contribution by rime, and snow chemistry as a function of crystal type and diameter distribution.

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Austin W. Hogan
and
Stephen Barnard

Abstract

Total aerosol concentrations have been observed twice daily at the South Pole since January 1974, using a Pollak photoelectric nucleus counter. The aerosol size distribution has been approximated using the Rich diffuser denuder and Spurny membrane filter techniques.

These observations show the seasonal variation in aerosol concentration at the surface to be of large relative amplitude. The concentrations reach a very low value, with monthly averages of less than 15 cm−3, beneath the strong nocturnal inversion in winter. The mean concentration begins to rise near the time of astronomical sunrise, and attains monthly average values of 100–200 cm−3 during summer months.

Variations in aerosol concentration and size accompany meteorological events. Extreme sky clarity and subsequent lowering of temperature and strengthening of the near-surface inversion in winter months are accompanied by nearly unmeasurable aerosol concentrations of 3–5 cm−3; winter warming is sometimes accompanied by increases in aerosol concentration to 50 cm−3.

Frontal passage aloft (at the 500 mb level) during summer months, and/or strong subsidence of dry air, is often accompanied by large increases in aerosol in the smaller size classes. Advection of warm moist air from the Wedell Sea and frontal passage at lower levels are accompanied by an increase in aerosol particles in the larger size classes. Several case studies are included to illustrate these phenomena.

Recent flight data indicates that the moist layer, just above the surface inversion at 650–550 mb over the South Pole, is relatively rich in particulate material. Mixing of this air downward can then be the source of increased aerosol concentration at the surface and increased particle precipitation to the ice cap.

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Benjamin Y. H. Liu
,
David Y. H. Pui
,
Austin W. Hogan
, and
Ted A. Rich

Abstract

The photoelectric condensation nucleus counter of Pollak with convergent light beam has been compared with an electrical aerosol detector using monodisperse aerosols with particle diameters between 0.025 and 0.15 μm, particle concentrations between 127 and 260,800 cm−3, and particles of two different chemical constituencies, e.g., NaCl and material volatilized from a heated nichrome wire. Very good agreement has been obtained. The discrepancy between these two methods was found to be less than 9% at concentration levels below 104 particles cm−2 and 17% at 2.5 × 105 particles cm−3. This discrepancy is well within the combined uncertainties in the two independent aerosol concentration measuring methods.

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Ramon J. Cipriano
,
Duncan C. Blanchard
,
Austin W. Hogan
, and
G. Garland Lala

Abstract

A laboratory model of a breaking wave or whitecap was constructed, and the aerosol produced by it was investigated intensively. Submicron- and even Aitken-sized particles were produced: the presence of salt particles of mass <10−17 g(r<0.01 μm) could be inferred. The evidence strongly suggests that the submicron fraction is composed of film drops, derived primarily from bubbles larger than 1 mm in diameter. The shape of the CCN spectrum and overall mass distribution of the model-produced aerosol were similar to what is observed in clean marine air.

Whether or not the production rate of such small particles is globally significant when the model results are applied to the oceans depends to a large extent on the set of assumptions one makes concerning aerosol residence time and fraction of sea surface covered by whitecaps. However, there are realistic choices of these parameters which suggest that appreciable fractions of both CCN and CN in the lower marine atmosphere are produced directly by the sea.

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