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Glenn E. Shaw

Abstract

Aerosols in central Alaskan winter air mass system were classified according to size by diffusive separation and light-scattering spectrometry. Particles entering central Alaska from the Pacific Marine environment had number concentrations ranging from 300 to 2000 cm−3 (geometric mean 685 cm−3) and unimodal size spectra, with maximum in number concentration near 1 × 10−6 cm radius.

Air masses entering Alaska from the Eurasian Arctic possessed a factor of two smaller aerosol number concentrations than Pacific Marine systems (e.g., 150–700 cm−3; geometric mean 386 cm−3) but contained a factor of two greater particle volume loading within the fine particle radius range ∼5 × 10−7 < r < 1 × 10−5 cm. The particles in Eurasian Arctic air masses were bimodally distributed, with maxima in the particle size spectra near r = 3 × 10−7 and 5 × 10−6 cm. Sulfur was the predominant element in all cases studied.

A particle depleted region was present in the size spectra obtained for Eurasian Arctic air masses. The deficiency of particles in the 10−6 cm radius range is interpreted as being the result of thermal coagulation taking place between sulfur-rich nuclei (produced at a rate of 10−20 to 10−18 g cm−3 s−1 and in sizes r < 10−6 cm) and “large” (r ∼ 10−5 cm) imported primary particles. The primary particles are in the removal-resistant Greenfield Gap (r ∼ 10−5 cm) and seem to originate in the central Eurasian region.

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Glenn E. Shaw

Multi-wavelength sun photometry has been a subject of interest in meteorology for two and a half centuries. The technique provides a quantitative index that relates to total suspended aerosol in the atmospheric air column above the observer; this aerosol changes continually in the atmosphere in response to many complicated physical processes. When used in conjunction with other aerosol and meteorological measurements, sun photometry has the capability of delineating characteristic features of different air masses and the aerosol sources that affect them.

This paper traces some of the early history of sun photometry and discusses a simple but modern filter wheel sun photometer and several sources of systematic error that have to be reckoned with. We provide examples of aerosol optical extinction spectra acquired at remote and pollution-prone stations with a simple and portable sun photometer.

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Glenn E. Shaw

Abstract

Analysis is presented of 800 measurements of atmospheric monochromatic aerosol optical depth made poleward of ∼65° latitude. The atmosphere of the southern polar region appears to be uncontaminated but is charged with a background aerosol having a mean size of 0.1 μm radius, an almost constant mixing ratio throughout the troposphere, a sea level optical depth (λ = 500 nm) of ∼0.025 and an inferred columnar mass loading of 4-15 × 10−7 g cm−2.

At around the time of spring equinox the northern polar region (all longitudes) is invaded with Arctic Haze, an aerosol showing a strong anthropogenic chemical fingerprint. The optical depth anomaly introduced by this man-caused haze is τ0 ≈ 0.110 and the associated columnar mass loading is ∼1.5 × 10−6 g cm−2. Turbidity measured seven decades ago at the solar observatory at Uppsala (60°N), suggests that Arctic optical depth has been rising at a rate of dτ/dt ≈ 0.01 ± 0.005 per decade.

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Glenn E. Shaw

Abstract

A cloud of aerosol with optical thickness τ ≈ 0.18 (500 nm wavelength), passed over the Hawaiian Islands from late April to early May 1979. Vertical profiles, taken by evaluating the optical extinction coefficient by sun photometry, showed that 80–90% of the aerosol was confined to a 1 km thick layer centered at 3.5–4km altitude. Trajectory analysis at the 500 mb pressure level (∼5 km) indicated that the aerosol probably had its origin in sandstorms in the eastern deserts of Asia nine days before the event. The dust cloud was first observed passing over Japan where sand particles fell out at Nagasaki on 21 April. By the time dust from the sandstone reached Hawaii, it has spread out to ∼1500 km and contained an estimated 1011 g of sand material, mainly in the 0.5 < r < 5.0 μm size range. This episode clearly indicates that substantial quantities of primary aerosol are being transported on global distance scales. The episode is used to obtain a desert aerosol surface particle flux which agrees to an order of magnitude with that suggested by Junge.

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Glenn E. Shaw

Abstract

Ozone content in the atmospheric column above Mauna Loa Observatory, Hawaii, was determined in experiments by measuring the attenuation of sunlight in the visible spectrum Chappuis bands (500 nm<λ<700 nm wavelength). The dominant source of uncertainty in such determinations is the accuracy with which the magnitude of the published Chappuis-band absorption coefficients are known. Values of ozone (for 42 clear days) obtained by using Vigroux's absorption coefficients differed systematically, and were 17.6% smaller than those derived simultaneously in time and space with a Dobson spectrophotometer. The agreement with the Dobson values improves, but is not perfect, when Inn and Tanaka's absorption coefficients are used in the data analysis. Until more accurate information becomes available on ozone absorption coefficients, spectrophotometric-derived values of atmospheric ozone must be considered uncertain to 10–20%.

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Glenn E. Shaw

Abstract

The spectral attenuation of sunlight passing through the atmosphere was determined with the Langley method for 110 clear days and at 11 wavelengths to an accuracy of δτ=±0.002 (τ is the optical thickness) at the Mauna Loa Observatory, Hawaii. Suspended aerosols above the observatory attenuated light by an average of 1.9% (in the vertical direction) at a wavelength of 5000 Å, and the average attenuation varied with wavelength as λ−1.6. Air masses from northerly directions were most turbid, τ¯=0.021±0.015, white those from southwesterly direction were least turbid, τ¯=0.017±0.005. The lowest values of optical extinction varied as λ−1.6 while those from directions of nearest continents varied as λ−2.5; the larger values of wavelength exponent for the continental aerosol is what would be expected for an aerosol cloud that had been carried by the winds from distant continents. It is deduced that aerosol from North America and Asia occasionally reaches the Hawaiian Islands. The explosive eruptions of Augustine Volcano in Alaska (January 1976) caused a perturbation of δτ≈0.01 at λ=500 Å on the optical extinction and decayed with a time constant of five months.

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Glenn E. Shaw

Abstract

Two recent volcanic eruptions (Volcan del Fuego in Guatemala, October 1974, and Ngauruhoe in New Zealand, 19 February 1975) may have populated the stratosphere with sufficient ash to create large-scale stratospheric dust veils. Optical effects on twilight sky radiance, apparently created by the eruption of Fuego, were observed at College, Alaska, three months after the eruption. In lower latitudes the Fuego dust veil effects were observed to peak and then decline one or two months following the eruption. Based on these observations, it is surmised that the Fuego dust veil underwent northerly meridional transport in winter and spring.

On 20 March 1975, however, a strong volcanic twilight was observed at Mauna Loa, Hawaii. Lidar echoes showed the presence of a two-layer dust cloud. It is suggested that this episode may have been caused by dust from Ngauruhoe that was mixed across the Hadley cell circulation system from the Southern Hemisphere.

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Glenn E. Shaw

The arctic atmosphere is the repository for surprisingly high concentrations of pollutants throughout the winter months. The polluted air mass in question includes virtually all the atmosphere above the Arctic Circle and also two great lobes that extend down over Eurasia and North America. In extent, this generally polluted airmass system is about as large as the African continent. The rather severe pollution throughout this airmass system in winter is, to a large extent, a result of the lowered rates of particle and gas removal in this cold, dark, and rather stable system.

The arctic haze possibly has important climatic and ecological and global change implications that are coming under investigation in a number of planned studies.

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Wolfgang E. Raatz and Glenn E. Shaw

Abstract

Noncrustal vanadium and manganese are used as chemical tracers for pollution-derived aerosols (collected over a period of four years in the near-surface air at Barrow, Alaska), in order to investigate tropospheric long-range transport of anthropogenic pollution from midlatitudes to the Alaskan Arctic. The analysis is based upon subjectively identifying characteristic transport pathway types using daily circumpolar weather maps. The transport occurs when the midlatitudinal and Arctic atmospheric circulations manifest quasi-persistent circulation patterns. Rapid transport of aerosols, on the order of 7–10 days, is dominated by quasi-stationary anticyclones and takes place along their peripheries where pressure gradients are relatively strong. The seasonal variation in concentration of the Arctic pollution-derived aerosol is related to the seasonal variation in the occurrence and position of midlatitude blocking anticyclones, of the Arctic anticyclone and of the Asiatic anticyclone. The positions of the major anticyclonic centers and their seasonal variation are responsible for the fact that different source regions contribute to the pollution-derived aerosol during different times of the year. Central Eurasian sources contribute predominantly during winter, Western Eurasian sources during spring, whereas North American and Far Eastern sources contribute little to the Arctic pollution aerosols collected in Alaska.

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Philip B. Russell and Glenn E. Shaw

Abstract

By analysing two sets of atmospheric solar measurements, Laulainen and Taylor conclude that conventional Volz sunphotometry is subject to considerable error arising from apparent day-to-day variation in J 0, the zero-airmass instrument response (“meter deflection”). They determined J 0 from the atmospheric data by using a curve-fitting and extrapolation procedure which is very similar to the familiar graphical “Langley plot” method. However, this method is subject to large errors under conditions of time-varying or horizontally inhomogeneous turbidity. We present an example of such errors and reinspect their data. This indicates that the apparent variation in J 0 was probably in large part (if not completely) due to changing turbidity conditions, rather than an actual change in instrument calibration. Thus their data do not necessarily support their conclusion that “much existing Volz turbidity data are of dubious value.” We emphasize that indiscriminate day-to-day determination of J 0 by curve fitting is risky at best, and recommend the use of artificial calibration sources of known radiance as the most conclusive method to determine the response of a photometer.

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