Alan P. Waggoner

Optical characteristics of atmospheric aerosols

Techniques have been developed to make point measurements of particle size, chemical nature, scattering and absorption extinction coefficients. These measurements have been shown to be sufficient to describe the optical or visual effects of trace materials in urban or rural air. These techniques and methods of analysis are described in this document. Conclusions include: scattering extinction and fine particle mass, absorption extinction and graphitic carbon are highly correlated. SO2−4 is usually the dominant scattering species and it occurs both in acid and neutral salt (with NH+4) forms. The role of organic carbon, especially in rural atmospheres appears small.

Chemical properties of tropospheric sulfur aerosols

Abstract Sulfur is widely recognized as an element present in atmospheric aerosol; however, only recently have data been acquired showing the dominance in industrial regions of submicrometer tropospheric aerosol by a family of sulfate compounds ranging from H2SO4 to (NH4)2SO4. It is possible to infer the presence of other molecular forms and oxidation states. The overall picture is as yet qualitative, with semiquantitative evidence showing that both the urban and rural aerosols in the eastern third of the United States consist mainly of impure sulfate compounds containing substantial amounts of water, with metal and organic compounds as trace inclusions. Among chief physico-chemical consequences of the dominance by sulfates are the fundamental nature of hygroscopic growth and predictable variations in refractive index. It is important to emphasize the role of omnipresent impurities and their possible effects.

H2SO4/(NH4)2SO4 background∗ aerosol: Optical detection in St. Louis region

Abstract Atmospheric H 2 SO 4 present as droplets of the free acid and the half neutralized acid, NH 4 HSO 4 mixed with other substances have proved difficult to identify by traditional air sampling methods and ionic analyses. Knowledge of their existence as a major or minor fraction of atmospheric aerosol is important to studies of health effects of air pollution, atmospheric optics, acid rain and to understanding the global sulfur cycle. The in situ method reported here utilizes a light scattering/humidity relation to detect reaction of the aerosol with NH 3 to form the deliquescent salt (NH 4 ) 2 SO 4 . Field measurements in and near Saint Louis, Missouri, confirm that H 2 SO 4 and/or NH 4 HSO 4 are frequently present as a major fraction ( ca. 1/2 or more) of the submicrometer particles, and that (NH 4 ) 2 SO 4 also is frequently found. At one site 35 km W.S.W. of the Arch, over 98 per cent of the measurements were dominated by H 2 SO 4 or its neutralization products and showed no dependence on wind direction or synoptic condition, thus indicating a regional as opposed to urban behavior. While the results to date are qualitative and semi-quantitative in nature, it appears possible to utilize gas phase titration with NH 3 to quantify the method.

Sulfuric Acid-Ammonium Sulfate Aerosol: Optical Detection in the St. Louis Region

Nephelometric sensing of the deliquescence of ammonium sulfate produced by the reaction of sulfuric acid or ammonium bisulfate aerosol with ammonia provides a means for detecting these substances in air. Field experiments show them to be the dominant substances in the submicrometer, light-scattering aerosol in the St. Louis region.

Sulfate Aerosol: Its Geographical Extent in the Midwestern and Southern United States

Sulfate particles (sulfuric acid and its neutralization products with ammonia) dominate the submicrometer-sized, light-scattering component of the aerosol in more than 90 percent of 2850 pairs of humidographic measurements made over a 3-month period at three rural midwestern and southern sites. The nearly continuous optical dominance by sulfate in the aerosol at these spatially varied locations, particularly in the Ozark Mountains, suggests that sulfate is a component of the submicrometer-sized aerosol that is distributed over a large geographical region and is not due to local sources.

Angular truncation error in the integrating nephelometer

Abstract The atmospheric light scattering extinction coefficient measured with an integrating nephelometer has a systematic error resulting from light lost at the angular extremes of the physical integration of scattered light. This error was calculated for typical atmospheric aerosols for the Ahlquist and Charlson instrument (integration range is 8° to 170°). The range of error resulting from angular truncation was from 0 to 22 per cent and depended primarily on the exponent of the Junge panicle size distribution and the large particle size cut-off. The error was essentially independent of the particle refractive index, small particle cut-off below 0.1 μm and the wavelength of light (436–600 nm). The calculated typical error of about 10 per cent agreed with previously published empirical and calculated results.

Measurements of some aerosol properties relevant to radiative forcing on the east coast of the United States

Airborne measurements of aerosol light-scattering efficiencies are presented for a portion of the northeast Atlantic seaboard of the United States during July 1993. The measurements suggest a value for the sulfate light-scattering efficiency in the range 2.2-3.2 m{sup 2} g{sup -1}, which is lower than the value used in recent modeling assessments of the climate impact of aerosols. In general, the sulfate light-scattering efficiency decreased with increasing altitude in a manner consistent with concurrent measurements of aerosol size distributions. Some limited measurements of cloud condensation nuclei and sea-salt particles are also presented. 28 refs., 8 figs., 4 tabs.

Chemical speciation of H2S04—(NH4)2S04 particles using temperature and humidity controlled nephelometry☆

Abstract Measurment of the response of sulfate compounds in aerosol particles tp thermal decomposition at specific relative humidities providies semiquantitative chemical analysis for sulfate compounds and ammonium to sulfate molar ratio. Laborotory data are presented and discussed . Experiments at one Eastern U.S. site show ambient aerosol NH4+/SO4= between 0.5 and 2.0.

Measurement of the Aerosol Total Scatter–Backscatter Ratio

In urban Seattle, simultaneous measurements were made of backscatter using a ruby laser radar and of the scattering portion of extinction using an integrating nephelometer. Both instruments were calibrated allowing separation of molecular and aerosol contributions to the two scatter coefficients. During the period of the experiment, backscatter and the scattering portion of extinction were well correlated at relative humidities less than 70%. For the aerosol, the ratio of backscatter to the scattering portion of extinction was only one-third that predicted from spherical particle Mie calculations for a power law size distribution aerosol.

In-situ, rapid response measurement of H2SO4/(NH4)2SO4 aerosols in urban Houston: a comparison with rural Virginia

Abstract We report measurements taken in Houston, Texas of the chemical composition and degree of hydration of haze particles and compare these results with previously reported measurements taken in rural Virginia. Our in-situ, real time measurements are based on detecting changes in particle light scattering extinction with changes in relative humidity and air temperature. With these methods we can determine fine particle mass and sulfate mass concentrations, and sulfate to ammonium ion molar ratio. In Houston, fine particle sulfate averaged 42 % of fine particle mass and the composition in terms of ammonium to sulfate molar ratio ranged from 0.5 to 2 with strong diurnal variation. The particles were most acid during the period 1500 to 2000 and neutral during 0200 to 0900. About 1 3 of the time the particles were droplets supersaturated in terms of salt content. In Virginia, the particles were on average more acidic and contained more water than those in Houston, but the particles were never observed to be supersaturated solution droplets. The difference in the hygroscopic behavior of the particles at the two sites is consistent with differences in both particle chemistry and ambient relative humidity.