Norman C. Ahlquist

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.

Modification, Calibration and a Field Test of an Instrument for Measuring Light Absorption by Particles

A filter-based single-wavelength photometer (Particle Soot Absorption Photometer, PSAP) for measuring light absorption by aerosols was modified to measure at three wavelengths, 467 nm, 530 nm, and 660 nm. The modified and an unmodified photometer were calibrated during the Reno Aerosol Optics Study (RAOS) 2002 against two absorption standards: a photoacoustic instrument and the difference between the extinction and scattering coefficient. This filter-based absorption method has to be corrected for scattering aerosol and transmission changes. A simple function for this was derived from the calibration experiment as a function of transmission and single-scattering albedo. For an unmodified PSAP at typical atmospheric absorption coefficients the algorithm yields about 5–7% lower absorption coefficients than does the usually used method. The three-wavelength PSAP was used for atmospheric measurements both during RAOS and during the New England Air Quality Study (NEAQS).

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.

On the generality of correlation of atmospheric aerosol mass concentration and light scatter

Abstract A series of simultaneous mass concentration and light scattering measurements has been conducted in several locations with widely varying types and amounts of aerosol. The results indicate that the relationship between mass concentration and the extinction coefficient due to scatter is the same for all locations. The approximately constant proportionality implies that the size distribution of the aerosol is relatively reproducible. If the results are put into the form of the product of meteorological range and mass concentration, the mass of material necessary to determine the meteorological range is distributed about a modal value of 1.2 g m −2 .

Monitoring of Atmospheric Aerosol Parameters with the Integrating Nephelometer

The integrating nephelometer of Beuttel and Brewer has been adapted successfully for continuous, objective monitoring of atmospheric aerosols. The paper includes a summary of work done to date with this improved device. In particular, the following points are discussed: (1) the design and operation of the instrument; (2) calibration of the instrument; (3) correlation of nephelometer output to the mass concentration of atmospheric aerosol; (4) correlation of nephelometer output with visual range; (5) the wavelength dependent nephelometer and the regularity of the wavelength dependence of haze; and (6) application of integrating nephelometer to mobile reconnaissance of aerosols. The utility of the integrating nephelometer for monitoring visibility and aerosol mass concentration is also emphasized.

Measurement of the wavelength dependence of atmospheric extinction due to scatter

Abstract Atmospheric haze usually acts as a minus-blue filter for either solar radiation or for viewing objects at a distance. Dark objects are obscured by a blue haze. The measurement of the actual wavelength dependence with a specially designed integrating nephelometer will be described. Three major points will be presented: 1. (1) The significance of the wavelength dependence of atmospheric light scatter; 2. (2) The design of an instrument to measure the wavelength dependence of the extinction coefficient due to scatter; 3. (3) Preliminary data to demonstrate the instrumental capability as well as to show the regularity of aerosol properties.

A New Instrument for Evaluating the Visual Quality of Air

Any device for assessing the visual quality of air must necessarily have an extremely high sensitivity since, at times, the Rayleigh scattering of air molecules is the determining factor. The integrating nephel-ometer of Brewer and Beuttell has been adapted for air quality measurement. It records the volume scattering coefficient and was reliable in over a month of continuous operation. The results may be interpreted in terms of visual range; if so the inherent noise in the system corresponds to more than 500 miles visual range with a 100-second response time. The low cost, simplicity, and sensitivity of the device make it appear useful for rapid evaluation of visual air quality. The design of the instrument will be presented, including both the optical and electronic components. The results of operation of the instrument in Seattle during days of moderate air pollution will be used to illustrate the utility of the device.

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.

Measurement of laboratory and ambient aerosols with temperature and humidity controlled nephelometry

Abstract The relationships between fine particle light scattering extinction coefficient, relative humidity and temperature can be used to quantify sulfate mass concentration and composition for laboratory generated and ambient aerosols. This measurement involves the use of an integrating nephelometer as well as a system for controlling the temperature and relative humidity of the air directly upstream of the nephelometer. Recent improvements in the control of these variables has subsequently enhanced our ability to detect the presence of sulfate compounds in complex atmospheric mixtures. Measurements at two urban sites (Seattle, WA and Riverside, CA) indicate the presence of fine presence of fine particle sulfate compounds mixed with more volatile compounds. Measurements at a ‘background’ site (Ozette Lake, WA) indicate a mixture of acidic sulfate compounds with compounds that are less volatile than (NH 4 ) 2 SO 4 .