Protecting Visibility in National Parks and Wilderness Areas (1993) / Chapter Skim
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Appendix B: Measurement Methods
Appendix B: Measurement Methods
Pages 325-358
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Gaseous Sulfur Dioxide Sulfur dioxide is the precursor to sulfuric acid (H2SO4) , ammonium bisulfate (NH4HSO4)
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Commercial instrumentation is available, but the instruments often must be mollified to improve their sensitivity in relatively clean air. Oxides of Nitrogen The oxides NO and NO2, collectively referred to as NOX, are important in the generation of nitric acid (HNO3)
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From page 327... ...
et al., 19851; molybdenum oxide annular denuder sampling followed by chemiluminescence detection (Langford et al., 19891; citric acid coated denuder sampling followed by liquid extraction and ion chromatographic analysis (R.B. Norton, pers.
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From page 328... ...
The subsequent evaporation of fog or cloud droplets results in the formation of particles of sulfuric acid or its salts. H2O2 can be measured by several techniques: scrubbing coupled with fluorometric detection ~azrus et al., 19X5, 1986~; diffusion scrubbing with fluorescence detection (Dasgupta et al., 1986; Hwang and Dasgupta, 19861; tunable diode laser absorption spectrometry (Slemr et al., 19861; and glass impinger collection followed by lumino!
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From page 329... ...
Further guidance on the choice of instrumentation to satisfy particular needs can be gained from the references cited in this section. MEASUREMENTS OF ATMOSPHERIC PARTICLES Airborne particle measurements are critical in any visibility program for several reasons: · Fine particles (i.e., smaller than 2.5 ,um in diameter)
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Particle Size Distribution Single-particle electro-optical counters and electrical mobility analyzers are the most commonly used instruments for measuring size clistributions of atmospheric particles. An extensive review of instrumentation is given in ACGTH (19891.
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To relate measurements to size distributions, a refractive index must be assumed. Indeed, measurements have shown that atmospheric particles of a given size can contain an external mixture of particles having two or more refractive indexes (Hering and McMurry, 1991; Covert et al., 1990~.
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From page 332... ...
The differential mobility analyzer (DMA) , also referred to as the electrostatic classifier, can be used to determine atmospheric particle size distributions from measurements of electrical mobility distributions (Liu and Pui, 1975; Knutson and Whitby, 1975; Fissan et al., 1983; Hoppel et al., 1990~.
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Because most atmospheric particle mass consists of particles larger than 0.05 ,um, such impactors can collect virtually all the particulate mass. Impactors are highly versatile devices and cover a wide range of particle sizes and sampling rates.
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The laboratory studies (Wang and John, 1988) involved parallel sampling of ammonium nitrate particles with a Berner impactor (size cuts ranging from 0.08 to 16 ,um)
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involved sampling Los Angeles airborne particles with a Berner impactor located downstream of a nitric acid denuder parallel with a dichotomous sampler with nylon filters. The dichotomous sampler is known to sample particulate nitrate quantitatively John et al., 1988~.
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From page 336... ...
That suggests that gas adsorption on quartz filters can lead to major uncertainty in determining the particulate organic carbon concentrations in samples collected in visibility monitoring networks, where concentrations often are low but can be a large fraction of the fine particle mass. Chemical reactions that involve the deposited particles can also lead to sampling artifacts.
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(1990) found that 50-90% of sampled atmospheric particles deposited within aircraft inlets of various designs.
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Recent work has shown that atmospheric particles are externally mixed to some extent; that is, particles in a given size range comprise of distincttypesofmaterials(CovertandHeintzenberg, 1984;Harrison, 1985; McMurry and Sto~zenburg, 1989; Covert et al., 1990; Hering and McMurry, 1991~. To understand optical properties of the atmosphere containing those particles, detailed knowledge is needed of the composition of the individual particles and the relative concentration of each in the ensemble (White, 1986~.
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The inconsistency of results among laboratories suggests that organic and elemental carbon analysis is a major source of uncertainty in aerosol composition for samples collected in visibility monitoring networks. Water Content of Airborne Particles Water often constitutes a substantial portion of atmospheric particles.
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to measure the sensitivity of particle size to relative humidity for particles in Los Angeles. With this technique, atmospheric particles of a known size are segregated using a differential mobility analyzer (DMA)
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If continuous real-time particulate data had also been available, it would have been possible to obtain valuable information about long-range transport of particulate species and about the effects of such species on visibility impairment at the remote receptor sites. Some work has been done to develop continuous or nearly continuous detectors for some particulate substances, such as sulfur (Gerber et al., 1983)
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The extinction coefficient is the sum of the scattering and absorption coefficients for both gases and particles. Although transmittance only partly characterizes the visibility along a particular sight path, it is an inherent atmospheric aerosol airquality factor and is unaffected by time of day, viewing angle, or characteristics of the natural illumination.
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From page 343... ...
, and, thus, light scattering by gases can be determined largely by the total air pressure. Because the relationship between air scattering and total air pressure is well known (Penndorf, 1957)
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The light-scattering coefficient of atmospheric particles is sensitive to relative humidity, especially for values exceeding 60-70%, because the amount of water absorbed by particles (and therefore particle size) depends on humidity (CharIson et al., 19781.
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If light interacts with more than one particle as it passes through the filter, the apparent absorption coefficient will exceed the correct value. Also, filter techniques are problematic because the optical properties of deposited particles may be different from those of airborne particles, especially if the particles undergo chemical reactions on the filter, either through exposure to gases passing through the filter or through contact with other particles on the surface of the filter.
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From page 346... ...
The apparent contrast between the target and sky is obtained from the radiances and can be used to determine the visual range under ideal measurement conditions. Atmospheric conditions that must be met to determine visual range from contrast measurements are well known (see AlIard and Tombach,
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Even though the apparent target-sky contrast can be measured, the information can be used only to infer atmospheric optical properties under ideal viewing conditions. Because ideal conditions are seldom encountered, slide data have limited utility for visibility monitoring.
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Because the measured light propagates through the open atmosphere, transmissometers measure the optical effects of the unperturbed airborne particle. That ensures that accurate measurements can be Marie at high relative humidities when the particle water content is large and that measurements include the effects of both fine and coarse particles.
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The average atmospheric extinction coefficient over a given sight path can be determined with a transmissometer. However, extinction does not completely characterize visibility.
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Information of that kind, however, provides information on atmospheric optical properties only under restricted measurement conditions that are seldom encountered in practice. Data from these instruments can provide valuable insights into air-quality levels necessary to achieve a desired visual quality for a particular view, but they are of limited use for routine compliance monitoring, because measurements are strongly affected by factors unrelated to air quality.
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Once the three isotopic versions of CO2 are thoroughly mixed, all three behave similarly (although not in precisely the same way because of the slight differences in mass of He carbon atom) , and measurement of the 14C content of a sample provides information about the other two forms of CO2.
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However, selenium and sulfur differ significantly in their chemical behavior under many atmospheric conditions, so their subsequent pathways might be different, Hereby undermining the purpose of the tracer. - Another tracer procedure for following the SO2 formed during coal combustion is the addition of another gaseous component intended to mix with SO2 and to follow its physical path, even if it is not capable of undergoing a chemical change analogous to that of SO2 changing to H2SO4.
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PERCEPTION MEASUREMENT The effect of haze on visibility is an important issue in local and regional air-quality management. In the past, visibility was primarily He concern of aviation and military operations where the most important aspect of visibility was visual range that is, the greatest distance at which an object could be discerned against the background sky.
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The ability to make quantitative connections between optical properties of the atmosphere and human judgments of visibility is still in the developmental stage because of the complexity of the physical and psychological phenomena. To quantify visibility impairment an index must be developed that can incorporate He complexity of those phenomena; He index also must be understandable and useful to the general public and policy makers as well as to scientific researchers.
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. Field Judgments by Passersby Another alternative to field judgments by experienced observers is to conduct on-site interviews of passersby to obtain their judgment of the VAQ at the time and location of the interview.
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(Visual range is defined as the maximal distance at which a large black object can be perceived on the horizon.) Even though viewing conditions were well cleaned and the viewers well-trained, airport visual range was only weakly correlated to judgments of overall VAQ made by experienced observers in a study by MiddIeton et al.
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From page 357... ...
Apparent contrast is computed from measurements of target and background radiances. Equivalent contrast, average landscape contrast, and modulation depth account for the spatial structure throughout a scene and require the measurement of radiance at many positions.
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