Aerosol ambient atmospheric

Particles in the atmosphere come from different sources, e.g., combustion, windblown dust, and gas-to-particle conversion processes (see Chapter 6). Figure 2-2 illustrates the wide range of particle diameters potentially present in the ambient atmosphere. A typical size distribution of ambient particles is shown in Fig. 2-3. The distribution of number, surface, and mass can occur over different diameters for the same aerosol. Variation in chemical composition as a function of particle diameter has also been observed, as shown in Table 4-3. 

Further studies are needed to give better dose-response information and to provide a frequency distribution of the population response to oxidants alone and in combination with other pollutants at various concentrations. Such studies should include the effects of mixed pollutants over ranges corresponding to the ambient atmosphere. The mixtures should be carefully characterized to be sure of the effects of trace pollutants on sulfate aerosol formation. The design of such studies should... 

Delumyea, R.G. Chu, L-C. Macias, E.S., Determination of Elemental Carbon Component of Soot in Ambient Aerosol Samples, Atmospheric Environment. 1980, 14, 647-652. 

FIGURE 9.1 Some characteristics of particles and aerosols in ambient atmospheres and industrial settings (adapted from Lapple, 196f). 

THE AMBIENT ATMOSPHERIC AEROSOL consists of liquid and solid particles that can persist for significant periods of time in air. Generally, most of the mass of the atmospheric aerosol consists of particles between 0.01 and 100 xm in diameter distributed around two size modes a coarse or mechanical mode centered around 10- to 20- xm particle diameter, and an accumulation mode centered around 0.2- to 0.8- xm particle diameter (1). The former is produced by mechanical processes, often natural in origin, and includes particles such as fine soils, sea spray, pollen, and other materials. Such particles are generated easily, but they also settle out rapidly because of deposition velocities of several centimeters per second. The accumulation mode is dominated by particles generated by combustion processes, industrial processes, and secondary particles created by gases converting to par-... 

Compositional analyses of size-segregated particles from ambient atmospheric aerosols are vital for understanding the sources and effects of these aerosols. Three challenges exist for analytical chemistry in the next decade ... 

Methods used to determine the strong acid content of aerosol particles in the ambient atmosphere are reviewed. These methods include those for generic determination of strong acid content and those in which the concentrations of individual strong acid species are determined. Difficulties in sampling these species due to their reactivity and occurrence under non-steady-state atmospheric conditions are discussed, and the methods currently used for resolving these difficulties are critically evaluated. 

Al fundamental question about the interpretation of acidic aerosol data is whether researchers can characterize past and current atmospheric concentrations and distributions (spatial and temporal) with sufficient accuracy for studies of their effects on ecosystems and human health. Part of the answer to this question can be provided by a review of the methods that have been used to measure the strong acid content of aerosol particles collected from the atmosphere. This chapter serves as such a review, and, in evaluating analytical procedures, it specifically assesses the ability of each procedure to overcome sampling artifacts, to distinguish between strong and weak acids, to properly partition strong acidity between gas-phase and aero-sol-phase species, and to quantitate strong acidity at the levels at which it is found in the ambient atmosphere. 

In addition to being oxidized by the hydroxyl radical, alkenes may react with the N03 radical as has been described by several investigators (52, 56, 66). Listed in Table I are some of the organic nitrates that have been predicted to be produced via reaction of OH and N03 with isoprene and pro-pene. Analogous compounds would be expected from other simple alkenes and from terpenes such as a- and (3-pinene. Other possible organic nitrates may be produced via the oxidation of aromatic compounds (53, 54) and the oxidation of carbonaceous aerosols (67). Quantitative determination of these species has not been made in the ambient atmosphere. 

PERSONAL MONITORING IS A RELATIVELY NEW CONCEPT in community air pollution measurement research (1-3). This fact is not surprising because most air pollution investigations have been directed toward the characterization of the ambient atmosphere, the observation of pollutant trends, the acquisition of data on chemical kinetic parameters and on the physical properties of aerosols, and the determination of compliance to national and other standards (4). Before the late 1970s, research on personal monitors was primarily conducted in industrial settings (5, 6) because American Confer-... 

An important parameter used to quantify dry deposition processes is the velocity of deposition (Kg) (Chamberlain, 1953). Vg is defined as the downward flux of aerosol or gas to a vegetation or soil surface, normalised to the ambient atmospheric gas or aerosol concentration above that surface. In the case of radionuclide deposition processes flux and concentration are, respectively, measured in units of radioactivity per unit area and volume, hence... 

Consequence modeling, for the purposes of the illustrations given in this chapter, means the prediction of ambient atmospheric concentrations using models for quantifying the release of fluids from containment, and the formation of vapor and liquid aerosol plumes using dispersion models. 

Sheldon started his Ph.D. studies at a time when the field of aerosol science was in its early stages of development. Working with H.F. Johnstone, he focused on how particles in turbulent airflow are deposited on the walls of pipes and ducts. Sheldon made important contributions right from the start he introduced the notion of a stopping distance of a particle injected into stagnant air, and then used this concept to predict particle motion through the viscous boundary layer to the surface. His thesis work laid the foundation for much of the later work on deposition of particles in industrial systems as well as dry deposition from the ambient atmosphere, where turbulent eddies impart velocities normal to the mean flow and enable particles to reach the surface. 

Dry gaseous deposition is a complex process which depends on the physical-chemical properties of the PCBs, characteristics of the adsorbing surface, and environmental conditions (e.g., windspeed). In the ambient atmosphere, dry particulate deposition is predominantly in the form of fine aerosols (<1 pm), which deposit on surfaces by rapid, vibratory (Brownian) diffusion (Holsen and Noll 1992). However, in urban areas, PCBs are associated with course aerosols (>1 pm), and these particulates represent the majority of the dry deposition flux even though PCBs are largely in the vapor phase (Holsen et al. 1991). 

Multicomponent aerosol particles exhibit behavior similar to that of single-component salts. As the ambient RH increases the salt mixture is solid, until the ambient RH reaches the deliquescence point of the mixture, at which the aerosol absorbs atmospheric moisture and produces a saturated solution. A typical set of data of multicomponent particle deliquescence, growth, evaporation, and then crystallization is shown in Figure 10.7 fora KCl-NaCl particle. Note that the DRH for the mixed-salt particle occurs at 72.7% RH, which is lower than the DRH of either NaCI (75.3%) or KC1 (84.2%). 

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

Baier, R.E., 1976. Infra-red spectroscopic analysis of sea fog water residues, ambient atmospheric aerosols and related samples collected during the USNS Hayes cruise off the coast of Nova Scotia, Canada 29 July—12 August, 1975. Calspan Corp. Internal Rep., VA-5788-M-2, 45 pp. 

After the reactor, the SO3 exhausted gas is separated from the organic acid. The exhaust gas, containing small amounts of non-converted SO2, unreacted SO3 and some entrained organic acid, has to be cleaned before emission to ambient atmosphere. The organic aerosol and fine SO3/H2SO4 droplets are separated fi om the exhaust gas flow in an electrostatic precipitator (ESP) and the gaseous SO2 and traces of SO3 gas are washed from the process air in a scrubber by dilute caustic solution, thus producing a mixed sulphite/sulphate solution. 

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