Rural continental aerosols

FIGURE 8.17 Typical rural continental aerosol number, surface, and volume distributions. 

The mass distribution of continental aerosol not influenced by local sources has a small accumulation mode and no nuclei mode. The PM10 concentration of rural aerosols is around 20 pg m-3. 

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A size distribution may refer to number density, volume, mass, or any other property of the aerosol that varies with particle size. The left-hand side of Fig. 7-1 shows somewhat idealized size spectra for the number densities associated with the marine and the rural continental aerosols. The distribution of sea-salt particles that contribute to the marine aerosol is added for comparison. Particle radii and number densities range over many orders of magnitude. In view of the need for a logarithmic representation of the data, it has been found convenient to use the decadic logarithm of the radius as a variable and to define the distribution function by /(log r) = dN/d( log r). A conversion of /(log r) to the more conventional form /(r), if needed, can be done by means of the rule d(log r) = dr/ r In 10) with In 10 = 2.302, which yields... 

Fig. 7-3. Average volume size distributions for continental aerosols. [Adapted from Whitby and Sverdrup (1980).] The measurement data were smoothed and idealized by fitting to them additive log-normal distributions. (1) Background aerosol, very clean (2) normal background aerosol (3) background aerosol disturbed by an urban plume (these data from measurements at Goldstone, California). (4) Average urban aerosol (from data taken at Minneapolis, Minnesota, Denver, Colorado, and various locations in California). The dashed curve gives the volume distribution resulting from the number density distribution for the rural continental aerosol shown in Fig. 7-1. The integrated volumina, given by the area underneath each curve, are shown in the insert.

Fig. 7-19. Distribution of water-soluble (ws), organic solvent-soluble (os), and insoluble mass fractions associated with the rural continental aerosol at Deuselbach, West Germany, according to measurements of Winkler (1974). The organic fraction comprises material soluble in cyclohexane, ether, acetone, and part of the methanol-soluble fraction. The uncertainty range assumes that 0-40% of the methanol-soluble fraction contains organic compounds, and the remainder is due to inorganic salts. The water-soluble fraction of organics averages about 0.66 for all size ranges combined.

Fig. 7-20. Mass-size distribution (A log r = 0.303) of individual anions and cations associated with the rural continental aerosol at Deuselbach, West Germany (nequiv/m3, = nmol/m3 times the ionic charge number). Left Individual contributions. Right Balance between cations and anions. A cation deficit must be balanced by protons leading to a corresponding acidity. [Data from Mehlmann (1986).]...

A fairly complete characterization of organic compounds in the lipid, that is, the solvent-soluble fraction of the rural continental aerosol, has been achieved by Simoneit and Mazurek (1982 Simoneit, 1984). They... 

Finally, Simoneit and Mazurek (1982) described n-fatty alcohols as constituents of rural continental aerosols. They ranged from Ci0 to C34 with a strong even-to-odd carbon number preference. The distribution generally favored the higher homologues, with a maximum in the range C26-C30. This... 

Fig. 8-7. Washout coefficients according to Slinn and Hales (1971) are shown in curves A and B (left-hand scale). They are based on rain drop size spectra of Zimin (1964) with r,max = 0.2 and 1 mm, respectively, and a precipitation rate of 10 mm/h (10 kg/m2 h). Curve C represents the first term and curves D and E the second term in the bracket of Eq. (8-6) in nonintegrated form (right-hand scale applies). These latter three curves are based on the mass-size distribution for the rural continental aerosol in Fig. 7-3. Curve C was calculated with eA(r2)=l for r2>0.5 ra and eA < I for r2<0.5(im, decreasing linearly toward zero at r2 = 0.06 p.m. This leads to eA = 0.8. Curves D and E were obtained by using the washout coefficients of curves A and B, respectively. Note that below-cloud scavenging (curves D and E) affect only giant particles, whereas nucleation scavenging (curve C) incorporates also submicrometer particles.

FIGURE 9 Left distribution of particle number concentration versus aerodynamically equivalent radius for the rural continental aerosol. Right The corresponding size distributions for the concentrations of particle surface and volume. The size distribution of mass concentration is nearly equivalent to that of volume. [From Zellner, R., ed. (1999). Global Aspects of Atmospheric Chemistry, Steinkopff/Springer, Darmstadt, Germany.]... 

Fig. 7-1. Left Idealized particle size distributions for the rural continental and the maritime aerosols. The distribution of sea-salt particles that contribute to the maritime aerosol is shown separately. The transition from the rural to the urban aerosol is indicated. Right Determination of remote tropospheric aerosol size distribution by a combination of instrumental techniques. [ Single-stage and free-wing impactors, O—O set of five double-stage impactors singleparticle optical scattering analyzer these data were obtained at the observatory lzana, Tenerife,...

The third section of Table 7-16 shows the relative composition of two continental aerosols to provide a direct comparison with the data in the second part of the table. The first aerosol given represents an average over samples taken at several remote sites in Europe, North America, and Africa (Rahn, 1975b). The second aerosol is the 1974 average obtained by Moyers et al. (1977) at two sites in rural Arizona. The (X)/(A1) ratios for the elements Si, Fe, Na, K, and Ca in the aerosols treated in Table 7-13 are shown at the bottom of that table. To further illustrate the behavior of important elements in the atmospheric aerosol, we show in Fig. 7-23 a number of scatter diagrams. The combination of these data will be discussed below. 

Aerosols in rural areas are mainly of natural origin but with a moderate influence of anthropogenic sources (Hobbs et al., 1985). The number distribution is characterized by two modes at diameters about 0.02 and 0.08 /xm, respectively (Jaenicke, 1993), while the mass distribution is dominated by the coarse mode centered at around 7 /xm (Figure 7.17). The mass distribution of continental aerosol not influenced by local sources has a small accumulation mode and no nuclei mode. The PM lo concentration of rural aerosols is around 20 /xg m . 

Rural aerosols, which are continental aerosols that are influenced to some degree by anthropogenic activities. 

Because a great variety of sources contribute materials to the tropospheric aerosol, it represents a complex mixture of marry substances that additionally depends on the size of the particles. Source characteristics are preserved only in the vicinity of somces. The mixtirre may be divided into three fractiorts water-solirble inorganic salts (electrolytes), water-irrsoluble minerals, and organic compoimds, both soluble in water and insoluble. Table II shows the chemical composition of two boimdary layer aerosols that are typical of marine and of rural continental air. 

Stemmier et al. [124] used humic acid aerosols as a proxy for HULIS (HUmic-Llke Substances) to study the photo-induced conversion of NO2 into HONO, which was previously observed on various organic condensed films [125-128]. The light-induced process was able to release more HONO than was obtained under dark conditions, similar to what was observed for other organic substrates. The amount of the enhancement is not dramatic even if the whole organic aerosol was composed of humic acids, for typical aerosol surface concentrations of 100 pm cm for rural and 1,000 pm cm for urban conditions, only 1.2 and 17 pptv h of HONO would be formed on aerosol surfaces in rural and urban environments, respectively. These values represent upper limits as in reality rural and urban continental aerosol is composed only of 20-50 mass % of organic matter. On the... 

The recent Sulfate Regional Experiment (SURE), promoted by the Electric Power Research Institute, has documented particle air chemistry at a large number of rural and semi rural sites distributed across the continental USA, but with major emphasis on the more polluted northeast. Mueller et. al. (13) have presented frequency distributions of sulfate concentrations measured at SURE sites, and have supported these data with nitrate and ammonium concentrations. From these measurements, distributions of acidic aerosol occurrences can be obtained but as yet are not available. 

SO2 0.2-10 ppb Continental rural Volcanoes, 16 Oxidation of sulfides, 45 Oxidation to 80 aerosol and wet deposition, 140 ... 

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