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FIGURE 8.14 Seasonal concentrations of PM2 5 and PM10 in four cities. The data show the lowest, lowest tenth percentile, lowest quartile, median highest quartile, highest tenth percentile, and highest daily average value. The dashed line shows the level of the annual PM2 5 standard in the United States.

8.2.2 Marine Aerosols

In the absence of significant transport of continental aerosols, particles over the remote oceans are largely of marine origin (Savoie and Prospero 1989). Marine atmospheric particle concentrations are normally in the range of 100-300 cm"3. Their size distribution is usually characterized by three modes (Figure 8.15): the Aitken (Dp <0.1 pm) the

Hoppel et al. (1989) Haaf and Jaentcke (1980) M^szdros and Vlssy (1974) Model Distribution

FIGURE 8.16 Measured marine aerosol volume distributions and a model distribution used to represent average conditions.

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FIGURE 8.16 Measured marine aerosol volume distributions and a model distribution used to represent average conditions.

accumulation (0.1 < Dp < 0.6 |im), and the coarse (Dp > 0.6 pm) (Fitzgerald 1991). Typically, the coarse-particle mode, representing 95% of the total mass but only 5-10% of the particle number (Figure 8.16), results from the evaporation of seaspray produced by bursting bubbles or wind-induced wave breaking (Blanchard and Woodcock 1957; Monahan et al. 1983). Typical seasalt aerosol concentrations in the marine boundary layer (MBL) are around 5-30 cm"3 (Blanchard and Cipriano 1987; O'Dowd and Smith 1993). For a comprehensive treatment of seasalt aerosols, we refer the reader to Lewis and Schwartz (2005).

Figures 8.15 and 8.16 show number and volume aerosol distributions in clean maritime air measured by several investigators (Meszaros and Vissy 1974; Hoppel et al. 1989; Haaf and Jaenicke 1980; De Leeuw 1986) and a model marine aerosol size distribution. The distributions of Hoppel et al. (1989) and De Leeuw (1986) were obtained at windspeeds of less than 5 m s"1 in the subtropical and North Atlantic, respectively. The distribution of Meszaros and Vissy (1974) is an average of spectra obtained in the South Atlantic and Indian Oceans during periods when the average windspeed was 12 m s"1. It is difficult to determine the extent to which the differences in these size distributions are the result of differences in sampling location and meteorological conditions such as windspeed (which affects the concentrations of the larger particles), or to uncertainties inherent in the different measurement methods.

8.2.3 Rural Continental Aerosols

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 pm, respectively (Jaenicke 1993), while the mass distribution is dominated by the coarse mode centered at around 7 pm (Figure 8.17).

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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.

8.2.4 Remote Continental Aerosols

Primary particles (e.g., dust, pollens, plant waxes) and secondary oxidation products are the main components of remote continental aerosol (Deepak and Gali 1991). Aerosol number concentrations average around 1000-10,000 cm-3, and PM10 concentrations are around 10 pg m 3 (Bashurova et al. 1992; Koutsenogii et al. 1993; Koutsenogii and Jaenicke 1994). For the continental United States PM10 concentrations in remote areas vary from 5 to 25 pg m"3 and PM2.5 from 3 to 17 pg m"3 (U.S. EPA 1996). Particles smaller than 2.5 pm in diameter represent 40-80% of the PM10 mass and consist mainly of sulfate, ammonium, and organics. The aerosol number distribution may be characterized by three modes at diameters 0.02, 0.1, and 2 pm (Jaenicke 1993) (Figure 8.18).

8.2.5 Free Tropospheric Aerosols

Background free tropospheric aerosol is found in the mid- and upper troposphere above the clouds. The modes in the number distribution correspond to mean diameters of 0.01 and 0.25 (Jaenicke 1993) (Figure 8.19). The middle troposphere spectra typically indicate

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