Coarse mode, atmospheric particles

Similar data for sulfate have been reported in many studies. Figure 9.36, for example, shows overall average sulfate distributions measured in marine areas as well as at continental sites (Milford and Davidson, 1987). The marine data show two modes, a coarse mode associated with sea salt and a fine mode associated with gas-to-particle conversion. Sulfate in seawater, formed, for example, by the oxidation of sulfur-containing organics such as dimethyl sulfide, can be carried into the atmosphere during the formation of sea salt particles by processes described earlier and hence are found in larger particles. The continental data show only the fine particle mode, as expected for formation from the atmospheric oxidation of the S02 precursors. 

The coarse mode (c/ae > 1pm) include particles generated by mechanical processes and introduced directly into the atmosphere from anthropogenic and natural sources (Horvath, 2000). A few examples include sea spray, erosion, resuspension, and industrial and agricultural activities. 

Most of the studies on size-resolved aerosol mass concentrations in areas with different levels of pollution show that particulate matter typically exhibit a bimodal distribution, with most of their mass being found in the submicron size range (dae < 1pm) and an additional minor mode in the coarse fraction (1 < dae < 10 pm) (Maenhaut et al., 2002 Smolfk et al., 2003 Wang et al., 2003 Gajananda et al., 2005 Samara and Voutsa, 2005). However, with instrumentation allowing more precise measurements, the aerosol mass size distribution was found to be multimodal with the preponderance of a fine mode (dae < 0.2 pm) and an accumulation mode (dae 0.5pm), with a minor coarse mode at d 3-4pm (Raes et al., 2000 Pillai and Moorthy, 2001 Berner et al., 2004). Traditionally, atmospheric researchers classify airborne particles into three size classes coarse (2.5 < c/ ie < 10pm), fine... 

Atmospheric particles have spherical equivalent diameters (Dp) ranging from 1 nm to 100 pm. Plots of particle number concentration (as well as surface area and volume) as a function of particle size usually show that an atmospheric aerosol is composed of three or more modes, as illustrated in Figure 1. By convention, particles are classified into three approximate categories according to their size Aitken (or transient) nuclei mode (Dp <0.1 pm), accumulation mode (0.1 < Dp < 2.5 pm), and coarse mode (Dp > 2.5 pm) (Seinfeld and Pandis 1998). Particles smaller than 2.5 pm are generally classified as fine. The terms PM2.5 and PMio refer to particulate matter with aerodynamic equivalent diameters under 2.5 and 10 pm, respectively. These terms are often used to describe the total mass of particles with diameters smaller than the cutoff size. 

As shown in Figure 1, within an atmospheric aerosol the smallest particles usually dominate the total number of particles, while the accumulation and coarse modes often determine the total surface area and volume (i.e., mass), respectively. For example. Figure 3 shows results from a study in Atlanta where nanoparticles (Dp = 3-10 nm) and nano- and ultrafme particles (Dp = 10-100 nm) contributed approximately 30 and 60%, respectively, to the total particle number concentration (Dp < 2 pm). However, in terms of particle mass, the accumulation mode particles were dominant, and nanoparticles with Dp < 10 nm contributed insignificantly. 

Three examples of chemistry occurring on or in atmospheric particles are provided below. These examples demonstrate how particulate-based chemical reactions can alter the composition both of particles and of the gas phase. However, the examples are by no means comprehensive of all chemical processes that occur in the troposphere. The particles involved in these examples are generally accumulation or coarse mode, though... 

The accumulation mode results largely from gas-to-particle conversion by chemical reaction, the condensation of water and other vapors, and the attachment of particles from the uitrafine range by coagulation. A smaller part of the accumulation mode is directly emitted as primary particles. This mode is. stable with respect to deposition, interacts little with the coarse mode, and has a relatively long atmospheric residence time. It ts for these reasons that it is called the accumulation mode. 

Processing of accumulation and coarse mode aerosols by clouds (Chapter 17) can also modify the concentration and composition of these modes. Aqueous-phase chemical reactions take place in cloud and fog droplets, and in aerosol particles at relative humidities approaching 100%. These reactions can lead to production of sulfate (Chapter 7) and after evaporation of water, a larger aerosol particle is left in the atmosphere. This transformation can lead to the formation of the condensation mode and the droplet mode (Hering and Friedlander 1982 John et al. 1990 Meng and Seinfeld 1994). 

An important feature of atmospheric aerosol size distributions is their multimodal character. Mass distributions, measured in urban centers, are characterized by three modes with a minimum between 1.0 and 3 The size range of particles larger than the minimum (supermicron particles) is termed coarse, while the smaller particles are called fine. The three modes present in the mass distribution of Figure 7.14 correspond to the nuclei mode (particles below 0.1 /zm), accumulation mode (0.1 < Dp < /.tm), and coarse mode Dp > )Lim) (Whitby and Sverdrup, 1980). Thus the fine particles include both accumulation and nuclei modes. The boundaries between these sections are not precise (recall in Chapter 2 that we divided fine and coarse modes at 2.5 /zm diameter). Note that our definition of modes has been based on the mass (or volume distribution). The location of modes may be different if they are based on the number or surface distribution. 

Coarse mode particles (>2.5 pm diameter) tend to result from mechanical processes such as construction, traffic, combustion, and the soil lifted and dispersed by wind action. The main competing mechanisms determining the stability of this size range of particles are turbulent mixing and sedimentation. For typical coarse particles in the atmosphere, the aerosol residence times range from several hours to about a day [121]. 

A term for the deposition of coarse mode particles has not been included in the proposed damage function. Coarse particles that are likely to deposit on zinc surfaces may increase the surface area as well as the time of surface wetness. The incorporation of terms for the effects of particles, as well as for other atmospheric gases, must await further work. 

Indeed, based on the number, surface, and volume distributions shown in Fig. 9.6, Whitby and co-workers suggested that there were three distinct groups of particles contributing to this atmospheric aerosol. Particles with diameters >2.5 yu,m are identified as coarse particles and those with diameters 2.5 pm are called fine particles. The fine particle mode typically includes most of the total number of particles and a large fraction of the mass, for example, about one-third of the mass in nonurban areas and about one-half in urban areas. The fine particle mode can be further broken down into particles with diameters between 0.08 and 1-2 yxm, known as the accumulation range, and those with diam-... 

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-... 

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