Accumulation mode, atmospheric particles

Table 2.12 shows washout ratios of radioactive and stable nuclides as measured in the UK. Fission products from distant bomb tests become attached to natural condensation nuclei in the atmosphere, and enter the accumulation mode of particle sizes (approximately 0.02 to 0.2 pm diameter). Washout ratios in the range 250typically present as particles in the 1-5 pm range, outside the normal accumulation mode, and this explains... 

Size distribution of the volume of aerosol particles (solid line) according to Whitby (1978). Dotted line corresponds to the power law of Fig. 26. V particle volume n nuclei mode a accumulation mode < coarse particle mode tlrGV geometric volume mean size. (By courtesy of Atmospheric EnvironmentI... 

The nuclei mode consists primarily of combustion particles emitted directly into the atmosphere and particles formed in the atmosphere by gas-to-particle conversion. This mode is not always present, but is usually found near highways and other sources of combustion. Because of their high number concentration, especially near their source, these small particles coagulate rapidly with each other and with particles in the accumulation mode. Consequently, nuclei particles have relatively short lifetimes in the atmosphere and end up in the accumulation mode. Nuclei particles may serve as sites for the formation of cloud droplets and may subsequently be removed fi om the atmosphere as rain droplets (rainout). 

FIGURE 9.7 Schematic of an atmospheric aerosol size distribution showing four modes. The original hypothesis of Whitby and co-workers is shown by the solid, trimodal curves, and the fourth, ultrafine particle mode, as well as the two peaks sometimes observed in the accumulation mode are shown by the dashed lines (adapted from Whitby and Sverdrup, 1980). 

In short, growth of sulfate particles at least in the accumulation mode and the presence of two peaks are both believed to be largely controlled by interactions with water in the atmosphere, including the aqueous phase oxidation of SOz to sulfate. 

Kerminen, V.-M., and A. S. Wexler, Growth Laws for Atmospheric Aerosol Particles An Examination of the Bimodality of the Accumulation Mode, Atmos. Environ., 29, 3263-3275 (1995). 

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-... 

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... 

However, not every aerosol particle serves as CCN. Accumulation mode aerosols provide the nuclei for most cloud drops (Penner et al., 2001). As in the case of anthropogenic and natural sulphate particles, OAs can also serve as CCN (Ramanathan et al., 2001). Additionally, the presence of water-soluble organic compounds in the particles and the presence of soluble gases (HN03) in the atmosphere can amplify the CCN activity of the aerosols and further increase the concentration of cloud droplets and the indirect forcing (Charlson et al., 2001). Also, biomass... 

It is likely that both the above mentioned effects - larger particle sizes and increase in airborne activity with height - contributed to the high values of W. At later times, the airborne activity from Chernobyl was mainly submicrometre in size and had equilibrated with the accumulation mode of natural nuclei. Over the period 10-90 d from the emission, 137Cs disappeared from the atmosphere with a half-life of 6 d, or mean life of 9 d (Fig. 2.8). The mass of air in the troposphere is 9000 kg per m2 of the earth s surface, and the average daily rainfall in the northern hemisphere is 3.1 mm. Using these data, it can be deduced that the washout ratio of 137Cs was... 

Because of similarity of the behavior of Be and Pb in the atmosphere, Be/ Pb is little affected by processes other than production and transport. Both Pb and Be are formed in the atmosphere as energetic single atoms. Since neither is volatile, each of them attaches to the first particles they encounter. The most abundant aerosols in terms of surface area are typically those with a diameter of 0.1 -0.5 p.m, the so-called accumulation mode. This size class carries many of the chemical species in the atmosphere that have low volatility and also have gaseous precursors, such as sulfate as discussed above. Accumulation-mode aerosols are most subject to long-distance transport. Scavenging by precipitation is the principal mechanism of removal of these aerosols from the atmosphere. 

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... 

This section provides a conceptual framework and several examples of modeling and fieldwork on the growth of atmospheric nanoparticles. The growth of nanoparticles is an important source of Aitken mode and accumulation mode particles, including cloud condensation nuclei, especially in remote regions with few primary particle sources. For more quantitative descriptions of growth processes, as well as their parameterizations in models, see Kulmala (1993), Kulmala et al. (1993), Kerminen et al. (1997), Mattila et al. (1997), Vesala et al. (1997), Seinfeld and Pandis (1998), and Friedlander (2000). 

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. 

The ultrafine range is usually composed of emissions from local combustion sources or particles generated by atmospheric photochemical activity that leads to homogeneous nucleation. The principal mechanism of decay of the ultrafine range is attachment to particles in the accumulation mode by diffusion. Neglecting the Brownian motion of the coarse particles compared with the fine particles, the fractional rate of decay of particles in the ultrafine range is given by (Chapter 7)... 

---