Coarse mode

When considering the impact of uptake by aerosol, the chemical composition of the aerosol is also likely to be significant. Bates et al. (1998, 2001) measured strong variations in the chemical composition of the Aitken, accommodation and sea-salt dominated coarse modes that would influence the free radical uptake rates, particularly the extent of aerosol acidification. Without data on the size segregated aerosol chemical composition during SOAPEX-2 and the relevant laboratory data, it is not possible to calculate accurate accommodation coefficients. 

It is seen that the distribution is bimodal, with the coarse mode dominating the aerosol volume concentrations. The 1979 average volume concentration of aerosol less than 10 ym diameter was 32.4 ymVcm. From its large standard deviation, it is clear that the coarse particle mode exhibited considerable variation throughout the year. Records show that high coarse mode volume concentrations accompanied moderate-to-high wind speeds. The coarse material was very likely wind-blown dust of crustal composition. 

Figure 4 presents particle size distributions for six elements which differ among themselves and also from those in Figure 3. Somewhat subjectively, we may identify three patterns in these distributions (a) A coarse mode, typified by Ca and the other elements of Figure 3, which may represent a terrestrial dust origin. This mode can account for coarse particle concentrations observed for Fe, K, Mn, and S. (b) A fine mode with somewhat greater concentrations in the 0.5-1 ymad fraction than in 1-2 ymad particles. The amounts in this <2 ymad range, in excess of those which can be attributed to a coarse crustal aerosol tail with the Ca distribution, show similarities in particle size distributions for Zn, Mn, and possibly Fe. Since the trends shown in Figure 2 point to these elements being characteristic of large scale air masses, their fine modes may be principally due to natural processes.

Figure 4. Particle size distributions of 6 elements that exhibit fine-mode concentrations, as distinct from a coarse-mode typified by Ca.

The S, Zn, and Pb are found only in a fine-mode centered at 0.5-1 fimad. The Mn and K can be resolved into both fine- and coarse-modes by reference to Ca, and Fe shows evidence of a possible fine mode superimposed on a much larger coarse mode similar to Ca (Xinglong aerosol, March 16-21,1980 means and standard errors of means). 

Fine-mode Cl and K may be mainly pollution-derived, coarse-mode Cl typical of marine air, and coarse-mode K typical of continental air (Xinglong aerosol, March 16—19, 1980 means and standard deviations). 

Concentrations K, Fe, Mn represent excess over that attributed to a coarse mode with Ca particle size distribution. 

A word of caution is also in order with respect to assigning a particular particle to the fine or coarse particle modes. Since the size distributions can generally be described as log-normal, they do not have sharp cutoffs. A few particles at the top end of the fine mode distribution will have diameters larger than 2.5 [jlm and a few at the bottom end of the coarse mode will have diameters smaller than this. For example, as Lodge (1985) points out, for a coarse particle distribution with a geometric mean diameter of 15 fim and a geometric standard deviation of 3, about 5% of the particles will have diameters below the 2.5-gm fine particle cutoff. This may be responsible for observations that while Si and Ca dominate the coarse particle mode, they are also often found at significant levels in fine particles (e.g., see Katrinak et al., 1995). 

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. 

Figure 1. Example of compositionally resolved bimodal and monomodal distributions of aerosols. The ordinate gives the percent of the species found in the given size fraction of the impactor. The mode near 0.3 xm is the accumulation mode , and that above 8 xm is the coarse mode The minimum of mass between 1 and 2 xm is typical the chlorine distribution is anomalous. Chlorine is in fact a coarse-mode marine aerosol that has lost its larger particles during transport from the ocean to Davis, California, a distance of roughly 100 km. (Reproduced with permission from reference 15. Copyright 1988.)...

PM 10 concentrations and constituents appear systematically higher at urban sites. Urban increments have been measured for most chemical constituents. Nearby (anthropogenic) sources and reduced dispersion in the urbanised areas are the main determining factors here. The observed increment for SIA is caused by more nitrate and sulphate. It is explained by depletion of chloride stabilising part of the nitrate and sulphate in the coarse mode. The question then arises how to assign the coarse mode nitrate (and sulphate) in the mass closure exercise as they replace the chloride. 

Only PM10 is discussed as data on PM2.5 are much less available. In our region, the average mass contribution of PM2.5 to PM10 is about 60-70%, and the relative distribution of the different chemical parts in PM2.5 usually resembles that of PM 10. All components are present in both the fine and coarse fraction. Whereas SIA, EC and OM dominate more in the fine fraction, SS and MD contribute more to the coarse mode. 

The reason why SIA is higher in urban areas is less obvious as these are secondary aerosols. The observed increment is predominantly caused by more nitrate and sulphate. The reaction of nitric acid and sulphuric acid with the sea-salt aerosol in a marine urbanised environment follows an irreversible reaction scheme. In essence, the chloride depletion stabilises part of the nitrate and sulphate in the coarse mode and may partly explain part of the observed increment. However, it also raises the question how to assign the coarse mode nitrate in the mass closure. The sea salt and nitrate contributions cannot simply be added any more as nitrate replaces chloride. Reduction of NOx emissions may cause a reduction of coarse mode nitrate, which is partly compensated by the fact that chloride is not lost anymore. A reduction would yield a net result of ((N03-C1)/N03 = (62-35)/62=) 27/62 times the nitrate reduction (where the numbers are molar weights of the respective components), and this factor could be used to scale back the coarse nitrate fraction in the chemical mass balance. A similar reasoning may be valid for the anthropogenic sulphate in the coarse fraction. Corrections like these are uncommon in current mass closure studies, and consequences will have to be explored in more detail. 

Air quality effects of aerosols have traditionally been studied in accumulation and coarse mode, concentrating largely in the supermicron particle range, usually by using particle mass as the main property of interest (e.g., PMX). However, submicron particles have also many important effects to both air quality and the climate... 

Number concentrations are dominated by submicron particles, whereas the mass concentrations are strongly influenced by particle concentrations in 0.1-10 pm diameter range [13]. Similarly, the variability of the number-based measurements is strongly dominated by variability in smaller diameter ranges, whereas the variability of mass-based properties, such as PM10, are dominated by variability in the accumulation mode (usually around 500 nm of mass mean diameter) and in the coarse mode. This means the variabilities of these properties are not necessarily similar in shorter timescales, due to sensitivity of variance from very different air masses and thus aerosol types. This is demonstrated in Fig. lb, where the variance of the each size class of particle number concentrations between 3 and 1,000 nm is shown for SMEAR II station in Hyytiala, Finland. The variance has similarities to the particle number size distribution (Fig. la), but there are also significant differences, especially on smaller particles sizes. Even though in the median particle number size distribution the nucleation mode is visible only weakly, it is a major contributor to submicron particle number concentration variability. 

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... 

Fine Mode Coarse Mode Fine Mode Coarse Mode ... 

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... 

Coarse mode particles generated by mechanical processes. 

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