Secondary aerosol material

The sources and chemical compositions of the fine and coarse urban particles are different. Coarse particles are generated by mechanical processes and consist of soil dust, seasalt, fly ash, tire wear particles, and so on. Aitken and accumulation mode particles contain primary particles from combustion sources and secondary aerosol material (sulfate, nitrate, ammonium, secondary organics) formed by chemical reactions resulting in gas-to-particle conversion (see Chapters 10 and 14). 

Review of the literature provides ample evidence that aerosol formation is an important part of the atmospheric chemistry linked with photochemical-oxidant production. The important chemical constituents of concern include sulfate, nitrate, and secondary organic material. 

In this paper we present results which reconcile the widely different results just discussed ranging from a carbon aerosol dominated by secondary organic material on the one hand to a carbon aerosol composed largely of primary carbon compounds on the other. We have employed an approach which uses lead or elemental carbon as a tracer for primary emissions and combines several analysis techniques to reexamine the published ACHEX data. We also present a new data set from St. Louis which is analyzed in a similar manner to contrast the aerosol in a midwestern city with that on the California coast. 

The major processes for creating atmospheric fine particles (diameter < 2.5 pm) are combustion and gas-to-particle conversion (GPC). Whereas combustion particles are emitted directly to the atmosphere (primary aerosol), gas-to-particle conversion refers to the chemistry that leads to particulate matter by converting volatile gases into condensable substances under atmospheric conditions. Gas-to-particle conversion leads to an increase in the mass of preexisting particles and under some circumstances may lead to the creation of new particles. Particulate material produced by GPC is referred to as secondary aerosol. 

Secondary organic aerosol material is formed in the atmosphere by the mass transfer to the aerosol phase of low vapor pressure products of the oxidation of organic gases. As organic gases are oxidized in the gas phase by species such as the hydroxyl radical (OH), ozone (03), and the nitrate radical (N03), their oxidation products accumulate. Some of these products have low volatilities and condense on the available particles in an effort to establish equilibrium between the gas and aerosol phases. 

Note that in this case if AROG > 0, then caer, > 0, that is, secondary aerosol is formed as soon as the ROG starts reacting. In this case the threshold AROG is zero. The aerosol concentration of i is inversely related to its vapor pressure and also depends on the preexisting organic material concentration. 

Formation of Binary Ideal Solution with Other Organic Vapor If the products of a given ROG cannot form a solution with the existing aerosol material, but they can dissolve in each other, then the formation of secondary aerosol can be described by a modification of the above theories. Assuming that two products are formed... 

Deviations from the theory outlined above may occur if the organic solution is not ideal. In this case the mole fraction x, should be replaced by the product y,jc where y is the activity coefficient of the component i in the given solution. Saxena et al. (1995) have shown that a series of organic aerosol compounds can interact also with the aqueous phase in atmospheric particles. Therefore, for high relative humidities, the secondary organic aerosol compounds may exist in three phases—namely, the gas, the organic aerosol material, and the aqueous phase. 

Sea spray, volcanic eruptions, soil dust, as well as some industries (cement manufacturing) produce the so called primary aerosols, i.e. the material is emitted directly in particulate state (Klockow, 1982), and they are both line and coarse. Secondary aerosols are produced in the atmosphere usually by eondensation after emission from high temperature sources, and they are fine as a rule. Considering the difference in the chemical composition it is recognized that the major components of the fine aerosols are toxie substances of anthropogenic origin such as As, Cd, Pb, Se, Zn etc. while the course aerosols are enriched in elements like Ca, Fe, Si coming from erosion, sea aerosols and other natural sources. 

Condensed-phase SOA formation water-soluble VOCs may dissolve into the aqueous phase of cloud droplets or wet aerosols. Subsequent aqueous-phase reactions (e.g., oxidation and/or oligomerization) can lead to the formation of low-volatility secondary organic material [82-87]. In particular, the dicarbonyl VOCs glyoxal and methylglyoxal have been studied as potential precursors for this SOA formation pathway. Recently, aqueous-phase reactions of isoprene-derived epoxydiols have also been shown to be efficient pathways to SOA formation in the aerosol aqueous phase [88-91]. 

Sareen N, Schwier AN, Shapiro EL, Mitroo DM, McNeill VF (2010) Secondary organic material formed by methylglyoxal in aqueous aerosol mimics. Atmos Chem Phys 10 997-1016... 

The presence of hydrocarbons, therefore, can accelerate the oxidation of nitric oxide to NO2, which in turn reacts in the light to produce more ozone. Atmospheric oxygen is the source of oxidation capacity for both this reaction and the transformation of propene to more oxidized compounds. While the ultimate fate of much of the hydrocarbon is oxidation to carbon dioxide, it is equally important to note that this complex web of reactions involves many volatile intermediates, such as formaldehyde (HCHO), acetaldehyde (CH3CHO), and PAN (peroxyacetyl nitrate, not shown in Fig. 4.40), all of which can cause human health effects and/or damage to materials. The oxidation process also leads to the production of secondary aerosols, which are responsible for the decreased visibility caused by smog. 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

Secondary Hazards Aerosols (contaminated material) Fomites (with vectors) Vector cycle. 

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-... 

Indoor Chemistry Various terpenes and terpenoids are emitted from household products and building materials. Ozone that has entered from outdoors or has been generated indoors can react with these compounds, either in the gas phase or on the surface of materials. The resulting oxidation products will contribute to the production and growth of meaningful quantities of secondary organic aerosols (SOA). The formation and growth of SOA can be studied under controlled conditions in test chambers (see also Chapter 13). 

Toftum, J., Freund, S., Salthammer, T. and Weschler, C.J. (2008) Secondary organic aerosols from ozone-initiated reactions with emissions from wood-based materials and a green paint. Atmospheric Environment, 42, 7632-40. 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

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