Atmospheric aerosols description

Descriptions of analytical methods for strong acid and acidic sulfate content of atmospheric aerosols have been reviewed (6-10). Methods for acidic aerosol determination are reviewed in this chapter according to the measurement principle either filter collection and post-collection extraction, deriv-atization or thermal treatment, and analysis or in situ collection (real-time or stepwise) and analysis. 

In this chapter, we made an attempt to provide a comprehensive review of the current state-of-the-art on sources, chemical nature, and physical properties of organic aerosols. This review begins with an overview of few basic concepts on atmospheric aerosols, followed by a description of the major constituents of atmospheric aerosols. The sources, transformations, and removal processes of organic aerosols are outlined and followed by an overview of the major environmental and human health issues associated with organic aerosols. The chemical and physical characterization of organic aerosols is then reviewed and is finally followed by a list of uncertainties and suggestions that require further studies. 

The development of the relationships between scattered light and aerosols has stimulated the use of radiation transfer theory for remote sensing of particles in planetary atmospheres. Highly sophisticated experimental and theoretical techniques have emerged for the interpretation of observations of sunlight and artificial light sources in the earth s atmosphere. A description of their application depends on further development of the concepts of radiant energy transfer. 

The Power-Law Distribution A variety of other mathematical functions have been proposed for the description of atmospheric aerosol distributions. The power law, or Junge, distribution was one of the first used in atmospheric science (Prup-pacher and Klett 1980),... 

An aerosol distribution can be described by the number concentrations of particles of various sizes as a function of time. Let us define Nk(t) as the number concentration (cm-3) of particles containing k monomers, where a monomer can be considered as a single molecule of the species representing the particle. Physically, the discrete distribution is appealing since it is based on the fundamental nature of the particles. However, a particle of size 1 pm contains on the order of 1010 monomers, and description of the submicrometer aerosol distribution requires a vector (N2, N-j,..., N10io) containing 1010 numbers. This makes the use of the discrete distribution impractical for most atmospheric aerosol applications. We will use it in the subsequent sections for instructional purposes and as an intermediate step toward development of the continuous general dynamic equation. 

The main advantage of the power-law distribution is simplicity, but it is often inadequate for the description of ambient aerosol distributions that have significant structure. Power-law expressions can provide reasonable fits to parts of the atmospheric aerosol number distribution but they are inadequate models of the surface and volume distributions. 

Some common size range descriptions for atmospheric aerosol particles and droplets are shown in Tables 1.5 and 1.6. These ranges and descriptions are based mostly on the techniques used to determine the sizes [122,124,125]. Aitken particles and droplets (diameters less than 0.2 pm) are typically detected using an Aitken nucleus counter (also called a Nolan-Pollak counter or a Poliak counter). Here, the aerosol is introduced into a chamber containing vapour-saturated gas. Rapid volume expansion and adiabatic cooling are used to induce supersaturation in the gas, which in turn causes condensation on the original particles, which act as nuclei [122, 125]. This makes the original, small particles or droplets easy to observe and count with a microscope. (The principle just described is somewhat similar to the operation of a Wilson cloud chamber (see Section 7.1.4).)... 

Despite numerous algorithms and experimental techniques for determining the fractal dimension of aerosols, a quantitative description of atmospheric aerosols is rare. Those which have been reported have involved image analysis of ambient aerosols collected on a Alter or TEM grid [23,101-103]. Kindratenko etal. [102] concluded that fractal analysis allows the unequivocal identiflcation of particles source. For samples from Siberia they quote a textural fractal dimension of 1.09 0.015 for fly ash and 1.04 0.015 for soil. However, the majority of the fly ash particles examined were spherical and did not exhibit any fractality. This suggests that, in this case, fly ash was formed by different mechanisms. 

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

System 10. soil (IV) plants (VIII) their biological reactions, endemic diseases (VIII) atmospheric air, aerosols (III, 28). During consideration of System 7-9-10, we have discussed the influence of the lower and upper limits of concentrations on plant metabolisms, including endemic disease. The study of link (VIII) should start with the correct selection of characteristic plant species. The following steps should include the different research levels, from floristic description up to biochemical metabolism. 

Assuming that some of the physical and chemical mechanisms just reviewed are predominant in the formation of organic aerosol, various schemes can be derived that permit a more quantitative description of the time evolution of atmospheric organic aerosol. For example, a kinetic scheme has been proposed recently (Grosjean and Friedlander, unpublished data) for aerosol formation from ole ic precursors that may be applied in principle to other hydrocarbon classes. Starting with this system. 

Particle Number Concentration and Size Distribution. The development of aerosol science to its present state has been directly tied to the available instrumentation. The introduction of the Aitken condensation nuclei counter in the late 1800s marks the beginning of aerosol science by the ability to measure number concentrations (4). Theoretical descriptions of the change in the number concentration by coagulation quickly followed. Particle size distribution measurements became possible when the cascade impactor was developed, and its development allowed the validation of predictions that could not previously be tested. The cascade impactor was originally introduced by May (5, 6), and a wide variety of impactors have since been used. Operated at atmospheric pressure and with jets fabricated by conventional machining, most impactors can only classify particles larger... 

Most instruments with an API source offer a choice of APCI or ESI. Detailed descriptions of these ionization methods are provided elsewhere a 3). Both rely on the formation of ions at atmospheric pressure in a source region separated from the high vacuum mass analyzer. Ions are transported to the analyzer through one or more differentially pumped skimmers or a heated capillary. Both APCI and ESI require aerosolization of the liquid eluent from the liquid chromatography (LC) column. The major difference lies in the method and phase of ionization. 

Wright D. L., Kasibhatla P. S., McGraw R., and Schwartz S. E. (2001) Description and evaluation of a six-moment aerosol microphysical module for use in atmospheric chemical transport models. J. Geophys. Res. 106, 20275-20291. 

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