Aerosol salt, chemical composition

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. 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

The chemical composition of PM10 shows a considerable conformity in these countries. Always, secondary inorganic aerosols (SIA) are the major constituent ( 40%) followed by the carbonaceous compounds ( 25%). Contributions of sea salt and mineral dust vary between 10% and 15% depending on presence and distance of respective sources. The unidentified mass is some 15% indicating that the composition of PM10 in this region is fairly well known. 

Atmospheric particles in the troposphere are composed of a complex mixture of highly water-soluble inorganic salts, insoluble mineral dust, and carbonaceous material (which includes organic compounds plus elemental carbon) (Jacobson et al., 2000). Studies in which the chemical composition has been determined as a function of particle size demonstrate a correlation between the chemical composition and the size mode of atmospheric aerosols (Meszaros et al., 1997 Krivacsy and Molnar, 1998 Alves et al.,2000 Maenhaut et al.,2002 Smolik et al., 2003 Samara andVoutsa, 2005). 

Cass GR, Hughes LA, Bhave P, Kleeman MJ, Allen JO, Salmon LG (2000) The chemical composition of atmospheric ultrafine particles. Phil Trans Roy Soc Lond A 358 2581-2592 Chameides WL, Stelson AW (1992) Aqueous-phase chemical processes in deliquescent sea-salt aerosols—... 

An important example related lo the atmospheric aerosol is the droplet containing dissolved sulfates that form as a result of the oxidation of SO2 in solution. The sulfates may be present a.s sulfuric acid or in a partially neutralized form as ammonium salts or metallic salts from sources such as flyash. The droplet size distribution and chemical composition are determined by a combination of thermodynamic and rate processes. In this section, we consider only equilibrium thermodynamics as it affects the vapor pressure of the drop. 

Relative Chemical Composition of Marine Aerosol Salt... 

Several workers (39, 49, 50) have shown that the atmospheric concentration of salt derived from the ocean surface decreases rapidly to small values at an altitude of about 2 km. Are there also changes in chemical composition in the salt fraction of the marine aerosol with... 

In many instances the chemical composition of oceanic rain has been used to draw conclusions concerning the chemical composition of marine aerosols (3, 52, S3, 54). The first two entries in Table V emphasize the trends with respect to altitude for our data on rain samples collected in Hawaii. The 100-meter sample was collected near the coast, and the 1000-meter sample was obtained several miles inland in the Hawaiian mountains. The results for F/Cl, Cl/Na, and Na/F in the 100-meter sample agree with the results found for marine aerosol salt fractions given in Table II, and differ only slightly from those of sea water. 

The source of the aerosol salt is the ocean. The elemental constituents occur in ratios which differ somewhat from those found in sea water. The chemical composition of the aerosol salt is influenced by several factors. When the salt aerosol is created at the sea surface by whitecaps, we And ion fractionation. That is, the sea water droplets injected into the atmosphere contain the elements in proportions different from those in sea water. Examples include different ratios for the halogens, alkali metals, sulfate, phosphate, and nitrogen (12, 44, 51, 62, 63), The causes for fractionation, both as physical and organic chemical processes are under... 

Study (4, 52). The aerosol salt composition may be further modified when the droplets are carried to higher altitudes where they lose water by evaporation, crystallize, and then lose other constituents such as chlorine gas (12, 26,64). Another possibility of change in the chemical composition is the reaction of the aerosol salt with atmospheric gases such as elemental iodine, which originates at the ocean surface (65). In addition, soluble particles are produced from gaseous compounds in the marine atmosphere, such as the production of (NH4)oS04 via the atmospheric oxidation of SO-j (9). 

The sources and chemical compositions of the fine and coarse particles are different. Coarse particles are generated by mechanical processes and consist of soil dust, sea salt, fly ash, tire wear particles, and so on. Nuclei 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 9 and 13). 

It is therefore mainly limited to the cocondensation of sulfuric acid and water. The acid in this case is produced in the atmosphere by gas phase oxidation of SO2. A special case is the production of sea salt aerosols over the oceans, which are effectively produced by the disintegration of air bubbles on the ocean. The atmospheric aerosol is a dynamic system the initially produced particles undergo various processes like heterogeneous condensation, coagulation, and deposition which lead to a change in their size and chemical composition and gives rise to the universality of the aerosol size-distribution. 

Because a great variety of sources contribute materials to the tropospheric aerosol, it represents a complex mixture of marry substances that additionally depends on the size of the particles. Source characteristics are preserved only in the vicinity of somces. The mixtirre may be divided into three fractiorts water-solirble inorganic salts (electrolytes), water-irrsoluble minerals, and organic compoimds, both soluble in water and insoluble. Table II shows the chemical composition of two boimdary layer aerosols that are typical of marine and of rural continental air. 

Figure 28.39 Data showing the change in chemical composition of aerosol particles obtained over a 4-day period, at a remote rural location, using ATOFMS. A polluted air mass, containing organic carbon particles, is seen to pass through the sampling site (i.e. the organic particles build up) on Tuesday and Wednesday and then depart swiftly. This is replaced by a clean air mass containing mainly sea salt (labelled Na)...

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