Metals atmospheric aerosols

Recently was estimated an expected impact on the global chemistry of the atmosphere of the indirect heterogeneous photocatalytic reactions under the much more abundant near ultraviolet, visible and near infrared solar light [2]. As photocatalysts may serve atmospheric aerosols, i.e. ultrasmall solid particles that sometimes are embedded into liquid droplets. Aerosols are known to contain Ti02, Fc203, ZnO and other natural oxides, as well as metal sulfides of volcanic or antropogenic origin, that may serve as semiconductor photocatalysts (see Fig.5). Aerosols are known to be concentrated mainly in the air layers near the surface of the Earth, i.e. in the troposphere, rather than stratosphere. 

The exploding wire method involves putting a large amount of energy into a wire suddenly, causing it to explode. If 02 is present, a metal oxide aerosol is produced, whereas particles of pure metal are formed in an inert atmosphere such as helium. Exploding wire generators and their size distribution characteristics have been discussed by Phalen (1972). 

The concentration of metals in atmospheric aerosols and rainwater (Table 7.1) is therefore a function of their sources. This includes both the occurrence of the metals in combustion processes and their volatility, as well as their occurrence in crustal dust and seawater. As a result of this, the size distribution of different metals is very different and depends on the balance of these sources. For a particular metal this distinction is similar in most global locations (Table 7.2), although some variability does occur as wind speed and distance from source exert an influence on the particle size distribution spectrum (Slinn, 1983). Once in the atmosphere particles can change size and composition to some extent by condensation of water vapour, by coagulation with other particles, by chemical reaction, or by activation (when supersaturated) to become cloud or fog droplets (Andreae et al., 1986 Arimoto et al., 1997 Seinfeld and Pandis, 1998). 

Maring, H.B. and Duce, R.A. (1987) The impact of atmospheric aerosols on trace metal chemistry in open ocean surface seawater. 1. Aluminium. Earth Planet. Sci. Letts, 84, 281-392. 

The reactions observed in the photochemical smog, especially those concerning decomposition and oxidation of volatile organic substances, are accelerated by atmospheric aerosols, eg oxidation of some halogenated hydrocarbons, isoprene, monoterpenes, and aromatic hydrocarbons is enhanced by the surfaces of metal oxides, desert sand, volcanic ash, and sea salt [8],... 

Figure 1 shows a schematic of a typical atmospheric aerosol particle (if such an entity can be assumed to exist). The particle consists of sulfates, nitrates, water, ammonium, elemental and organic carbon, metals, and dust. After a primary particle is emitted, gas-phase reactions occur, converting oxides of nitrogen to nitric acid, sulfur dioxide to sulfuric acid, and hydrocarbons to oxidized, low-vapor-pressure condensable organics. 

We focus initially on the photochemical behaviour of complexes of Fe(III) with simple carboxylic acids and give particular attention to oxalic acid. This compound is prevalent in atmospheric aerosols [28], provides a simple example of environmentally important light-mediated ligand-to-metal charge transfer (LMCT) processes which result in ligand decarboxylation [27] and is used to initiate the degradation of contaminants both in the absence and presence of added hydrogen peroxide (via the so-called modified photo-Fenton process [29,30]). In addition, the photochemistry of Fe(III)-oxalate complexes has been studied in detail, as it is the basis of... 

There has been recent interest in a somewhat different aspect of adsorption and reaction on metal oxides photocatalysis. The interest stems partially from that role that some transition-metal oxides can play in photochemical reactions in the atmosphere. Atmospheric aerosol particles can act as substrates to catalyze heterogeneous photochemical reactions in the troposphere. Most tropospheric aerosols are silicates, aluminosilicates and salts whose bandgaps are larger than the cutoff of solar radiation in the troposphere (about 4.3 eV) they are thus unable to participate directly in photoexcited reactions. However, transition-metal oxides that have much smaller bandgaps also occur as aerosols — the most prevalent ones are the oxides of iron and manganese — and these materials may thus undergo charge-transfer excitations (discussed above) in the pres-... 

Particulate emissions are by-products of fuel combustion, industrial processes, and motor vehicles and are believed to have a significant potential for causing adverse health effects. Carbonaceous material present in atmospheric aerosols is a combination of elemental carbon and organic and inorganic compounds. Particulate matter may also consist of fly ash, minerals, or road dust and contain traces of a number of heavy metals. Population-based studies have consistently found that the association between adverse respiratory effects and particulate concentrations occurs in a number of regions throughout the United States. This association is strongest for PM]o and PM2.5 indices (particulate matter less than 10 and 2.5 pm in diameter, respectively). The observed adverse effects include increases in total mortality, mortality due to respiratory and cardiovascular causes, chronic bronchitis, and hospital visits and admissions for asthma. Elderly or unhealthy individuals and infants appear to comprise subpopulations that are most sensitive to the adverse health effects of PM. 

Certain metals present in the atmospheric aerosol, including Fe, Mn. and V, may serve as catalysts for the oxidation of substances such as SOi. which is converted to HjSOj in the... 

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. 

Sulfate ion i.s the chemical component usually present in highest concentration in the submieron atmospheric aerosol. Almo.st all of the sulfate results from the atmospheric oxidation of SO either by homogeneous gas-phase reactions or by aerosol- or droplet-phase reactions. Reaction with the hydroxyl radical OH is thought to be the major ga.s-phase mechanism. Many solution-phase processes are possible, including reaction with dissolved HiO and reactions with 0 catalyzed by dissolved metals such as Fe and Mn (Seinfeld and Pandis, 1998). 

In 1998, there were 7 monitoring stations for regional air pollution monitoring, characterized as pollution of a rural type. The measurement programme includes SO2, NO2 and heavy metal concentrations in atmospheric aerosol PM, Pb, Mn, Cu, Cd, Zn, Ni, V, Cr. 

At the National Institute of Chemistry (NIC), in the frame of CMD subproject of EUROTRAC-2, experimental studies of the role of soluble constituents of atmospheric aerosols in the aqueous-phase autoxidation mechanisms of S(IV) was studied. The research focused on atmospheric water droplets (clouds, fog), where soluble constituents of atmospheric particles may be important in aqueous SO2 oxidation under non-photochemical conditions. In the frame of CMD project laboratory experiments in a semi-batch continuous stirred tank reactor under controlled conditions (T, air flow rate, stirring), were made in order to study the autoxidation of S(IV)-oxides catalyzed by transition metal ions (Fe(III), Fe(II), Co(II), Cu(II), Ni(II), Mn(II)). These studies were carried out at the National Institute of Chemistry. 

A small but important class of atmospheric aerosol particles are the ice nuclei. These nuclei promote the freezing of water drops in clouds (see Fletcher, 1962). In this way they play a definite role in the formation of precipitation in mixed clouds containing both water drops and ice crystals. This kind of precipitation formation is due to the fact that the saturation vapour pressure over ice is smaller than over liquid water. In this way ice crystals grow by condensation while drops tend to evaporate. Thus, if human activity emits ice nuclei to the atmosphere the precipitation distribution can be modified. Results of measurements show that in the vicinity of steel works and aluminum foundries the concentration of ice nuclei active at a temperature of — 20 °C is unusually high. It is believed that this is caused by the presence of some metal oxides in the air (Pruppacher, 1973). More recent studies on ice nuclei also showed that lead compounds (e.g. Pbl 2) in exhaust gases of vehicles also have ice nucleating ability. It is believed, however, that anthropogenic ice nuclei cannot play an important role, except in local scale processes (see Pruppacher, 1973). 

Weschler, C. J., and T. E. Graedel (1982). Theoretical limitations on heterogeneous catalysis by transition metals in aqueous atmospheric aerosols. In Heterogeneous Atmospheric Chemistry (D. R. Schryer, ed.), Geophys. Monogr. 26, 196-202. Amer. Geophys. Union, Washington, D.C. 

Lowest concentrations of trace elements have been found in the Antarctic (Cunningham and Zoller 1981), and very low levels in the maritime atmosphere over the Pacific ocean (Gordon etal. 1978 Duce etal. 1983 Farrington and Zoller 1984). A series of elements (V, Cr, Mn, Cu, Zn, Co, Ag, Cd, Ba, Pb, Bi, U) have been measured by Planchon et al. (2002) in snow samples collected at remote, low accumulation sites in Coats Land, Antarctica. Heavy metal concentrations were found to be extremely low, down to 3 pgkg confirming the high purity of Antarctic snow. A review of the data on heavy metals in aerosols over... 

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