Combustion sources aerosols

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

Finally, while elemental carbon is a common constituent of urban aerosols due to its combustion source, it is also found in particles in remote areas due to long-range transport (e.g., see O Dowd et al., 1993a and Saxena et al., 1995). 

We discuss in this section four key aspects of heterogeneous reactions (1) theoretical and experimental structure and reactivity relationships (2) held measurements of relative and absolute PAH decay rates in near-source ambient air and during downwind transport (3) laboratory studies of the photolysis/photo-oxidation and gas-particle interactions with 03 and NOz of key 4- and 6-ring PAHs adsorbed on model substrates or ambient aerosols and (4) environmental chamber studies of the reactions of such PAHs associated with several physically and chemically different kinds of combustion-generated aerosols (e.g., diesel soot, wood smoke, and coal fly ash). Where such data are available, we also briefly consider some toxicological ramifications of these reactions. 

The fact that fine atmospheric particles are enriched in a number of toxic trace species has been known since the early 1970s. Natusch and Wallace (20, 21) observed that the fine particles emitted by a variety of high-temperature combustion sources follow similar trends of enrichment with decreasing particle size as observed in the atmosphere, and they hypothesized that volatilization and condensation of the trace species was responsible for much of the enrichment. Subsequent studies of a number of high-temperature sources and fundamental studies of fine-particle formation in high-temperature systems have substantiated their conclusions. The principal instruments used in those studies were cascade impactors, which fractionate aerosol samples according to the aerodynamic size of the particles. A variety... 

A receptor model analysis in western Germany separated nitrate-rich from sulphate-rich secondary aerosols, with the latter being accompanied with vanadium and nickel [7]. Such factor composition pinpoints to heavy oil combustion sources which can be found, e.g. in oil refineries, off-shore platforms and overseas ships. In addition, trans-boundary pollution from eastern European countries is a significant source. 

Nitrate-rich secondary aerosol, mainly composed of ammonium nitrate, on the other hand, is predominantly formed on medium spatial scale and hence may have its major origin within Germany. Large sources for NH3, one of the aerosol precursor compounds, are located in the agricultural areas of north-western Germany ( Swine belt ). The other precursors, oxides of nitrogen, stem mainly from industrial and traffic-related combustion sources. 

Nolan J. L., Measurement of light absorbing aerosols from combustion sources, pp. 265 269 in Novakov (1979) ref. cited. 

The observed difference in the values of pH, with smaller values especially during the winter season, probably originates from combustion sources that include a considerable seasonal contribution from heating. The study further of pH indicates that for the summer and autumn of 2003 rainfalls were more alkaline with less H ions, but as these seasons preceded precipitation showed lower pH and higher ion concentrations (Figure 5). Probably in this period the particulate load in the atmosphere declined substantially and hence aerosol rich in basic ions declined leading to an increase in the H ion concentration and a subsequent lowering of rainfall pH. 

Formation of combustion particles also involves nucleation and condensation of vapors, although the processes occur at elevated temperatures inside the combustion source and during cooling of the plume. Like secondary aerosols, combustion particles have a major semivolatile component composed of sulfates from sulfur dioxide oxidation and organic oxidation products, and of unburned fuel and oil as well. Furthermore, they contain a large non-volatile component consisting of soot, metals, and metal oxides. 

Table VI summarizes aerosol mass concentrations and composition in different regions of the troposphere. It is interesting to note that average total fine particle mass (that associated with particles of diameter less than about 2 /im) in non-urban continental, i.e., regional, aerosols is only a factor of two lower than urban values. This reflects the relatively long residence time of particles (recall the estimate of a lifetime of fine particles by dry deposition of 10 days). Correspondingly, the average composition of non-urban continental and urban aerosols is roughly the same. The average mass concentration of remote aerosols is a factor of three lower than that of non-urban continental aerosols. The elemental carbon component, a direct indicator of anthropogenic combustion sources, drops to 0.3% in the remote aerosols, but sulfate is still a major compo-...

The mass concentrations of the accumulation and coarse particle modes are comparable for most urban areas. The Aitken and nucleation modes, with the exception of areas close to combustion sources, contain negligible volume (Figures 8.11 and 8.13). Most of the aerosol surface area is in particles of diameters 0.1-0.5 pm in the accumulation mode (Figure 8.11). Because of this availability of area, transfer of material from the gas phase during gas-to-particle conversion occurs preferentially on them. 

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). 

This class of compounds is ubiquitous in the atmosphere with more than 100 PAH compounds having been identified in urban air. PAHs observed in the atmosphere range from bicyclic species such as naphthalene, present mainly in the gas phase, to PAHs containing seven or more fused rings, such as coronene, which are present exclusively in the aerosol phase. Intermediate PAHs such as pyrene and anthracene are distributed in both the gas and aerosol phases. The major compounds that have been identified in the atmosphere with their abbreviations and some physical properties are shown in Table 14.15 (Baek et al. 1991). The ambient concentrations of PAHs vary from a few ngm with values as high as 100 ngm-3 reported close to their sources (traffic, combustion sources). Concentrations are usually higher during the winter compared to the summer months (Table 14.16). 

The characteristics of several QCM instruments for aerosol measurement have been reviewed (ll). Particles are collected by impaction, electrostatic precipitation or both. The mass sensitivity is reported to be affected by the location of deposited particles on the crystal, the size of the particles, and the type of coating. In addition, the sensitivity changes as the crystal becomes loaded. Despite some limitations, most of the studies Indicated that QCMs can be successfully used for aerosol measurement with good correlation coefficient with the reference filtration method. Applications included measurement of aerosol in ambient air, particulate emission from automobiles and diesel engines, smoke plume from a coal-fired power plant, solid fueled rocket plvune, and particulate matter in the effluents in combustion sources. 

The morphology and size of PM from combustion sources has been investigated extensively and there are numerous publications on the subject (Harrison and van Grieken 1999 Donaldson and Borm 2006). Combustion carbonaceous aerosols... 

Organic compounds are a major constituent of the FPM at all sites. The major sources of OC are combustion and atmospheric reactions involving gaseous VOCs. As is the case with VOCs, there are hundreds of different OC compounds in the atmosphere. A minor but ubiquitous aerosol constituent is elemental carbon. EC is the nonorganic, black constituent of soot. Combustion and pyrolysis are the only processes that produce EC, and diesel engines and wood burning are the most significant sources. 

A smaller factor in ozone depletion is the rising levels of N2O in the atmosphere from combustion and the use of nitrogen-rich fertilizers, since they ate the sources of NO in the stratosphere that can destroy ozone catalyticaHy. Another concern in the depletion of ozone layer, under study by the National Aeronautics and Space Administration (NASA), is a proposed fleet of supersonic aircraft that can inject additional nitrogen oxides, as weU as sulfur dioxide and moisture, into the stratosphere via their exhaust gases (155). Although sulfate aerosols can suppress the amount of nitrogen oxides in the stratosphere... 

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