Enrichment aerosol elemental

Aerosol Elemental Enrichment Factors, 1979, China Lake... 

As expected, B, S, V, Zn, As, Se, Sb, and the halogens were found to be strongly enriched. The elements of interest in this study, B, S, and Se have EF values of 35 22, 910 420, and 1940 980, respectively. Preliminary results show the lowest B(g), SO2, Se(p), and sulfate concentrations during N to E winds and the highest values during SW to NW winds. A summary of the data is shown in Figure 4, and periods apparently affected by coal-derived aerosol are clearly seen. 

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. 

Since the majority of the elements in surface dust arise from deposited aerosol and added soil it is not surprising to find strong linear relationships between the concentrations of the elements in an atmospheric dust and street or house dust. This is illustrated by the two examples given in Fig. 8 for remote house dust vs urban atmospheric dust and street dust vs rural atmospheric dust. As discussed above crustal/soil material is a major component of atmospheric dust and the soil based elements in the atmospheric dust are Al, Ca, Fe, Mg, Mn, Ni, K, Si and Ti. The elements As, Br, Cd, Cl, Co, Cu, Pb, Rb, Se, V, and Zn are, on the other hand, enriched in atmospheric dust. The same elemental distribution applies to surface dust, but in this case their concentrations (compared on a mass basis) are reduced presumably due to dilution with soil. However, the elements enriched in the atmosphere remain enriched in the surface dusts. 

Figure 1. Enrichment factors with respect to crustal abundances (39) for elements attached to urban aerosols from (9) Washington, D.C. (16), (O) Tucson, AZ f40j, (y,) St. Louis, MO (based on data from Loo et al. (41)),(A) Charleston, WV (42), (Ls) Portland, OR (21), and fB) Boston, MA (3,43). See Table IV, Footnote a...

To estimate the contribution of wind-blown dust of crustal origin to the fine aerosol concentration, elemental enrichment factors were calculated using the method of Macias et al. (20). The enrichment factor, EF., for an element i was calculated as follows ... 

Iron was chosen as the reference element because its major source is likely to be soil and it is measured with good accuracy and precision by FIXE. Crustal abundances were taken from Mason (21). Enrichment factors greater than 1 indicate an enrichment of that element relative to crustal abundances values less than 1 indicate a depletion. The results of this calculation are shown in Table 4. For this calculation it was assumed that ammonium and nitrate accounted for all aerosol nitrogen. It is seen that Si and Ca are near their crustal abundance, indicating a probable soil dust source. The low EF for Al is probably due to a systematic error in the Al measurement rather than a true depletion. Potassium, although present in small concentrations, is slightly enriched relative to crust. The other fine aerosol species, C, N, S, and Pb are enriched by factors of thousands over their natural crustal abundance, indicating that they are not due to wind-blown dust. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

Figure 2 indicates Mn/Fe to be somewhat above the crustal ratio through 19 March, and thereafter a marked Increase is seen. The aerosol ratio Zn/Fe averages about 20 times greater than in the earth crust (somewhat greater on 20-21 March), showing "anomalous" atmospheric enrichment of Zn first recognized by Rahn (7). Since particle size distribution measurements, discussed below, show substantial fine particle concentrations of both Zn and Mn, the processes for their transfer to the atmosphere must be different from those for the other six elements of Figure 2. However, their concentration variations in time still resemble those of Fe shown in Figure 1 and therefore these elements may also be relatively large scale characteristics of air masses, in contrast to S where regional pollution sources and aerosol formation processes must be Important.

Because of the nature of the enrichment process during combustion, potential tracer elements such as As, Se, and I are more enriched on fine than on coarse particles. Therefore, analyzing the fine particle fraction of source emissions and ambient aerosols increases the source-discriminating power of receptor models. 

Afterfilter data. As indicated in Table I, the minimum D50 in this study was about 0.5 pm, and particles smaller than this were collected on an afterfilter. Aerosols from combustion of pulverized coal typically are distributed bimodally, with a fine-particle mode at about 0.1 pm and a large-particle mode at supermicrometer sizes the modal diameter of the latter depends strongly on the efficiency characteristics of the control device. The elemental concentrations in the fine-particle mode are of interest in health-impact and source-apportionment studies because of the typically high enrichment of the concentrations of many potentially toxic elements and useful tracer elements in particles in this size range. Large-particle con-taimination of the afterfilter due to particle bounce can, however, limit the value of these data. 

Natural radionuclides are present in all plants and animals and in man. The activity of is 31 Bq per g of K, and the average activities in meat and in milk are about 120Bq/kg and 50Bq/l, respectively. The transfer of U and Th to plants and animals is very small due to the low solubility and the low resorption of these elements. Their activities in milk are of the order of 10 Bq/1 and in meat and fish of the order of 5 10 Bq/kg. Ra has better access to the food chain and, due to its similarity to Ca, Ra is enriched in bones, where it is found in amounts of the order of 10 g/g. The activity of Ra in other parts of animals and man is about 10 Bq/kg. Pb and Po, decay products of Rn, are present in aerosols and deposited with precipitations on plants. Their uptake from the air is much higher than that by the roots. They enter the food chain and are found in concentrations of 1 to 10 Bq/kg in meat. In reindeer livers values >100 Bq/kg have been measured. 

There have been many measurements of the elemental composition of urban aerosols stimulated by the need for large databases in aerosol source apportionment (discussed in a later section). Table 13.2 compares concentrations in the fine and coarse fractions for various U.S. cities. The results show remarkable similarities in the order of magnitude of the concentrations from city to city for each element. Soil dust is a major component of the coarse fraction as indicated by the strong enrichment in aluminum and silicon in every city. The coarse fraction is much less active chemically both with respect to its mechanisms of formation and as a site for reaction, compared to the fine fraction discussed next. 

In the recent decade, evidence has been mounting that volcanic emissions lead to an enrichment of so-called volatile elements in the particulate matter ejected, compared with the relative abundances found in bulk material of the earth s crust (Cadle et al., 1973 Mroz and Zoller, 1975 Lepel et al., 1978 Buat-Menard and Arnold, 1978). Figure 7-18 summarizes the results of Buat-Menard and Arnold (1978) for samples from the plume of Mt. Etna, Sicily, and compares them with similar data for aerosols collected... 

Fig. 7-18. Left Enrichment factors for volatile elements in aerosols from volcanoes. Data were compiled from Cadle el al. (1973) and Duce et al. (1976) for the volcano Kilauea Mroz and Zoller (1975) for Heimaey Lepel era/. (1978) for St. Augustine Buat-Menard and Arnold (1978) for Etna. Right Enrichment factors for volatile elements in background aerosols. Data are from Duce et al. (1976), triangles, Buat-Menard and Chesselet (1979), diamonds, Zoller et al. (1974), circles, and Maenheut et al. (1979), vertical bars.

We turn next to consider the nonvolatile alkali and alkaline earth elements and the insoluble components of mineral origin. Their major natural sources are the Earth s crust and the ocean, respectively. We expect the chemical composition of the aerosol to reflect the relative contributions of elements from both reservoirs, provided other contributions from anthropogenic or volcanic sources are negligible. In Section 7.4.4 it has been noted, however, that his premise does not hold for all constituents of the aerosol. Some trace components are considerably enriched compared with their crustal abundances. It is appropriate, therefore, to inquire whether the observations confirm our expectations at least for the major elements listed in Table 7-13, or whether deviations occur also in these cases. As Rahn (1975a,b) has shown, the problem may be approached in two ways, either by calculating enrichment factors defined by... 

Hoffman et al. (1974) found the same procedure applicable to data obtained from measurements on board of ships in the central Atlantic Ocean. Table 7-15 includes mean (X)/(Na) ratios from their work. Shown in parentheses are the values derived from the slopes of regression lines. They are distinctly lower than the averaged data. Hoffman et al. (1974) measured also the abundance of iron in the aerosols. Since the samples were taken in a region partly affected by fallout from the Saharan dust plume, iron serves as a convenient indicator for the contribution of material from continental sources. Not surprisingly, the enrichment of the elements Mg, Ca, K, and Sr was well correlated with the iron content. The (X)/(Na) ratios approached those of sea water only when the Fe concentrations were very low. These results demonstrate that materials from both marine and crustal sources are present over the open ocean. In addition, they provide some verification for the existence of a tropospheric background aerosol having the continents as a source, and they confirm the absence of a significant fractionation of alkali and alkaline earth elements in the production of sea salt. 

As Table 7-16 shows, the relative abundances of the major elements in the aerosol do not differ greatly from those in bulk soil, crustal rock, or average shale—that is, the elements are neither greatly enriched nor seriously depleted. A good match with any of the three reference materials is not obtained, however. The differences must be significant, since they are greater than conceivable analytical errors. Consider silicon as an example. Tables 7-13 and 7-16 indicate an average Si/Al ratio of 2.7, which is lower than that for either bulk soil or crustal rock and is more similar to that in shales. Fly ash exhibits a particularly low Si/Al ratio. It is possible that the low aerosol value in heavily industrialized Tees-side (Table 7-13) is due to a mixture of natural and combustion aerosols, but this explanation cannot be extended to the remote continental aerosol. A more likely explanation for the silicon deficiency is the size distribution of the Si/Al ratio in soil particles. The very coarse quartz particles, which are rich in silicon, are not readily mobilized. Since only the fine fraction of soil particles contributes to aerosol formation, the Si/Al ratio in the aerosol will be determined by that of silts and clays (see Table 7-7 for definitions). Common clay... 

Table 7-lb. Average Absolute and Relative Abundances of Major Elements in Crustal Rock, Soil, and Shale-, Relative Abundances of Elements in Fly Ash from Coal and Fuel-Oil Combustion and Relative Abundances of Major Elements in the Remote Continental Aerosol, with Enrichment Factors (Aerosol) EF= (X)/(AI)aeroso,/(X)/(AI)crusta, rock ... 

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