Aerosol crustal

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. 

The major ions have two main escape routes from the ocean (1) incorporation into sediments or pore water and (2) ejection into the atmosphere as seasalt spray. This spray is caused by bursting bubbles that produce small particles, called aerosols, that range in diameter from 0.1 to 1000 pm. The annual production rate of seasalt aerosols is large, on the order of 5 x lO kg/y, but virtually all of it is quickly returned when the spray fells back onto the sea surfece. A small fraction (about 1%) is deposited on the coastal portions of land masses and carried back into the ocean by river runoff. As shown in Table 21.6, seasalts represent a significant fraction of dissolved solids in river runoff, especially for sodium and chloride. Due to the short timescale of this process, seasalt aerosol losses and inputs are considered by geochemists to be a short circuit in the crustal-ocean-atmosphere fectory. The solutes transported by this process are collectively referred to as the cyclic salts. ... 

K-means cluster analysis is an excellent method for the reduction of individual-partide datai if extra clusters are used to allow for the non-spherical shape and natural variability of atmospheric particles. The "merge" method for choosing seedpoints is useful for detecting the types of lew abundance particles that are interesting for urban atmospheric studies. Application to the Phoenix aerosol suggests that the ability to discriminate between various types of crustal particles may yield valuable information in addition to that derived from particle types more commonly associated with anthropogenic activity. 

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...

It is seen that the distribution is bimodal, with the coarse mode dominating the aerosol volume concentrations. The 1979 average volume concentration of aerosol less than 10 ym diameter was 32.4 ymVcm. From its large standard deviation, it is clear that the coarse particle mode exhibited considerable variation throughout the year. Records show that high coarse mode volume concentrations accompanied moderate-to-high wind speeds. The coarse material was very likely wind-blown dust of crustal composition. 

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. 

As a first cut, univariate regression was used to see relationships between pairs of measured variables. The results are summarized in Table 6. It is seen that the particle scattering coefficient is highly correlated with the total fine aerosol mass concentrations, sulfate, and ammonium. To a lesser degree it is correlated with the particle absorption coefficient, potassium, and the unaccounted mass concentration. My. The correlations between the particle scattering coefficient and the nitrate, total carbon (Ct) and crustal species are poor. The poor correlation, r=0.6J, between bgp and the soot concentration is probably an indication of the error in the soot measurement. Although the sulfate and the unaccounted mass are highly correlated with the total mass, they are not well correlated with each other (r=0.49). 

On the basis of a preliminary regression analysis in which it was found that the crustal species, e.g., Fe, Ca, and Si, were well correlated with each other, the fine aerosol was grouped into four species sulfates, organics, crustal species, and the unaccounted species. The concentrations of these four species were calculated in the following manner ... 

In summary, the variance in the measured fine particle scattering coefficient, bgp was dominated by sulfate concentrations. Organics and crustal species were much less important statistically. The inferred mass scattering effeciency for sulfates was intermediate between other desert values and Los Angeles. The statistical results for China Lake are quite similar to those by other investigators at other locations, even though only the fine aerosol was sampled at China Lake. 

Sin summary, both techniques present a qualitatively similar picture of the contributions of aerosol chemical species to bgp. Ammonium sulfate is the most important measured species, while the contributions of organics and crustal species are smaller, but not negligible. The species which could not be identified by chemical analysis also contributed to bsp. but roughly half of this contribution is statistically linked to sulfate. 

The measured fine aerosol species were grouped into sulfates, organics, and crustal species, each having annual average mass concentrations of about 2-2.5 micrograms per cubic meter. An... 

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.

The average particle size distributions for four predominantly crustal elements, Al, Si, Ca, and Ti, are shown in Figure 3. They are essentially identical. It should be pointed out that the downturn of the relative concentrations above 8 ymad (impactor stage 6) is the combined result of the actual distribution of particle sizes in the atmosphere and the efficiency with which these very coarse particles can enter (upward) into the cascade impactor. This efficiency must decrease with increasing particle size and generally depend on inlet design and wind speed. Nevertheless, it is important to note here that the patterns of the four elements are similar, implying a common aerosol source. 

Figure 4 presents particle size distributions for six elements which differ among themselves and also from those in Figure 3. Somewhat subjectively, we may identify three patterns in these distributions (a) A coarse mode, typified by Ca and the other elements of Figure 3, which may represent a terrestrial dust origin. This mode can account for coarse particle concentrations observed for Fe, K, Mn, and S. (b) A fine mode with somewhat greater concentrations in the 0.5-1 ymad fraction than in 1-2 ymad particles. The amounts in this <2 ymad range, in excess of those which can be attributed to a coarse crustal aerosol tail with the Ca distribution, show similarities in particle size distributions for Zn, Mn, and possibly Fe. Since the trends shown in Figure 2 point to these elements being characteristic of large scale air masses, their fine modes may be principally due to natural processes.

In a search for sources of alkaline materials in rural air and rain, we have sampled and performed multi-element analyses on ambient particulate matter and potential source materials. Ambient aerosols were sampled daily using single Nuclepore filters or Florida State University "streakers." Samples of soil and unpaved road materials were also collected and analyzed. The samples were analyzed by various multi-element methods, including ion-and proton-induced X-ray emission and X-ray fluorescence, as well as by atomic absorption spectrophotometry. Visual observations, as well as airborne elemental concentration distributions with wind direction and elemental abundances in aerosols and source materials, suggested that soil and road dust both contribute to airborne Ca. Factor analysis was able to identify only a "crustal" source, but a simple mass balance suggested that roads are the major source of Ca in rural central Illinois in summer. 

Table I. Mean abundances of crustal elements in aerosols, soil, and road dust in a rural area near Champaign, IL.

Table II. Mean concentrations of four Important crustal elements in four aerosol sample groups.

Abundances. Additional evidence of possible multiple sources comes from an examination of the mass percentages (abundances) of crustal elements in aerosol samples. Figures 6 through 11 show abundances for Al, Si, K, Ca, Ti, and Fe. 

Table 1) and the aerosol element abundances measured in the 36 Individual vane samples. The distributions of crustal element abundances in aerosols were shown in Figures 6-11 along with the respective abundances of the elements in the source materials. 

The results presented a variety of evidence for the identity of Ca sources near our rural sampling site. The distribution of mean crustal element concentrations as a function of wind direction in summer and fall, from the streaker data, suggest a combination of road and soil sources. This agrees with a comparison of crustal abundances in aerosols and source materials. The comparison showed that most of the elements examined had abundances in the aerosol that often fell between those characteristic of roads and soil. This was not the case for Si, but Si may be expected to be less abundant in aerosol samples than in bulk surficial materials because of the preponderance of quartz (Si02) in the larger particles. 

Urban aerosol 105 Three modes nuclei, accumulation, and coarse larger particles contain crustal elements (Fe, Si, etc.), smaller contain nitrate, sulfate, ammonium, and elemental and organic carbon and are formed by combustion processes and gas-to-particle conversion... 

TABLE 9.12 Abundance in Earth s Crust of Some of the Most Common Crustal Elements and Those Commonly Found in Atmospheric Aerosols or Used as a Tracer"... 

Patterson, E. M., Optical Properties of the Crustal Aerosol Relation to Chemical and Physical Characteristics, J. Geophys. Res., 86, 3236-3246 (1981). 

A hypothetical aerosol size/composition distribution is shown in Figure 12.1, indicating that crustal materials (e.g., COf, Si, Al, Fe, Ca, and Mn), sea spray (e.g., Mg, Na, and Cl), and biogenic organic particles (e.g., pollen, spores, and plant fragments) are usually found in the coarse aerosol fraction (2.5 < r/ae < 10pm) (Meszaros et al., 1997 Krivacsy and Molnar, 1998 Matsumoto et al., 1998 Seinfeld and Pandis, 1998 Maenhaut et al., 2002 Smolik et al., 2003). Wind erosion, primary emissions, mechanical disruption, sea spray, and volcanic eruptions all contribute to the concentrations of these species (Seinfeld, 1986 Seinfeld and Pandis, 1998). 

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). 

---