Chemistry of the natural aerosol : a case study in South America / Douglas R. Lawson.

Tese (PhD)- Florida State University, 1978

An intensive aerosol sampling program has been carried out at remote continental and marine, as well as urban, locations on the South American continent. Samples were taken at 28 sites-during July-August 1976 and January-March 1977 with cascade impactors and time sequence filter samples. 113 impactor runs and 19 time sequence filter runs provided nearly 1600 discrete samples, including blanks, which were analyzed by proton induced X-ray emission (PIXE) for 20 elements: Al, Si, P, S,-Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Br, Sr, Pb, and Bi. Analysis of the elemental composition of the marine aerosol at Salvador, Brazil and Punta Arenas, Chile shows elemental fractionation in the aerosol relative to seawater composition. S and Ca are enriched in coarse particles (1 µm diameter) by factors of 1.8 and 1.6, respectively, relative to Cl. K and Br show no fractionation in the marine aerosol, while Ti and Fe are enriched on the order of 10 relative to Cl. These results indicate that the sea surface may be an important source of trace element enrichment in the marine aerosol. Cascade impactor samples from remote sites show a SO ng/m³ fine particle mode of S present at nearly all locations. Although some coastal regions exhibit slightly higher values, owing to the intense sea spray formation, the fine particle mode of S was not evident in southern Chile winter measurements, suggesting that even a lower value of -10 ng/m³ may be representative of true background levels of fine particles. The 50 ng/m³ fine particle mode of S otherwise generally observed, and not related to proximity to the ocean, is one-fourth the published nonurban values from the Northern Hemisphere. Consequently, revisions in sulfur budget calculations may be necessary. Tropical rain forest aerosols in Bolivia and Brazil, sampled before and during rainfall episodes, indicate that significant portions of S and K in coarse particles may be biogenically produced. Concentrations of these elements are not greatly suppressed during periods of intermittent rainfall, while concentrations of soil-derived aerosol Al, Si, Ca, and Fe are greatly reduced during rainy periods. The proportions of aerosol S and K resemble those found in biological matter, and differ greatly from corresponding values in earth's crustal materials. These results suggest that biological processes might ultimately be responsible for anomalous trace element enrichments in the atmospheric aerosol. The crustal elements Al, Si, K, Ca, Ti, and Pe in 4 µm aerosol particles from remote continental locations are distributed in similar relative proportions, although their concentrations vary 100-fold. The continental aerosol is apparently a uniform mixture, and may serve as a new reference material, the standard crustal aerosol (SCA) for enrichment factor calculations. The crustal elements in the SCA have weight ratios to Fe: Al, 2.0; Si, 4.5; K, 0.20; Ca, 0.41; Ti, 0.10. Enrichment factors for urban aerosols from major South American cities show that very little enrichment of crustal elements, even in the largest cities, is present in the urban aerosol. Field comparisons using cascade impactors have been made for uncoated and paraffin, Apiezon L grease, and Vaseline coasted impaction surfaces. Uncosted and paraffin osted impaction surfaces were only 25-45% and 5C-75% as efficient in particle collection, respectively, as the equally efficient Apiezon L and Vaseline coatings. The relative humidity and aerosol particle source are important factors determining collection efficiencies in cascade impactors. Calculated mass median diameter (MMD) values differ greatly from one location to another; more effective impaction surface coatings produce higher and more accurately determined MMD's than inefficient coatings.