Aerosol Chemical Composition and Hg

R. Arimoto

New Mexico State University

NMSU will make measurements at only the SP site in both 2003 and 2005. This will involve a continuation of the aerosol chemical composition investigation began during the ISCAT program but with the addition of a few new species. The analytes for this study will include key compounds in the cycles of nitrogen (nitrate) and sulfur (sulfate and methanesulfate) as well as two important chemical tracers, Na for sea salt, and Al for mineral dust. And as in ISCAT 2000 study three radionuclides (Be-7, Po-210 and Pb-210) will be measured as a way to track stratospheric air and volcanic emissions from Mt Erebus. To this earlier list has been added the metallic pollutants Hg and Pb.. These studies will serve two major purposes. First they will continue the ISCAT assessments of the sources for non-sea salt aerosol sulfate, including contributions from biogenic and anthropogenic sources, e.g., science objective 3. And second, the studies will contribute to ANTCI by serving as a point of comparison, i.e., measurements of aerosol nitrate can be compared with gas-phase HNO3, a critical species in the reactive nitrogen budget. Finally, there will be a significant effort to gain a better understanding of the processes influencing Antarctic plateau Atmospheric mercury levels.
For the NMSU studies, total suspended particle samples will be collected daily using separate high volume (>50 cubic meters per hour) samplers. One filter (Whatman 41®) from each sampling interval will be cut into two to four aliquots, with separate portions used for studies of trace elements and another for the major ion studies. The second filter (glass fiber) for each collection interval will be used for studies of Be-7, Po-210 and Pb-210.
A variety of methods will be used for the analyses of the aerosol samples, including; inductively-coupled plasma mass spectrometry (ICP-MS), atomic absorption spectrometry (AAS), ion chromatography (IC), and alpha- and gamma-spectroscopy. The filters will be prepared for the elemental analyses using a mixture of acids (HNO3, HCl, and HF,) plus H2O2 in a microwave digestion unit. The acid digests of the aerosol samples will be analyzed using a Perkin-Elmer Elan 6000 inductively-coupled plasma mass spectrometer following US EPA Method 200.8 or with a Perkin-Elmer 5100 atomic absorption unit for elements such as As, Se, and Hg for which AAS is especially sensitive. Detection limits for trace elements typically are of the order of several tens of nanograms per cubic meter. For the aerosol ion analyses, the filters will be extracted with 18 MO de-ionized water. The anion (sulfate, nitrate, methanesulfonate, and chloride) and cation (Na⁺, K⁺, Mg²⁺, and Ca²⁺) concentrations will be determined using a Dionex 500 ion chromatography system with the appropriate guard and separation columns. The detection limits for the aerosol ions also are in the tens of nanograms per cubic meter range. The uncertainties in the elemental and ion analyses are on the order of ~ 10-20%. These aerosol ion measurements will complement those of Rodney Weber on the C-130 in 2005 and allow a comparison of long-term ground level measurements to aircraft vertical profile results. Such data should provide new insight into particle transport and deposition rates to the surface.
A system was developed at NMSU for the analysis of Be-7 and Pb-210 activities in aerosol filters by gamma spectroscopy, using a proprietary phoswich detector. Techniques also were developed for the determination of Po-210 activities in the aerosol samples based on a procedure developed by the Environmental Measurements Laboratory of the US DOE.
The studies of the filterable Hg, which is a combination of reactive gaseous mercury and particulate Hg, will be complemented by collection of gas-phase Hg samples on gold-coated glass beads packed in quartz tubes. Analyses will be conducted by cold vapor atomic absorption after the sample tubes are returned to NMSU.