J1.5
Scanning Electron Microscope and Micro-Chemical Speciation of Sulfate-dominated, lead-bearing aerosols, New York City, 2007
Nicholas C. Steiner, City College of New York, New York, NY; and S. St. John, D. Breger, and J. C. Steiner
Iron- and sulfur-enriched aerosols are common industrial pollutants transported preferentially from the Midwestern United States. By some estimates, sulfates and desert dust comprise up to 50% of the PM2.5 aerosol mass reaching New York City. The major influx and mixing with urban air occurs during the summer months. Test cases carried out to confirm sourcing and to establish aerosol speciation presently focus on collections compiled May through July, 2007. Aerosols enriched in iron and sulfur associated with westerly wind patterns reach maximum levels in the period June 26 to 28. Aerosols impacted on 0.2 micron polycarbonate filters are used for baseline estimates of chemical composition and to establish possible associations with transported lead, arsenic and other trace metals.
X-ray fluorescence and atomic absorption spectrometry (AAS) analyses demonstrate that an approximate 20% upsurge in iron, sulfur, calcium, titanium, potassium and lead occurred in the test period while aluminum and silica, considered crustal components, remain at background levels. A principle discovery shows that lead is well correlated (60%) with calcium over the study interval.
Scanning electron microscope and energy dispersive (SEM/EDS) micro-analytical analyses of particulates accumulated over 24 hour periods for June 26-28 show that iron is present in two principle associations: iron with alumina-silicates and iron-enriched particles. The commonly spherical iron particles are present as discrete 2 to 5 micron-size grains and as irregular grains associated in iron-carbon-oxygen complexes. Filters are also characterized by the presence of PM2.5 iron oxide, barium, chromium, copper metals, and irregular masses and blades of gypsum. The net iron levels obtained by AAS are approximately 20% reduced over those obtained by x-ray fluorescence using a Panalytical PW1400 wavelength-dispersive x-ray fluorescence spectrometer operated at 40 Kilovolts and 40 milliamps counted over 100 seconds and calibrated using Panalytical aerosol standards. Based on the bimodal occurrence of iron in aerosol particulates, the difference is assigned to the probable differential solubility of Fe metals and FeO oxides relative to Fe-sulfate, Fe-carbonate and organic compound associations.
A search for trace metals using an FEG FEI XL30 Environmental SEM/EDS system in high-vac mode shows a variety of associations. Lead particulates are typically present as 0.4 to 1.2 micron trace grains in 5 to 20 micron composite grains. The common aggregate matrix by SEM/EDS is carbon-rich, presumably organic compounds, containing reduced levels of major elements, although Pb is also embedded in silica-enriched composites.
Recorded presentationJoint Session 1, Measurements and Modeling of Aerosols and Clouds-I
Tuesday, 22 January 2008, 1:30 PM-3:00 PM, 220
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