title: Development of a wet-chemical method for the speciation of iron in atmospheric aerosols.
abstract: The ability to quantify the chemical and physical forms of transition metals in atmospheric particulate matter (PM) is essential in determining potential human health and ecological effects. A method for the speciation of iron in atmospheric PM has been adapted which involves extraction in a well-defined solution followed by oxidation state specific detection. The method was applied to a suite of environmental aerosols. Ambient atmospheric aerosols in an urban area of St. Louis (the St. Louis-Midwest Supersite) were collected on Teflon substrates and were leached in one of four different solutions: (1) >18.0 Momega water; (2) 140 microM NaCl solution; (3) pH = 7.4 NaHCO3 solution; and (4) pH = 4.3 acetate buffering system. Fe(ll) was determined directly using the Ferrozine method as adapted to liquid waveguide spectrophotometry using a 1 m path-length cell. Fe(lll) was determined similarly after reduction to Fe(ll). It was found that, at low ionic strength, pH exerted a major influence on Fe(ll) solubility with the greatest Fe(ll) concentration consistently found in the pH = 4.3 acetate buffer. Soluble Fe(lll) (as defined by a 0.2 microm filter) varied little with extractant, which implies that most of the Fe(lll) detected was colloidal. To characterize well-defined materials for future reference, NIST standard reference materials were also analyzed for soluble Fe(ll) and Fe(lll). For all SRMs tested, a maximum of 2.4% of the total iron (Urban Dust 1649a) was soluble in pH = 4.3 acetate buffer. For calibration curves covering the ranges of 0.5-20 microg/L Fe(ll), excellent linearity was observed in all leaching solutions with R2 values of > 0.999. Co-located filters were used to test the effect of storage time on iron oxidation state in the ambient particles as a function of time. On two samples, an average Fe(ll) decay rate of 0.89 and 0.57 ng Fe(ll) g(-1) PM day(-1) was determined from the slope of the regression, however this decrease was determined not to be significant over 3 months (95% confidence). As an application of this method to mobile source emissions, size-resolved PM10 samples were collected at the inlet and outlet of the Caldecott Motor Vehicle Tunnel in northern California. These samples indicate that the coarse fraction (PM10-PM2.5) contains almost 50% of the total soluble Fe(ll) in the aerosol.
authors.affiliation: Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 North Park Street, Madison, Wisconsin 53706, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Majestic
authors.forename: James J
authors.initials: JJ
authors.lastname: Schauer
authors.forename: Martin M
authors.initials: MM
authors.lastname: Shafer
authors.forename: Jay R
authors.initials: JR
authors.lastname: Turner
authors.forename: Philip M
authors.initials: PM
authors.lastname: Fine
authors.forename: Manisha
authors.initials: M
authors.lastname: Singh
authors.forename: Constantinos
authors.initials: C
authors.lastname: Sioutas
mesh_terms: D000336:Aerosols / Q000737:chemistry*; D001272:Atmosphere; D002138:Calibration; D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D012015:Reference Standards; D015203:Reproducibility of Results; D012995:Solubility
chemical_list: D000336:Aerosols; D007501:Iron