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Supplemental Text:
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Microwave digestion:
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Digestion of the filters and source materials was done by microwave (25 mL
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MARSXpress vessels, MARS 5, CEM) following a procedure modified from Upadhyay
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et al. [2009]. Sample preparation was done in metal free Class 10 laminar flow hoods in
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the W. M. Keck Foundation Laboratory for Environmental Biogeochemistry. The
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microwave procedure used 10 mL HNO3, 1 mL HF, and 4 mL 18 MΩ H2O. The program
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ramped to 140 °C over 6 minutes, held for 2 minutes, ramped to 165 °C over 5 minutes,
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held for 6 minutes, and finally ramped to 180 °C over 4 minutes, and held for an
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additional 15 minutes. Microwave vessels were cleaned between uses by following the
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above procedure using blank acid. Samples were dried down following microwave
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digestion and further digestion was done on a hotplate using HNO3 and H2O2 (ACS
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grade) until all material was digested.
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Soluble element analysis:
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Sample aliquots were prepared for major ion analysis by sonicating a 25 mm diameter
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circular filter subsection in 10 mL 18 MΩ H2O.1.5ml of filtered (0.22µm, Millipore)
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sample was pipetted into a polypropylene auto-sampler vial. Inorganic anion (NO3-, NO2-,
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SO42-, and Cl-) concentrations were determined using a Dionex IC20 ion chromatograph
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equipped with an IonPac®AG12Aguard column, IonPac® AS12A analytical column, a
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Dionex Anion Self-Regenerating Suppressor (ASRS), and a conductivity detector.
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Separation was achieved using a 2.7 mM Na2CO3/0.3mM NaHCO3 eluent at a flow rate
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of 1.2 ml/min. Inorganic cation (Na+, NH4+, and K+) concentrations were determined
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using a second IC20 ion chromatograph equipped with an IonPac® CG12A guard
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column and an IonPac® CS12A analytical column, a Dionex Cation Self Regenerating
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Suppressor (CSRS), and a conductivity detector. Separation was achieved using a 20 mM
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methanesulfonic acid eluent at a flow rate of 1.0 ml/min. Both IC systems were calibrated
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daily using a series of laboratory-prepared ion standards. For all of the major and minor
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elements listed above, the soluble phase was isolated using an acetate buffer extraction
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and analyzed by ICP-MS analysis of the same elements detailed previously [Upadhyay et
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al., 2011]. Filter subsections were added to 8 mL of a 5 mM acetate buffer at pH 4.3 on a
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shaker table for 2.5 h. The solutions were filtered with a 0.22 µm polyethersulfone (PES)
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syringe filter (Millipore Millex).
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References:
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Upadhyay, N., B. J. Majestic, P. Herckes (2011), Solubility and speciation of atmospheric
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iron in buffer systems simulating cloud conditions, Atmos. Environ., 45, 1858-1866,
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doi:10.1016/j.atmosenv.2011.01.010.
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