Airborne nanoparticulate matter (nPM) poses a major health risk world wide, but also represents a global mechanism for trace element contributions to biogeochemical cycles. Knowledge of element presence, speciation, and bonding in crystalline or amorphous phases in nPM mixtures is fundamental to deciphering mechanisms of adverse health impacts, assessing potential element bioavailability or cycling in aquatic and terrestrial ecosystems, and informing pollution regulations and health advisories. Air sample filters (PM 2.5) from the San Joaquin Valley consisted of paired sets collected during either summer or winter. Analyses included bulk and microfocused (u) X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF) mapping, and complementary characterization by scanning and transmission electron microscopy (SEM, TEM) with energy-dispersive X-ray spectrometry (EDXS), electron energy-loss spectrometry (EELS) and particle size analysis. Total filter particulate mass was ~10 times higher in winter than summer due to high ammonium nitrite, but the mass fraction of the geological (mineral) component was ~3 times higher in summer than winter. Size distributions determined by SEM and TEM indicated individual particles generally between 10-100 nm and aggregates between 1-2 um. Elemental spot analyses with EDXS or u-XRF mapping indicated mixtures containing Al, Si, Fe, Na, Mg, Ca and/or K in some particles, consistent with the presence of aluminosilicate minerals, mostly likely smectitic clays. Hotspots of Si were correlated with Al in some but not all particles, consistent with the presence of clay minerals. Results of u-XRF mapping showed complex mixtures of Fe, Zn, and Cu in particle aggregates; Cu hotspots may be from brake pad residue. Sulfur speciation determined by u-XANES was dominated by sulfate as expected, but reduced S species were detected in a few spectra. Comparison of bulk Fe XAS and u-XANES showed that Fe was primarily a mixture of Fe(III)-oxide as either ferrihydrite (amorphous Fe(OH)<sub>3</sub>) or goethite (FeOOH) and a low-Fe clay mineral (smectite-type) with mostly Fe(III) rather than Fe(II). Spectral differences suggest slightly different proportions of Fe-oxide and clay minerals in different particle aggregates, even in u-XANES spectra. Results suggest a significant fraction of labile or reactive Fe associated with the Fe(III)-oxide component, which comprised half or more of the total iron in bulk spectra.

Element speciation in atmospheric nanoparticles and implications for human health and biogeochemical cycling

253rd National Meeting (2017)

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