10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Cyclic Siloxane Oxidation over North America: Quantifying the Strength, Properties, and Lung Cytotoxicity of Widespread Silicon from Personal Care Products
Nathan Janechek, Benjamin King, Rachel Marek, Andrea Adamcakova-Dodd, Traci Lersch, Kristin Bunker, Gary Casuccio, Kaj Hansen, William Brune, Peter Thorne, Keri Hornbuckle, Jennifer Fiegel, CHARLES STANIER, University of Iowa
Abstract Number: 1489 Working Group: Aerosols in Earth System
Abstract Cyclic volatile methyl siloxanes (cVMS) are anthropogenic chemicals present in a range of consumer personal care products such as antiperspirants and lotions. They are highly volatile, but rather non-reactive – and thus found in high concentrations in indoor environments, and then are transported long distances through the atmosphere. A major removal pathway for these silicon-containing gases is reaction with the OH radical, which has been recently shown to yield secondary Si-containing aerosol compounds in addition to the established gas phase products. To investigate this further, we integrate research on the source, environmental loading, transport, and potential health impacts of this compound class. Specifically, a series of related laboratory and modeling investigations were performed. These include laboratory aerosol generation in an Oxidation Flow Reactor (OFR), physical and chemical particle characterization, in vitro lung cytotoxicity studies, and modeling using the Community Multiscale Air Quality (CMAQ) atmospheric transport/chemistry model.
While the parent compounds such as decamethylcyclopentasiloxane (D5) have been well-studied for potential health and ecological impacts, the oxidation products and potential aerosol species have received much less attention, with almost no ambient measurements or experimental physical property data. We generated Si-C-O aerosols by supplying D5 to an OFR. The particles were characterized for concentration, aerosol yield, size, morphology, volatility, individual particle chemical composition, and CCN formation potential (using a DMT-CCN or Droplet Measurement Technologies Cloud Condensation Nuclei Counter).
Aerosol yield sensitivity to D5 and OH concentrations, residence time, and seed aerosols were analyzed. TEM-EDS and STEM-EDS analysis show spherical particle morphology for primary particles, elemental composition consistent with aerosols derived from cyclic siloxane sources, and aggregate formation. Measured aerosol yields were 20-50% with typical aerosol concentrations 300,000 particles cm-3, up to 200 μg m-3, and diameters of 30-90 nm. Particles sized 10-110 nm did not experience any significant reduction in diameter after being heated up to 200°C suggesting low volatility. Particles exhibited slight hygroscopic behavior with calculated effective kappa parameters ranging between 0.006-0.02 which is comparable to the lower range of previously reported secondary organic aerosols. Smaller particles were found to have increased CCN potential, suggesting the aerosols are non-homogenous with smaller particles likely more oxidized. Acute exposure cellular toxicity was tested by exposing A549 lung epithelial cells to the generated aerosols using an in vitro air-liquid interface exposure system. No significant difference in cell viability could be attributed exclusively to the aerosol species and no increase was observed in biomarkers of inflammation and oxidative stress.
Three cVMS compounds and their oxidation reactions were added to the CMAQ model. Typical concentrations, seasonal variability, spatial patterns, and vertical profiles were quantified at 36 km spatial resolution over North America during four separate seasons. Typical model concentrations showed parent compounds were highly dependent on population density as cities had monthly averaged peak D5 concentrations up to 432 ng m−3. Peak oxidized D5 concentrations were significantly less, up to 9 ng m−3, and were located downwind of major urban areas. Model results were compared to available measurements and previous simulation results. Parent compound concentrations in urban and peri-urban locations were sensitive to transport factors, while parent compounds in rural areas and oxidized product concentrations were influenced by large-scale seasonal variability in OH.
Key uncertainties with cVMS aerosol production is the yield of aerosol and semivolatile compounds at ambient OH concentrations, and the chronic health effects of the particles. This work only addressed yield at elevated reactant concentrations, and acute in vitro health effects.