AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Comprehensive Atmospheric Modeling of Gas-phase Cyclic Volatile Methyl Siloxanes and Their Oxidation Products
NATHAN JANECHEK, Kaj Hansen, Charles Stanier, University of Iowa
Abstract Number: 470 Working Group: Regional and Global Air Quality and Climate Modeling
Abstract Cyclic volatile methyl siloxanes are common chemicals in personal care products, especially antiperspirants and lotions. In the atmosphere, cyclic siloxanes are oxidized by hydroxyl radicals (OH) forming oxidation products that likely contribute to secondary aerosol loadings. In this work, we use the Community Multiscale Air Quality (CMAQ) atmospheric chemistry transport model to simulate gas phase cyclic siloxane behavior over North America, including first-ever comprehensive predictions of the potential for secondary aerosol from this source. This may be of interest to aerosol measurement groups, as modeled silicon SOA concentrations may guide measurement efforts in the absence of measurements. The model has been modified to include the most common cyclic siloxanes (D4, D5, and D6) and their OH oxidation products, emissions and seasonally varying boundary conditions calculated using measurement ratios accounting for differential oxidative aging, and wet and dry deposition for the cyclic siloxane species. This work represents the highest resolution modeling to date, and for the first time expected concentrations and the spatial distribution of the oxidation products, which some fraction likely exists in the particle phase, are reported. Using the model, four 1-month periods are modeled to quantify seasonal variability, expected concentrations, spatial patterns, and vertical profiles. Modeling results show urban parent cyclic siloxane seasonal concentrations are insensitive to regional OH but are rather controlled by the degree of localized vertical and horizontal dispersion. Monthly averaged peak concentrations for D5 were up to 432 ng m-3 occurring in major urban areas while the oxidized D5 product was much lower, up to 9 ng m-3 and occurred downwind of cities. Model results were evaluated against available measurement and modeling studies. Error metrics remain high but are improved compared to previous modeling attempts, especially for urban locations.