AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Expanding the Modeling of Semivolatile Aerosols within the CMAQ Framework: Development and Application to Oxidized Cyclic Siloxanes and Polychlorinated Biphenyl Compounds
NATHAN JANECHEK, Scott N. Spak, Keri Hornbuckle, Charles Stanier, University of Iowa
Abstract Number: 473 Working Group: Regional and Global Air Quality and Climate Modeling
Abstract Cyclic volatile methyl siloxanes (cVMS) and polychlorinated biphenyl (PCB) are well-studied anthropogenic chemical classes with some important similarities in atmospheric behavior. PCB and cVMS oxidation products can exist as semivolatile compounds that partition to the aerosol phase. For example, the saturation (c*) concentrations of PCB congeners range from 0.1 to 105 µg m-3, while the oxidized cVMS compounds have been estimated at 0.01 to 100 µg m-3. PCBs were important industrial chemicals used in electrical capacitors and transformers, fire retardants, and plasticizers. Banned in the 1970s, they are still present in the environment and pose health concerns. Cyclic siloxanes on the other hand are also important industrial chemicals common in personal care products, especially antiperspirants and lotions. In the atmosphere, parent cVMS are primarily degraded by reaction with hydroxyl radicals (OH) to form semivolatile oxidation products which contribute to atmospheric secondary organic aerosol and may be an important source of ambient nanoparticles. In this work, semivolatile aerosol species for cVMS and PCBs are added to the Community Multiscale Air Quality (CMAQ) model using an equilibrium-partitioning model (subroutine orgaer). The secondary aerosol species for the oxidation products of the most common cVMS (D4, D5, and D6) are added using experimentally derived yield and volatility data. Mass stoichiometric yield, effective saturation concentration, enthalpy of vaporization, and molar mass were used in the model parameterization. Using the model, aerosol phase partitioning of the cVMS oxidation products are quantified, as well as the first reported ambient concentrations and the spatial distribution, which can be used to guide future measurements to confirm secondary aerosols. Similarly, semivolatile PCB and PCB oxidation species are added using new urban emission estimates to the CMAQ model to simulate gas and aerosol species. PCB concentrations and deposition are quantified to determine air exposure, an important tool in determining cost-effective cleanup strategies.