AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Implementing Volatility Basis Set Algorithm for Organic Aerosol Formation in CMAQ 5.0
BONYOUNG KOO, Greg Yarwood, Eladio Knipping, ENVIRON International Corporation
Abstract Number: 521 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract A volatility basis set (VBS) approach is implemented in the Community Multiscale Air Quality (CMAQ) model version 5.0 to provide an alternative framework of organic particulate matter (PM) formation. Of the various components of atmospheric PM, organic aerosol (OA) is among the most abundant and the least understood and is usually under-predicted substantially in current air quality models. Traditionally, primary organic aerosols (POA) have been treated as non-volatile and non-reactive. However, environmental chamber studies that vary temperature and aerosol dilution have shown that POA can be semi-volatile and undergo gas-particle partitioning in the atmosphere similar to secondary organic aerosols (SOA). Also, it is now known that further oxidation of semi-volatile organic compounds (SOA and POA) can create products with lower volatility leading to increased OA formation downwind. The organic aerosol (OA) module in CMAQ 5.0 uses a two-product model for semi-volatile SOA with polymerization decreasing SOA volatility over time and assumes non-volatile POA with oxidative aging increasing the mass of POA over time. The VBS approach provides a unified framework for gas-aerosol partitioning of both POA and SOA including chemical aging. The VBS implementation in CMAQ 5.0 uses four separate basis sets to differentiate anthropogenic POA and SOA, biogenic SOA, and OA from biomass burning. Each basis set consists of five volatility bins including zero-volatility bin for non-volatile compounds. Molecular weight increases as volatility decreases to account for mass gain from chemical aging. The new organic PM module is evaluated with a modeling database provided by US EPA which simulates a 2006 summer episode over the entire continental US, and its effect on modeled OA formation is discussed.