1.5-Dimensional volatility basis set approach for modeling organic aerosol in CAMx and CMAQ

A hybrid volatility basis set (VBS) approach to modeling atmospheric organic aerosol (OA) is developed that combines the simplicity of the 1-dimensional (1-D) VBS with the ability to describe evolution of OA in the 2-dimensional space of oxidation state and volatility. This 1.5-D scheme uses four basis sets to describe varying degrees of oxidation in ambient OA: two basis sets for chemically aged oxygenated OA (anthropogenic and biogenic) and two for freshly emitted OA (from anthropogenic sources and biomass burning). Each basis set has five volatility bins including a zero-volatility bin for essentially non-volatile compounds. The scheme adjusts oxidation state as well as volatility in response to chemical aging by simplifying the 2-dimensional VBS model. The 1.5-D VBS module is implemented in two widely used photochemical grid models (CAMx and CMAQ) and evaluated for summer and winter 2005 episodes over the eastern U.S. CAMx performs reasonably well in predicting observed organic carbon (OC) concentrations while CMAQ under-estimates OC, with differences between models being attributed to science algorithms other than the VBS. Oxygenated OA accounts for less than half of the modeled OA mass in winter but about 80% of total OA in summer due to more rapid chemical aging in summer.