Theoretical method for lumping multicomponent secondary organic aerosol mixtures

Atmospheric organic aerosol mixtures are composed of hundreds of individual semivolatile organic components, each with its own partitioning properties. In most atmospheric models, these components are grouped together into a few lumped compounds. A theoretical method is described for converting multiple organic aerosol components into one or more lumped groups using a set of equations that calculates the properties of lumped compounds from individual component properties. Expected organic aerosol mass and temperature range values are specified and components are grouped together according to relative volatility. A universal set of volatility dividing lines is presented for dividing components into multiple lumped groups. The lumping method is evaluated with three different mixtures: a base case mixture, a set of 1000 random mixtures with varying properties, and a mixture of products formed from the reaction of alpha-pinene and ozone. Modeling results suggest that lumping a multicomponent mixture into two or three groups may be sufficient to represent partitioning behavior over a wide range of temperature and organic aerosol mass. The temperature dependence of lumped groups is predicted to be lower than that of their individual components.