PNNL

Abstract

Diffusivity of semivolatile organic compounds (SVOCs) in the bulk particle phase of viscous atmospheric secondary organic aerosol (SOA) can have profound impacts on aerosol growth and size distribution dynamics. Here we investigate the bulk diffusivity (Db) of SVOCs formed from photooxidation of isoprene as they partition to a bimodal aerosol consisting of an Aitken (potassium sulfate) and accumulation mode (aged a-pinene SOA) particles as a function of relative humidity (RH). Model analysis of the observed size distribution evolution shows that liquid-like Db > 10-10 cm2 s-1 fail to explain the growth of the Aitken mode. In contrast, much lower values between 3 X 10-15 cm2 s-1 at 32% RH and 8 X 10-15 cm2 s-1 at 80% RH were needed inside the aged a-pinene SOA to successfully reproduce the growth of both modes. The hindered uptake of SVOCs by the highly viscous accumulation mode particles allow the relatively less viscous Aitken mode particles to effectively absorb the available SVOCs and grow much faster than would be possible otherwise. These results have important implications for modeling SOA formation and aerosol growth in the ambient atmosphere where the accumulation mode SOA particles can age for several days.