Growth Kinetics of Viscous Secondary Organic Aerosol

Atmospheric aerosol particles larger than about 80 nanometers serve as cloud condensation nuclei (CCN). Growth of ultrafine aerosol particles formed to CCN active sizes occurs largely by condensation of myriad oxidation products of biogenic and anthropogenic volatile organic compounds (VOCs) forming secondary organic aerosol (SOA). While the initial growth is driven by irreversible condensation of nearly nonvolatile oxidation products, the much more abundantly formed semi-volatile oxidation products are traditionally assumed to rapidly equilibrate with pre-existing organic aerosol mass, thus favoring the growth of particles that are already large. However, recent studies indicate that SOA exists as a semisolid at low to moderate relative humidity. In this talk, we will present insights into the growth kinetics of SOA formed from photooxidation of isoprene in the laboratory and in a forest during the 2010 CARES field campaign in California. A significant fraction of isoprene SOA was composed of a complex mixture of oligomers, which are responsible for the increased viscosity of the particulate organic phase. Model analysis of the observed growth kinetics indicated that the condensing semivolatile vapors did not rapidly equilibrate, but rather their time- and size-dependent partitioning was controlled by diffusion into the viscous particulate organic phase. Because the particle-phase diffusion time scale increases with increasing particle size, the substantially hindered growth of large organic particles effectively enhanced the growth of ultrafine particles that were competing to absorb the semivolatile organics. These results suggest that a greater fraction of ultrafine aerosols may grow to CCN active sizes due to diffusion-controlled condensation of semivolatile organics under low to moderately humid conditions.