Evaporation kinetics and phase of laboratory and ambient secondary organic aerosol and the effect of adsorbed spectator gases

Description

Field measurements of secondary organic aerosol (SOA) find significantly higher mass loads than predicted by models, sparking intense effort focused on finding additional SOA sources but leaving the fundamental assumptions used by models unchallenged. Current air-quality models use absorptive partitioning theory assuming SOA particles are liquid droplets, forming instantaneous reversible equilibrium with gas phase. Further, they ignore the effects of adsorption of spectator organic species during SOA formation on SOA properties and fate. Using an accurate and highly sensitive experimental approach for studying evaporation kinetics of size-selected single SOA particles, we characterized room-temperature evaporation kinetics of laboratory-generated α-pinene SOA and ambient atmospheric SOA. We found that even when gas phase organics are removed, it takes ~24 hours for pure α-pinene SOA particles to evaporate 75% of their mass, which is in sharp contrast to the ~10-minute timescale predicted by current kinetic models. SOA formed in the presence of “spectator” hydrophobic organic vapors like dioctyl phthalate, dioctyl sebacate, pyrene, or their mixture, were shown to adsorb noticeable amounts of these organics, forming what we term here “coated” SOA particles. We find that these adsorbed coatings further reduce evaporation rates of SOA particles. Moreover, aging of coated SOA particles dramatically reduces evaporation rates, and in some cases nearly stops it. To address the question of how closely the laboratory observations described above reflect reality in the atmosphere, we characterized the evaporation kinetics of size-selected atmospheric SOA particles sampled in situ during the recent Carbonaceous Aerosols and Radiative Effects Study (CARES) field campaign. Ambient SOA was found to exhibit evaporation behavior very similar to that of laboratory-generated coated and aged SOA. Like laboratory SOA, their evaporation is size-independent and does not follow the kinetics of liquid droplets, in sharp contrast with model assumptions. The findings about SOA phase, evaporation rates, and the importance of spectator gases and aging all indicate that there is a need to reformulate the way SOA formation and evaporation are treated by models. Presently we are developing a new modeling approach that takes into account these new findings. Some preliminary modeling results will be presented.