PNNL

Abstract

Partitioning of secondary organic aerosols (SOA) is a critical process for determining their particle size and mass concentration. SOA particles were reported to evaporate slowly when they adopt highly viscous phase state, but the underlying mechanism of their size-independent evaporation kinetics remains controversial. Here we apply kinetic multilayer modeling to simulate the observed evaporation of ?-pinene SOA which exhibits relatively fast evaporation followed by very slow evaporation that has minimal size dependence and limonene SOA which exhibits very slow evaporation. We demonstrate that surface crust formation, emerging from swift evaporation of semi-volatile compounds and accumulation of low volatility compounds at the particle surface, leads to slow evaporation with reduced size dependence. While decomposition of oligomers in SOA followed by evaporation of monomers can also contribute to this process, the role of surface crust formation is further confirmed by observation and modeling of size-independent slow evaporation of polyethylene glycol mixtures containing high molecular weight oligomers that do not decompose. We present experimental evidence using a surface-based mass spectrometry technique that the particle surface becomes enriched in high molecular weight compounds upon evaporation of low molecular weight compounds. Our findings imply that the presence of highly viscous surface crust may also limit SOA growth and heterogeneous processes, affecting the particle size distribution dynamics, fate, and chemistry of SOA.