Insights into the O : C-dependent mechanisms controlling the evaporation of α-pinene secondary organic aerosol particles

The volatility of oxidation products of volatile organic compounds (VOCs) in the atmosphere is a key factor to determine if they partition into the particle phase contributing to secondary organic aerosol (SOA) mass. Thus, linking volatility and measured particle composition will provide insights into SOA formation and its fate in the atmosphere. We produced α-pinene SOA with three different oxidation levels (characterized by average oxygen-to-carbon ratio; O:C‾=0.53, 0.69, and 0.96) in an oxidation flow reactor. We investigated the particle volatility by isothermal evaporation in clean air as a function of relative humidity (RH <2 %, 40 %, and 80 %) and used a filter-based thermal desorption method to gain volatility and chemical composition information. We observed reduced particle evaporation for particles with increasing O:C‾ ratio, indicating that particles become more resilient to evaporation with oxidative aging. Particle evaporation was increased in the presence of water vapour and presumably particulate water; at the same time the resistance of the residual particles to thermal desorption was increased as well. For SOA with O:C‾=0.96, the unexpectedly large increase in mean thermal desorption temperature and changes in the thermogram shapes under wet conditions (80 % RH) were an indication of aqueous phase chemistry. For the lower O:C‾ cases, some water-induced composition changes were observed. However, the enhanced evaporation under wet conditions could be explained by the reduction in particle viscosity from the semi-solid to liquid-like range, and the observed higher desorption temperature of the residual particles is a direct consequence of the increased removal of high-volatility and the continued presence of low-volatility compounds.