Isothermal evaporation of α-pinene secondary organic aerosol particles formed under low-NO_x and high-NO_x conditions

Abstract. Many recent secondary organic aerosol (SOA) studies mainly focus on biogenic SOA particles formed under low-NO_x conditions and thus are applicable to pristine environments with minor anthropogenic influence. Although interactions between biogenic volatile organic compounds and NO_x are important in, for instance, suburban areas, there is still a lack of knowledge about volatility and processes controlling the evaporation of biogenic SOA particles formed in the presence of high concentrations of NO_x. Here we provide detailed insights into the isothermal evaporation of α-pinene SOA particles that were formed under low-NO_x and high-NO_x conditions to investigate the evaporation process and the evolution of particle composition during the evaporation in more detail. We coupled Filter Inlet for Gases and AEROsols-Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) measurements of the molecular composition and volatility of the particle phase with isothermal evaporation experiments conducted under a range of relative humidity (RH) conditions from dry to 80 % RH. Very similar changes were observed in particle volatility at any set RH during isothermal evaporation for the α-pinene SOA particles formed under low-NO_x and high-NO_x conditions. However, there were distinct differences in the initial composition of the two SOA types, possibly due to the influence of NO_x on the RO₂ chemistry during SOA formation. Such compositional differences consequently impacted the primary type of aqueous-phase processes in each type of SOA particles in the presence of particulate water.