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

Many recent secondary organic aerosol (SOA) studies mainly focus on biogenic SOA particles formed under low NOx conditions and thus are applicable to pristine environments with minor anthropogenic influence. Although interactions between biogenic volatile organic compounds and NOx are important in, for instance, suburban areas, there is still a lack of knowledge about the volatility and processes controlling the evaporation of biogenic SOA particles formed in the presence of high concentrations of NOx. Here we provide detailed insights into the isothermal evaporation of alpha-pinene SOA particles that were formed under low NOx and high NOx 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 low RH (< 7 % RH) to high RH (80 % RH). Very similar changes were observed in particle volatility at any set RH during isothermal evaporation for the alpha-pinene SOA particles formed under low NOx and high NOx 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 RO(2 )chemistry during SOA formation. Such compositional differences consequently impacted the primary type of aqueous-phase processes in each type of SOA particle in the presence of particulate water.

Automated summary

Many recent studies on secondary organic aerosols (SOA) have focused on biogenic SOA particles formed in pristine environments with low NOx conditions. However, there is a lack of knowledge about the evaporation and volatility of biogenic SOA particles formed in the presence of high concentrations of NOx. This study investigates the isothermal evaporation and composition evolution of alpha-pinene SOA particles formed under both low NOx and high NOx conditions, providing detailed insights into the processes controlling their evaporation.