Following Particle-Particle Mixing in Atmospheric Secondary Organic Aerosols by Using Isotopically Labeled Terpenes

We used unlabeled and deuterium-labeled precursors to generate and characterize secondary organic aerosol (SOA), a class of atmospheric constituents that rank among the least understood in the climate system, while circumventing the typical problems caused by spectral similarity of SOA mass fragments in aerosol mass spectrometry. We used highly sensitive single-particle mass spectrometers to measure mixing via semi-volatile gas-phase exchange between SOA from biogenic precursors (terpenes) and an anthropogenic precursor (toluene). These are common laboratory mimics for ambient SOA. The experiments showed that particles derived from isoprene and alpha-pinene ozonolysis undergo fast exchange via evaporation and condensation of semi-volatile constituents without any signs of diffusion limitations, even when the relative humidity (RH) is <10%. Particles derived from limonene ozonolysis showed slower exchange than those from alpha-pinene because of their more polar constituents and resulting lower diffusivity. Finally, particles derived from beta-caryophyllene ozonolysis showed limited vapor uptake even for RH >> 30%.