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 α-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 α-pinene because of their more polar constituents and resulting lower diffusivity. Finally, particles derived from β-caryophyllene ozonolysis showed limited vapor uptake even for RH ≫ 30%. The exchange of constituents between distinct types of aerosols is relevant to many processes important to atmospheric chemistry, combustion, bio-threat detection, and consumer-product formulations. However, because of the high similarity of aerosol mass spectrometer signals, it is difficult to distinguish between different aerosol populations and to track constituent exchange. We have overcome this hurdle by synthesizing deuterium-labeled terpenes as precursors for secondary organic aerosols and studying mixing driven by semi-volatile vapor exchange with particles formed from other unlabeled terpenes as well as toluene. We found that particles from isoprene and α-pinene ozonolysis absorbed vapors rapidly. Particles from limonene ozonolysis showed slower exchange, and particles from β-caryophyllene ozonolysis showed limited exchange. Our results show that molecular exchange among particles from terpene oxidation becomes slower and less extensive as the precursor carbon number increases. Atmospheric fine particles contain thousands of organic compounds. Natural compounds from trees are often terpenes, consisting of multiple isoprene units, which when oxidized produce hundreds of poorly understood product compounds, many of which have extremely low vapor pressures and partition to particles. The interactions of these compounds control many particle properties, but it is difficult to distinguish them from each other. By synthesizing isotopically labeled terpenes, we were able to follow the interactions of individual particles with precision.