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Mixing of secondary organic aerosols versus relative humidity

Significance Recent studies called into question whether diffusion in “glassy” atmospheric secondary organic aerosols (SOA) is fast enough for phase partitioning to equilibrate, as commonly assumed in models; however, most of those studies relied on rheological measurements or uptake of small molecules such as water and ammonia. Here, we conduct unique SOA mixing experiments using single-particle mass spectrometry to probe the diffusion of semivolatile organics into SOA derived from toluene and α-pinene at different relative humidities. Results show that, if the SOA systems are representative, equilibrium partitioning likely does have time to occur in the boundary layer. This work directly probes uptake of semivolatile organics by SOA particles over a wide humidity range. Atmospheric aerosols exert a substantial influence on climate, ecosystems, visibility, and human health. Although secondary organic aerosols (SOA) dominate fine-particle mass, they comprise myriad compounds with uncertain sources, chemistry, and interactions. SOA formation involves absorption of vapors into particles, either because gas-phase chemistry produces low-volatility or semivolatile products that partition into particles or because more-volatile organics enter particles and react to form lower-volatility products. Thus, SOA formation involves both production of low-volatility compounds and their diffusion into particles. Most chemical transport models assume a single well-mixed phase of condensing organics and an instantaneous equilibrium between bulk gas and particle phases; however, direct observations constraining diffusion of semivolatile organics into particles containing SOA are scarce. Here we perform unique mixing experiments between SOA populations including semivolatile constituents using quantitative, single-particle mass spectrometry to probe any mass-transfer limitations in particles containing SOA. We show that, for several hours, particles containing SOA from toluene oxidation resist exchange of semivolatile constituents at low relative humidity (RH) but start to lose that resistance above 20% RH. Above 40% RH, the exchange of material remains constant up to 90% RH. We also show that dry particles containing SOA from α-pinene ozonolysis do not appear to resist exchange of semivolatile compounds. Our interpretation is that in-particle diffusion is not rate-limiting to mass transfer in these systems above 40% RH. To the extent that these systems are representative of ambient SOA, we conclude that diffusion limitations are likely not common under typical ambient boundary layer conditions.