Self-limited uptake of α-pinene oxide to acidic aerosol: the effects of liquid–liquid phase separation and implications for the formation of secondary organic aerosol and organosulfates from epoxides

Abstract. The reactive uptake of α-pinene oxide (αPO) to acidic sulfate aerosol was studied under humid conditions in order to gain insight into the effects of liquid–liquid phase separation on aerosol heterogeneous chemistry and to elucidate further the formation of secondary organic aerosol and organosulfates from epoxides. A continuous flow environmental chamber was used to monitor changes in diameter of monodisperse, deliquesced, acidic sulfate particles exposed to αPO at 25% and 50% RH (relative humidity). In order to induce phase separation and probe potential limits to particle growth from acidic uptake, αPO was introduced over a wide range of concentrations, from 200 ppb to 5 ppm. Uptake was observed to be highly dependent on initial aerosol pH. Significant uptake of αPO to aerosol was observed with initial pH < 0. When exposed to 200 ppb αPO, aerosol with pH = -0.5 showed 23% growth, and 6% volume growth was observed at pH = 0. Aerosol with pH = 1 showed no growth. The extreme acidity required for efficient αPO uptake suggests that this chemistry is typically not a major route to formation of aerosol mass or organosulfates in the atmosphere. Effective partition coefficients (K^{p, eff}) were in the range of (0.1–2) x 10^-4 m³μg^-1 and were correlated to initial particle acidity and particle organic content; particles with higher organic content had lower partition coefficients. Effective uptake coefficients (γ_eff) ranged from 0.1 to 1.1 x 10^-4 and are much lower than recently reported for uptake to bulk solutions. In experiments in which αPO was added to bulk H₂SO₄ solutions, phase separation was observed for mass loadings similar to those observed with particles, and product distributions were dependent on acid concentration. Liquid–liquid phase separation in bulk experiments, along with our observations of decreased uptake to particles with the largest growth factors, suggests an organic coating forms upon uptake to particles, limiting reactive uptake.