Mixing order of sulfate aerosols and isoprene epoxydiols affects secondary organic aerosol formation in chamber experiments

The reactive uptake of isoprene epoxydiols (IEPOX) is a significant source of isoprene-derived secondary organic aerosols (SOA). Multiple field studies have reported that summertime isoprene-derived SOA in the Southeastern U.S. correlated strongly with sulfate mass concentration. However, previous laboratory studies have focused largely on the effect of aerosol acidity on the reactive uptake of IEPOX. In this study, we investigated the role of inorganic sulfate aerosols in SOA formation arising from the reactive uptake of trans-β-IEPOX (the predominant IEPOX isomer) at 50–56% RH in laboratory chamber experiments. Our measurements showed that the SOA mass concentration increased with the sulfate mass for both highly acidic and less acidic seed aerosols. This was due to the roles that sulfate played in SOA formation as a particle-phase reactant and as a contributor to aerosol surface area and volume. Higher concentrations of SOA were formed when highly acidic seed aerosols were used, consistent with previous laboratory studies. SOA mass concentration and composition were also observed to be dependent on the injection order of IEPOX and sulfate seed aerosols (i.e., injection of IEPOX first vs. Injection of seed aerosols first) in the chamber experiments. Higher SOA mass concentrations were measured in experiments where sulfate seed aerosols were introduced into the chamber first, followed by IEPOX. Volatility measurements showed that the SOA formed in the “seed aerosols first” experiments likely contained larger quantities of low volatility organic matter compared to SOA formed in the “IEPOX first” experiments. These results showed that the mass concentration and composition of IEPOX-derived SOA formed in chamber experiments can be sensitive to mixing conditions in the chamber brought about by slight differences in experimental methodology (in this case injection procedure). The sensitivity of SOA formation to the amount of seed aerosols and injection procedure used in chamber experiments indicated that caution should be exercised when extrapolating laboratory data to ambient conditions.