Heating-Induced Evaporation of Nine Different Secondary Organic Aerosol Types

The volatility of the compounds comprising organic aerosol (OA) determines their distribution between the gas and particle phases. However, there is a disconnect between volatility distributions as typically derived from secondary OA (SOA) growth experiments and the effective particle volatility as probed in evaporation experiments. Specifically, the evaporation experiments indicate an overall much less volatile SOA. This raises questions regarding the use of traditional volatility distributions in the simulation and prediction of atmospheric SOA concentrations. Here, we present results from measurements of thermally induced evaporation of SOA for nine different SOA types (i.e., distinct volatile organic compound and oxidant pairs) encompassing both anthropogenic and biogenic compounds and O₃ and OH to examine the extent to which the low effective volatility of SOA is a general phenomenon or specific to a subset of SOA types. The observed extents of evaporation with temperature were similar for all the SOA types and indicative of a low effective volatility. Furthermore, minimal variations in the composition of all the SOA types upon heating-induced evaporation were observed. These results suggest that oligomer decomposition likely plays a major role in controlling SOA evaporation, and since the SOA formation time scale in these measurements was less than a minute, the oligomer-forming reactions must be similarly rapid. Overall, these results emphasize the importance of accounting for the role of condensed phase reactions in altering the composition of SOA when assessing particle volatility

Multiphase Reactions between Organic Peroxides and Sulfur Dioxide in Internally Mixed Inorganic and Organic Particles: Key Roles of Particle Phase Separation and Acidity

Organic peroxides (POs) are ubiquitous in the atmosphere and particularly reactive toward dissolved sulfur dioxide (SO2), yet the reaction kinetics between POs and SO2, especially in complex inorganic–organic mixed particles, remain poorly constrained. Here, we report the first investigation of the multiphase reactions between SO2 and POs in monoterpene-derived secondary organic aerosol internally mixed with different inorganic salts (ammonium sulfate, ammonium bisulfate, or sodium nitrate). We find that when the particles are phase-separated, the PO-S(IV) reactivity is consistent with that measured in pure SOA and depends markedly on the water content in the organic shell. However, when the organic and inorganic phases are miscible, the PO-S(IV) reactivity varies substantially among different aerosol systems, mainly driven by their distinct acidities (not by ionic strength). The second-order PO-S(IV) rate constant decreases monotonically from 5 × 105 to 75 M–1 s–1 in the pH range of 0.1–5.6. Both proton catalysis and general acid catalysis contribute to S(IV) oxidation, with their corresponding third-order rate constants determined to be (6.4 ± 0.7) × 106 and (6.9 ± 4.6) × 104 M–2 s–1 at pH 2–6, respectively. The measured kinetics imply that the PO-S(IV) reaction in aerosol is an important sulfate formation pathway, with the reaction kinetics dominated by general acid catalysis at pH > 3 under typical continental atmospheric conditions