Hygroscopicity Depends on Aerosol Acidity and Sulfate Content during the Reactive Uptake of Isoprene Epoxydiols

Aerosol liquid water content has a significant but highly uncertain effect on atmospheric radiative forcing. Hygroscopicity of organic–inorganic mixed aerosols is complex, especially when they are also phase-separated, and little is understood about their dependence on acidity. We conducted cloud condensation nuclei (CCN) measurements during smog chamber studies, where secondary organic aerosol (SOA) was generated from the acid-driven reactive uptake of isoprene epoxydiols (IEPOX) onto sulfate seed aerosols at pH 0.9 (pure H₂SO₄), 1.1, and 2 [mixtures of H₂SO₄ and (NH₄)₂SO₄]. Direct CCN measurements were compared to predictions of the hygroscopicity parameter κ using a weighted-sum model with measured κ values of authentic standards for the three major particle constituents, including inorganic sulfate (Sulf_inorg), 2-methyltetrols (2-MT), and methyltetrol sulfates (MTS). Sulf_inorg was quantified using ion chromatography (IC), while 2-MT and MTS were quantified using hydrophilic interaction liquid chromatography interfaced to high-resolution quadrupole time-of-flight mass spectrometry and equipped with electrospray ionization (HILIC/ESI–HR-QTOFMS). SOA κ values ranged from 0.2 to 0.6, while single-component aerosols generated from authentic standards of 2-MT and MTS had κ values of 0.11 and 0.15, respectively. We found that predicted and measured κ values matched well at high IEPOX/Sulf_inorg, but the discrepancy varied with initial IEPOX/Sulf_inorg and seed solution pH and changed over the course of an experiment. The density of 2-MT and MTS was measured using an aerodynamic aerosol classifier and used to calculate total mass loadings from the measured volume concentration, revealing that, although 2-MT and MTS make up the bulk of IEPOX-derived SOA, other constituents may be significantly denser.