Supporting Information: Heterogeneous Uptake Kinetics of Limonene and Limonene Oxide by Sulfuric Acid Solutions

The heterogeneous uptake of limonene and limonene oxide (also known as 1,8-cineole) by a range of different aqueous sulfuric acid (H₂SO₄) solutions (30%-80% (w)) was investigated to develop an understanding of the reactivity of biogenic organic compounds in the atmosphere towards acidic aerosols. Experiments were performed using a rotating wetted-wall reactor coupled to a single photon ionization time-of-flight mass spectrometer. The heterogeneous uptake of the compounds into H₂SO₄ followed first-order kinetics and the corresponding steady-state uptake coefficients (γ) were calculated for the first time. Limonene oxide was found to be more reactive than limonene towards H₂SO₄. Reactive uptake was observed for limonene oxide in acidic solution containing greater than 30% (w). The steady-state uptake coefficients of limonene oxide in 30%-50% (w) H₂SO₄ solutions at room temperature ranged from (7.100± 0.023)×10^-5 to (8.150±0.162)×10^-3. Furthermore, the reactions of limonene oxide with sulfuric acid in bulk solution were investigated using gas chromatography-mass spectrometry (GC-MS) and electron spray ionization-mass spectroscopy (ESI-MS). Analysis of the products revealed the presence of monoterpenes, terpineols, terpin hydrates, and terpin hydrate diorganosulfate from the bulk solution reaction of limonene oxide with H₂SO₄. The formation of significantly more hydrophobic organic compounds with lower volatilities suggested that limonene oxide is a significant precursor in the formation of atmospheric secondary organic aerosols. A transformation mechanism has been proposed based on the products.