Heterogeneous Uptake Kinetics of Limonene and Limonene Oxide by Sulfuric Acid Solutions

Abstract

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.