14:45 〜 15:00
[AAS11-22] Heterogeneous kinetics of the ozonolysis of oleic acid with SO2
キーワード：不均一反応、エアロゾル、オゾン酸化反応、クリーギー中間体
Heterogeneous reactions at gas-condensed phase interfaces play a vital role in atmospheric chemistry, altering aerosol properties and influencing pollutant lifetimes and distributions. Oleic acid (OA), a common unsaturated fatty acid in aerosols, is widely used as a model for studying heterogeneous oxidation due to its atmospheric relevance. Recent studies have examined factors like relative humidity and coexisting pollutants, yet the influence of sulfur dioxide (SO2) on OA ozonolysis remains largely unexplored, despite its potential to interact with organic compounds and scavenge reactive intermediates. This study investigates the effects of SO2 on OA ozonolysis, focusing on its impact on reaction kinetics and product distribution. By integrating experimental and computational approaches, it aims to enhance understanding of SO2's role in organic aerosol transformations and refine atmospheric chemical models.
First, OA is dissolved in ethanol, and the solution is uniformly deposited onto a ZnSe window to form a uniform OA thin film. The thickness of the film is determined using an optical thickness measurement device and is further calibrated based on Fourier transform infrared spectroscopy (FTIR) data. Once the OA thin film is prepared, the ZnSe window is installed into a gas flow cell to facilitate controlled exposure to reactive gases. Thin films of oleic acid with varying volumes were prepared, and their thicknesses were measured optically. The oxidation experiments are conducted under three different conditions: exposure to sulfur dioxide (SO2) alone, exposure to ozone (O3) alone, and simultaneous exposure to both SO2 and O3. SO2 is introduced from a high-purity gas cylinder and diluted with dry air before entering the reaction chamber, with its concentration measured using a dedicated chemical sensor. The reaction kinetics are analyzed under the assumption of pseudo-first-order form and rate constants are derived by performing regression analysis on the experimental data. Additionally, quantum chemical calculations are carried out using Gaussian 16 software at the CCSD(T)/aug-cc-pV(D+d)Z//M06-2X/aug-cc-pV(T+d)Z level of theory. These calculations aim to trace the reaction pathways, identify transition states, and determine the energy barriers associated with key reaction steps.
First, the oxidation of OA by ozone (O3) alone was examined. The results confirmed that OA ozonolysis followed pseudo-first-order kinetics with respect to OA concentration, consistent with previous studies. By tracking the decay of the 1710 cm-1 peak over time, reaction rate constants were determined for different OA film thicknesses. The results showed that reaction rates decreased as film thickness increased, highlighting the dominant role of interfacial reactions. In thinner films, the oxidation rate was significantly higher, likely due to enhanced gas-phase reactant diffusion and greater surface accessibility. When OA was exposed to both O3 and SO2, a distinct inhibition effect was observed. Compared to the OA-O3 system, the overall reaction rate decreased in the presence of SO3. This suppression was attributed to SO2 acting as a scavenger of Criegee intermediates (CIs), which are key reactive species in the ozonolysis pathway. In the absence of SO2, CIs can react with other oxidation products to form ester form oligomer. However, when SO2 was introduced, it preferentially reacted with CIs, forming sulfur trioxide (SO3) instead. This competing pathway altered the overall reaction mechanism and reduced OA consumption rates. The shift in reaction products suggests that SO2 not only affects reaction kinetics but also influences the atmospheric fate of oxidized organic compounds.
This study was funded by a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (JSPS), Grant Number JP21K12286.
First, OA is dissolved in ethanol, and the solution is uniformly deposited onto a ZnSe window to form a uniform OA thin film. The thickness of the film is determined using an optical thickness measurement device and is further calibrated based on Fourier transform infrared spectroscopy (FTIR) data. Once the OA thin film is prepared, the ZnSe window is installed into a gas flow cell to facilitate controlled exposure to reactive gases. Thin films of oleic acid with varying volumes were prepared, and their thicknesses were measured optically. The oxidation experiments are conducted under three different conditions: exposure to sulfur dioxide (SO2) alone, exposure to ozone (O3) alone, and simultaneous exposure to both SO2 and O3. SO2 is introduced from a high-purity gas cylinder and diluted with dry air before entering the reaction chamber, with its concentration measured using a dedicated chemical sensor. The reaction kinetics are analyzed under the assumption of pseudo-first-order form and rate constants are derived by performing regression analysis on the experimental data. Additionally, quantum chemical calculations are carried out using Gaussian 16 software at the CCSD(T)/aug-cc-pV(D+d)Z//M06-2X/aug-cc-pV(T+d)Z level of theory. These calculations aim to trace the reaction pathways, identify transition states, and determine the energy barriers associated with key reaction steps.
First, the oxidation of OA by ozone (O3) alone was examined. The results confirmed that OA ozonolysis followed pseudo-first-order kinetics with respect to OA concentration, consistent with previous studies. By tracking the decay of the 1710 cm-1 peak over time, reaction rate constants were determined for different OA film thicknesses. The results showed that reaction rates decreased as film thickness increased, highlighting the dominant role of interfacial reactions. In thinner films, the oxidation rate was significantly higher, likely due to enhanced gas-phase reactant diffusion and greater surface accessibility. When OA was exposed to both O3 and SO2, a distinct inhibition effect was observed. Compared to the OA-O3 system, the overall reaction rate decreased in the presence of SO3. This suppression was attributed to SO2 acting as a scavenger of Criegee intermediates (CIs), which are key reactive species in the ozonolysis pathway. In the absence of SO2, CIs can react with other oxidation products to form ester form oligomer. However, when SO2 was introduced, it preferentially reacted with CIs, forming sulfur trioxide (SO3) instead. This competing pathway altered the overall reaction mechanism and reduced OA consumption rates. The shift in reaction products suggests that SO2 not only affects reaction kinetics but also influences the atmospheric fate of oxidized organic compounds.
This study was funded by a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (JSPS), Grant Number JP21K12286.