講演情報
[AAS08-P50]Ozonolysis Mechanism of BVOC at the Air/Water Interface
*黒川 啓1、江波 進一1、沼館 直樹1 (1.国立大学法人筑波大学)
キーワード：
界面反応、大気化学、エアロゾル、生物由来揮発性有機化合物、オゾン分解、室内実験
Introduction
Understanding the various processes occurring at the air-water interface of aerosols-microscopic liquid droplets present in the atmosphere-is a key to elucidate the mechanism of global climate change and adverse health effects of air pollution. Terpenoids, which are unsaturated organic compounds synthesized by plants and emitted into the atmosphere, can react with ozone to form oxygenated products, resulting in the formation of aerosols[1]. Among these compounds, nerolidol is a representative sesquiterpene-alcohol emitted from snapdragon and other plants[2]. Nerolidol has been reported to exhibit an extremely high reactivity toward ozone at the air-liquid interfaces[3], suggesting that it may significantly contribute to aerosol formation in the atmosphere. However, the factors that determine the reaction between nerolidol and ozone at the air-water interface, as well as whether and how their reactivity can be modulated, remain unclear. Especially, it is uncertain whether the reactions rates and products could be changed between the nerolidol(aq) + O3(g) reaction vs the nerolidol(g) + O3(aq) reaction. In this study, we therefore establish an experimental system distinct from those used in previous research to elucidate the detailed reaction mechanism between nerolidol and ozone.
Experiment
In the previous study[3], nerolidol and NaCl were dissolved in a mixed solvent of water (W) and acetonitrile (AN) (W/AN = 1:4, v/v), and droplets of the solution were generated using a nebulizer. Ozone gas was then introduced onto these droplets to measure the air-water interfacial reaction between nerolidol and ozone.
In this study, we measured reactions occurring at the air-water interface by introducing gaseous nerolidol onto droplets of a solution in which ozone and NaCl were dissolved in a mixed solvent of water and acetonitrile (W/AN = 1:4, v/v), as well as onto pure water droplets in which ozone was dissolved. Furthermore, experiments adding NaClO4 to the solutions were also conducted because hydroperoxide products were previously detected as the Na+-complex by positive ion mass spectrometry. During the experiments, reactants and products at the gas-liquid interface were detected using quadrupole mass spectrometry (Agilent 6130 Quadrupole LC/MS Electrospray System)[3].
Results
When gaseous nerolidol was introduced onto droplets of the W/AN solution in which ozone and NaCl were dissolved, no new peaks appeared, implying that the ozonolysis similar to that reported in the previous studies did not occur under the present conditions. In contrast, when gaseous nerolidol was introduced onto pure water droplets in which ozone was dissolved, peaks not reported in the previous studies were observed. These results indicate that new reactions may be occurring in this system. Furthermore, the absence of products similar to those reported in previous studies in experiments using NaClO4 also supports this result. Our new data showed the reactions rates and products are indeed changed between the nerolidol(aq) + O3(g) reaction vs the nerolidol(g) + O3(aq) reaction. Based on these results, we propose that differences in the interfacial state of nerolidol may be responsible for the observed differences in reactivity.
Reference
[1] Johnson & Marston., The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere, Chem. Soc. Rev., 2008, 37, 699-716 [2] Dudareva et al., The nonmevalonate pathway supports both monoterpene and sesquiterpene formation in snapdragon flowers. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 933-938., [3] Qiu et al., Interfacial vs Bulk Ozonolysis of Nerolidol, Environ. Sci. Technol. 2019, 53, 5750-5757.
Understanding the various processes occurring at the air-water interface of aerosols-microscopic liquid droplets present in the atmosphere-is a key to elucidate the mechanism of global climate change and adverse health effects of air pollution. Terpenoids, which are unsaturated organic compounds synthesized by plants and emitted into the atmosphere, can react with ozone to form oxygenated products, resulting in the formation of aerosols[1]. Among these compounds, nerolidol is a representative sesquiterpene-alcohol emitted from snapdragon and other plants[2]. Nerolidol has been reported to exhibit an extremely high reactivity toward ozone at the air-liquid interfaces[3], suggesting that it may significantly contribute to aerosol formation in the atmosphere. However, the factors that determine the reaction between nerolidol and ozone at the air-water interface, as well as whether and how their reactivity can be modulated, remain unclear. Especially, it is uncertain whether the reactions rates and products could be changed between the nerolidol(aq) + O3(g) reaction vs the nerolidol(g) + O3(aq) reaction. In this study, we therefore establish an experimental system distinct from those used in previous research to elucidate the detailed reaction mechanism between nerolidol and ozone.
Experiment
In the previous study[3], nerolidol and NaCl were dissolved in a mixed solvent of water (W) and acetonitrile (AN) (W/AN = 1:4, v/v), and droplets of the solution were generated using a nebulizer. Ozone gas was then introduced onto these droplets to measure the air-water interfacial reaction between nerolidol and ozone.
In this study, we measured reactions occurring at the air-water interface by introducing gaseous nerolidol onto droplets of a solution in which ozone and NaCl were dissolved in a mixed solvent of water and acetonitrile (W/AN = 1:4, v/v), as well as onto pure water droplets in which ozone was dissolved. Furthermore, experiments adding NaClO4 to the solutions were also conducted because hydroperoxide products were previously detected as the Na+-complex by positive ion mass spectrometry. During the experiments, reactants and products at the gas-liquid interface were detected using quadrupole mass spectrometry (Agilent 6130 Quadrupole LC/MS Electrospray System)[3].
Results
When gaseous nerolidol was introduced onto droplets of the W/AN solution in which ozone and NaCl were dissolved, no new peaks appeared, implying that the ozonolysis similar to that reported in the previous studies did not occur under the present conditions. In contrast, when gaseous nerolidol was introduced onto pure water droplets in which ozone was dissolved, peaks not reported in the previous studies were observed. These results indicate that new reactions may be occurring in this system. Furthermore, the absence of products similar to those reported in previous studies in experiments using NaClO4 also supports this result. Our new data showed the reactions rates and products are indeed changed between the nerolidol(aq) + O3(g) reaction vs the nerolidol(g) + O3(aq) reaction. Based on these results, we propose that differences in the interfacial state of nerolidol may be responsible for the observed differences in reactivity.
Reference
[1] Johnson & Marston., The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere, Chem. Soc. Rev., 2008, 37, 699-716 [2] Dudareva et al., The nonmevalonate pathway supports both monoterpene and sesquiterpene formation in snapdragon flowers. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 933-938., [3] Qiu et al., Interfacial vs Bulk Ozonolysis of Nerolidol, Environ. Sci. Technol. 2019, 53, 5750-5757.