Browsing by Subject "O-2"

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  • Joshi, Satya P.; Pekkanen, Timo T.; Timonen, Raimo S.; Eskola, Arkke J. (2019)
    The kinetics of (CH3)(2)CCH + O-2 (1) and (CH3)(2)CCCH3 + O-2 (2) reactions have been measured as a function of temperature (223-600 K) at low pressures (0.4-2 Torr) using a tubular laminar flow reactor coupled to a photoionization mass spectrometer (PIMS). These reactions are important for accurate modeling of unsaturated hydrocarbon combustion. Photolysis of a brominated precursor by a pulsed excimer laser radiation at 248 nm wavelength along the flow reactor axis was used for the production of radicals. The measured bimolecular rate coefficient of reaction 1 shows a negative temperature dependence over the temperature range 223-384 K and becomes temperature independent at higher temperatures. The bimolecular rate coefficient of reaction 2 exhibits a negative temperature dependence throughout the experimental temperature range. The bimolecular rate coefficients of reactions 1 and 2 are expected to be at the high-pressure limit under the current experimental conditions, and the following values are obtained at 298 K: k(1)(298 K) = (4.5 +/- 0.5) x 10(-12) cm(3) s(-1) and k(2)(298 = (8.9 +/- 1.0) x 10(-12) cm(3) s(-1). The observed products for reactions 1 and 2 were CH3COCH3 and CH3 + CH3COCH3, respectively. Substituting both beta-hydrogens in the vinyl radical (CH2CH) with methyl groups decreases the rate coefficient of the CH2CH + O-2 reaction by about 50%. However, the rate coefficient of the triply substituted (CH3)(2)CCCH3 radical reaction with O-2 is almost identical to the CH2CH + O-2 rate coefficient under the covered temperature range.
  • Döntgen, Malte; Pekkanen, Timo T.; Joshi, Satya P.; Timonen, Raimo S.; Eskola, Arkke J. (2019)
    The kinetics and thermochemistry of the pent-1-en-3-yl radical reaction with molecular oxygen (CH2CHCHCH2CH3 + O-2) has been studied by both experimental and computational methods. The bimolecular rate coefficient of the reaction was measured as a function of temperature (198-370 K) and pressure (0.2-4.5 Torr) using laser photolysis-photoionization mass-spectrometry. Quantum chemical calculations were used to explore the potential energy surface of the reaction, after which Rice-Ramsperger-Kassel-Marcus theory/master equation simulations were performed to investigate the reaction. The experimental data were used to adjust key parameters, such as well depths, in the master equation model within methodological uncertainties. The master equation simulations suggest that the formation rates of the two potential RO2 adducts are equal and that the reaction to QOOH is slower than for saturated hydrocarbons. The initial addition reaction, CH2CHCHCH2CH3 + O-2, is found to be barrierless when accounting for multireference effects. This is in agreement with the current experimental data, as well as with past experimental data for the allyl + O-2 reaction. Finally, we conducted numerical simulations of the pent-1-en-3-yl + O-2 reaction system and observed significant amounts of penta-1,3-diene being formed under engine-relevant conditions.
  • Li, Yuanyuan; Nie, Wei; Liu, Yuliang; Huang, Dandan; Xu, Zheng; Peng, Xiang; George, Christian; Yan, Chao; Tham, Yee Jun; Yu, Chuan; Xia, Men; Fu, Xiao; Wang, Xinfeng; Xue, Likun; Wang, Zhe; Xu, Zhengning; Chi, Xuguang; Wang, Tao; Ding, Aijun (2020)
    Titanium dioxide (TiO2) is extensively used with the process of urbanization and potentially influences atmospheric chemistry, which is yet unclear. In this work, we demonstrated strong production of Cl-2 from illuminated KCl-coated TiO2 membranes and suggested an important daytime source of chlorine radicals. We found that water and oxygen were required for the reactions to proceed, and Cl-2 production increased linearly with the amount of coated KCl, humidity of the carrier gas, and light intensity. These results suggested that water promotes the reactivity of coated KCl via interaction with the crystal lattice to release free chloride ions (Cl-). The free Cl- transfer charges to O-2 via photoactivated TiO2 to form Cl-2 and probably the O-2(-) radical. In addition to Cl-2, ClO and HOCl were also observed via the complex reactions between Cl/Cl-2 and HOx. An intensive campaign was conducted in Shanghai, during which evident daytime peaks of Cl-2 were observed. Estimated Cl-2 production from TiO2 photocatalysis can be up to 0.2 ppb/h when the TiO2-containing surface reaches 20% of the urban surface, and highly correlated to the observed Cl-2. Our results suggest a non-negligible role of TiO2 in atmospheric photochemistry via altering the radical budget.
  • Song, Shuang; Li, Changyou; Shi, Xiaohong; Zhao, Shegnan; Tian, Weidong; Li, Zhijun; Bai, Yila; Cao, Xiaowei; Wang, Qingkai; Huotari, Jussi; Tulonen, Tiina; Uusheimo, Sari; Leppäranta, Matti; Loehr, John; Arvola, Lauri (2019)
    Winter is a long period of the annual cycle of many lakes in the northern hemisphere. Low irradiance, ice, and snow cover cause poor light penetration into the water column of these lakes. Therefore, in northern lakes, respiration often exceeds primary production leading to low dissolved oxygen concentrations. This study aimed to quantify under-ice metabolic processes during winter in an arid zone lake with little snow cover. This study was carried out in a mid-latitude lake in Inner Mongolia, northern China. The study lake receives relatively high incoming solar radiation on the ice in mid-winter, and radiation can penetrate down to the bottom sediment as the lake is shallow and the ice lacks snow cover. Primary production and respiration were estimated during two winters using high-frequency sensor measurements of dissolved oxygen. To quantify under-ice metabolic processes, sensors were deployed to different depths. During both winters, sensors collected data every 10 min over several weeks. The amount of solar radiation controlled photosynthesis under ice; temperature and photosynthesis together appeared to control respiration. The balance between gross primary production and ecosystem respiration was especially sensitive to changes in snow cover, and the balance between P and R decreased. Our data suggest that photosynthesis by plankton, submerged plants, and epiphytic algae may continue over winter in shallow lakes in mid-latitudes when there is no snow cover on the ice, as may occur in arid climates. The continuation of photosynthesis under ice buffers against dissolved oxygen depletion and prevents consequent harmful ecosystem effects.