10:00 AM - 10:15 AM
[AAS05-04] Investigation of gaseous and heterogeneous reactions of RO2 radicals using FAGE-LIF method
Keywords:RO2 radical, reaction rate coefficient, NO2, aerosol, uptake coefficient
Peroxy (RO2) radicals are produced by oxidation of various volatile organic compounds (VOCs) in the atmosphere and play an important role. They are mainly removed by self-reactions under clean air conditions. On the other hand, they oxidize NO and produce NO2. Since NO2 produces O3 via photochemical reactions, RO2 radicals act as a part of the cyclic reaction producing tropospheric O3. RO2 radicals also react with NO2 and generate a RO2NO2 complex, which can reproduce NO2 via thermal decomposition and act as an atmospheric NOx reservoir. Furthermore, in recent years, it has been suggested that the loss processes of ROx radicals via heterogeneous reactions on the aerosol surface may be important. Despite these importance, there are limited kinetics data about RO2. In particular, there were few reports of detection under atmospheric conditions, and it was limited to some RO2 radicals. In this study, we improved a combination technique of Fluorescence Assay by Gas Expansion and Laser Induced Fluorescence (FAGE-LIF) system for detection of various RO2 radicals and investigated their reactions including not only gaseous reactions but aerosol uptake processes.
All experiments were performed at 1 bar, 298 K. O3/H2O/sample VOC/N2 carrier mixture were added into the reaction cell. By irradiation of pump laser pulses, OH radicals were generated by the photolysis of O3 and subsequent reaction with H2O. OH radicals reacted with VOC rapidly and generated RO2 radicals. A part of main flow was added into the detection cell. In order to convert RO2 radicals to OH radicals again, NO and O2 were added into the detection cell. Then, OH radicals were detected by LIF method. The relative concentration of RO2 radicals were measured from the LIF intensity of OH radicals.
Figure 1 shows time profiles of CH3O2 radicals, which were produced by the reaction of OH with CH4, measured at various NO2 concentrations. CH3O2 radicals decay single exponentially in the absence of NO2. With the existence of NO2, however, profiles include two decay components due to the reverse reaction. In order to determine the rate constants, double exponential fitting including the reverse reaction was performed. The fitting results are shown in figure as red lines. All lines reproduce the experimental results very well. The reaction rate constants including also reverse reaction have been determined from the NO2 concentration dependence. Those rate constants agree with previous reported values. This indicates that the improved FAGE-LIF system allows us to measure the kinetics of RO2 radicals appropriately. We also performed same experiments using C2H5O2 radicals. We will discuss at the presentation.
Additionally, we investigated uptake processes of RO2 radicals onto aerosols. Sample aerosols were deliquesced NaCl particles generated from 0.1 − 0.3 g L−1 aqueous NaCl solutions via a collision-type atomizer. Those particles were added into the reaction cell with zero air carrier and another sample gases. The decay rates of RO2 radicals due to aerosols were determined by the difference between the fitting results with and without aerosols. By using the decay rates due to the aerosols and aerosol size distributions, uptake coefficients have been determined for some RO2 radicals (as shown in Fig.2). The aerosol size distributions were employed from the separate experiments under the almost same conditions. As a result, the increase of the uptake coefficients with the size of RO2 radicals was observed for the first time. In addition, even small RO2 radicals were uptaken into aerosols, although their values were small. This indicates that the heterogeneous reactions of RO2 radicals can affect the O3 production mechanisms. Consequently, there is possibility that the considering the interaction between RO2 radicals and aerosols should improve the accuracy of the atmospheric model.
This work was supported by JSPS KAKENHI Grant Nos. JP16H06305, JP18K18179, JP19J40218, and JP19H04255.
All experiments were performed at 1 bar, 298 K. O3/H2O/sample VOC/N2 carrier mixture were added into the reaction cell. By irradiation of pump laser pulses, OH radicals were generated by the photolysis of O3 and subsequent reaction with H2O. OH radicals reacted with VOC rapidly and generated RO2 radicals. A part of main flow was added into the detection cell. In order to convert RO2 radicals to OH radicals again, NO and O2 were added into the detection cell. Then, OH radicals were detected by LIF method. The relative concentration of RO2 radicals were measured from the LIF intensity of OH radicals.
Figure 1 shows time profiles of CH3O2 radicals, which were produced by the reaction of OH with CH4, measured at various NO2 concentrations. CH3O2 radicals decay single exponentially in the absence of NO2. With the existence of NO2, however, profiles include two decay components due to the reverse reaction. In order to determine the rate constants, double exponential fitting including the reverse reaction was performed. The fitting results are shown in figure as red lines. All lines reproduce the experimental results very well. The reaction rate constants including also reverse reaction have been determined from the NO2 concentration dependence. Those rate constants agree with previous reported values. This indicates that the improved FAGE-LIF system allows us to measure the kinetics of RO2 radicals appropriately. We also performed same experiments using C2H5O2 radicals. We will discuss at the presentation.
Additionally, we investigated uptake processes of RO2 radicals onto aerosols. Sample aerosols were deliquesced NaCl particles generated from 0.1 − 0.3 g L−1 aqueous NaCl solutions via a collision-type atomizer. Those particles were added into the reaction cell with zero air carrier and another sample gases. The decay rates of RO2 radicals due to aerosols were determined by the difference between the fitting results with and without aerosols. By using the decay rates due to the aerosols and aerosol size distributions, uptake coefficients have been determined for some RO2 radicals (as shown in Fig.2). The aerosol size distributions were employed from the separate experiments under the almost same conditions. As a result, the increase of the uptake coefficients with the size of RO2 radicals was observed for the first time. In addition, even small RO2 radicals were uptaken into aerosols, although their values were small. This indicates that the heterogeneous reactions of RO2 radicals can affect the O3 production mechanisms. Consequently, there is possibility that the considering the interaction between RO2 radicals and aerosols should improve the accuracy of the atmospheric model.
This work was supported by JSPS KAKENHI Grant Nos. JP16H06305, JP18K18179, JP19J40218, and JP19H04255.