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For the purpose of future visualization of the flow field in superfluid helium-4, clusters of the triplet state excimer 4He2* are generated along the micro-scale recoil tracks of the neutron-absorption reaction n + 3He → 3T + p. This reaction is induced by neutron irradiation of the 3He fraction contained in natural isotopic abundance liquid helium with neutron beams either from the Japan Proton Accelerator Research Complex, Materials and Life Science Experimental Facility (JPARC)/Materials and Life Science Experimental Facility or from the Kyoto University Institute for Integrated Radiation and Nuclear Science. These 4He2* clusters are expected to be ideal tracers of the normal-fluid component in superfluid helium with several advantageous properties. Evidence of the excimer generation is inferred by detection of laser induced fluorescence emitted from the 4He2* clusters excited by a purpose-built short pulse gain-switched titanium:sapphire (Ti:sa) laser operating at a wavelength of 905 nm. The setup and performance characteristics of the laser system including the Ti:sa and two continuous wave re-pumping lasers are described. Detection at the fluorescence wavelength of 640 nm is performed by using optical bandpass filtered photomultiplier tubes (PMT). Electrical noise in the PMT acquisition traces could successfully be suppressed by post-processing with a simple algorithm. Despite other laser-related backgrounds, the excimer was clearly identified by its fluorescence decay characteristics. Production of the excimer was found to be proportional to the neutron flux, adjusted via insertion of different collimators into the neutron beam. These observations suggest that the apparatus we constructed does function in the expected manner and, therefore, has the potential for groundbreaking turbulence research with superfluid helium. [ABSTRACT FROM AUTHOR]

The present study aimed to investigate OH-reactivity change in the OH-initiated photo-oxidation of isoprene, α-pinene and m -xylene under high NO x conditions in the smog chamber at the National Institute for Environmental Studies, Japan. In all OH-initiated photo-oxidation experiments, first-generation oxidation products measured by Fourier-transform infrared spectroscopy (FTIR) could not fully explain the OH reactivity of the products measured by the laser photolysis/laser-induced fluorescence technique, and ∼20%, ∼40%, and ∼70% of OH reactivity attributed to these volatile organic compound (VOC) products, respectively, were missing. A master chemical mechanism (MCM) calculation demonstrated that the observed OH reactivity during photo-oxidation could be explained with secondary VOC products. It was found that vapor wall loss (VWL) and aerosol formation (AF) strongly affected the OH reactivity when the photochemical reaction was prolonged, i.e., multiple oxidations were involved. The MCM calculation showed that aromatics have almost twice the productivity of OH reactivity attributed to secondary VOC products, P s_VOC , compared with isoprene and α-pinene at the beginning of the period not affected by VWL/AF. The evaluation of the missing OH reactivity from O 3 oxidation and the sequential multiple oxidations of secondary products still involves an uncertainty that originates from an incomplete knowledge of VWL/AF, and thus, further investigation is required to understand OH-reactivity evolution in both chamber and ambient observations. The error in photolysis rate and possibly unconsidered reactions of secondary products, such as the pinonaldehyde case, also presents potential uncertainty in the OH reactivity prediction and thus requires evaluation. ● OH-reactivity budget in the isoprene, α-pinene and m-xylene oxidation with OH was investigated in high NO x conditions. ● Master Chemical Mechanism (MCM) calculations were compared with experiments. ● Secondary produced VOCs predicted by MCM model could explain the missing OH reactivity. ● The estimation error in MCM prediction became large when multiple oxidations were involved due to vapor wall loss and aerosol formation. [ABSTRACT FROM AUTHOR]

Abstract: In situ measurements of nitrogen dioxide (NO2) concentrations were conducted at an urban site in Tokyo (35°71′N, 139°76′E), Japan during the summers of 2007–2009. A chemiluminescence instrument equipped with a molybdenum oxide converter (Mo-CL) and a laser-induced fluorescence (LIF) instrument were used simultaneously for determining the ambient NO2 concentrations. A tight linear correlation between the NO2 concentrations determined by the LIF and Mo-CL techniques was evident except for high ozone concentration periods over 50 ppbv. During the high ozone episodes, correlation plots of the NO2 concentrations between Mo-CL and LIF techniques indicated an overestimation of Mo-CL NO2. To examine in detail the systematic differences between two techniques, a thermal dissociation laser-induced fluorescence (TD-LIF) instrument was employed in the summer of 2009 to measure total peroxy nitrates (ΣPNs) and total reactive nitrogen (NO z ). We found that the ΣPNs concentrations correlated well with the discrepancies in NO2 concentrations determined by the Mo-CL and LIF techniques. This result suggests that peroxy nitrate species significantly contribute to the observed interference in the Mo-CL measurements under high O3 concentrations in our observation at the urban site in Tokyo. [Copyright &y& Elsevier]

HO 2 radicals play important roles in tropospheric chemistry. The large discrepancies among field measurements and sophisticated model predictions for the overall HO 2 concentrations can be attributed to the HO 2 uptake coefficients onto ambient aerosols (γ) have not yet been properly quantified. This study presents the first online measurement of the total HO 2 reactivity caused by the ambient gas phase (k g ') and aerosol phase ( k a ') in the summer of 2018 in Kyoto, Japan, using a combined laser-flash photolysis and laser-induced fluorescence (LFP–LIF) technique coupled with a versatile aerosol concentration enrichment system (VACES) that enriches ambient aerosols. The results denote that k g ' ranged from 0.1 s−1 (25th percentile) to 0.32 s−1 (75th percentile) with an average value of 0.22 ± 0.16 s−1 (1 σ), which can be primarily explained by the reaction of HO 2 with NO 2. With the application of VACES and the auto-switching aerosol filter, k a ' ranged from 0.004 s−1 (25th percentile) to 0.028 s−1 (75th percentile) with an average value of 0.017 ± 0.015 s−1. When converted k a ' to ambient conditions (by dividing with the enrichment factor), this result was ~10 times higher than the HO 2 reactivity caused by its self-reaction under ambient concentration levels (~5 ppt) at 298 K. The related γ ranged from 0.08 (25th percentile) to 0.36 (75th percentile), with an average value of 0.24, which is comparable with the values used in previous modeling studies (~0.2); however, it presents a large variation of ±0.20 (1 σ) within the measurement time. This indicates that a large bias may exist with respect to the estimation of HO 2 concentrations when using a constant γ value. Further, we suggest that different γ values should be applied in modeling studies depending on the environment conditions; the γ value obtained here can be used as an upper limit value for urban areas in the summer due to possible aerosol phase changes. The analysis of ambient air backward trajectories indicates that the predominant NO 2 emission sources originated from the mainland of Japan. However, no significant differences regarding HO 2 uptake coefficients were observed when air masses came through the mainland or from the coastal direction. This study provides considerably reliable γ , which could increase the modeling accuracy of heterogeneous processes in tropospheric chemistry. However, combining online and offline methods appears to be more suitable for addressing the variation in γ and model discrepancies in further studies. Image 1 • Gas phase HO 2 reactivities in an urban area in Kyoto (Japan) were mostly attributed to NO 2. • The real-time HO 2 uptake kinetics onto ambient aerosols were measured. • The HO 2 reactivity caused by ambient aerosols was higher than by HO 2 self-reaction. • The average γ is comparable with the value used in previous modeling studies. • Using a constant γ value to estimate HO 2 concentrations may cause large bias. [ABSTRACT FROM AUTHOR]