Fig. 1 Overview of the field campaign at the Tsukuba observation site for a) PM2.5 concentrations and chemical compositions of filer samples; b) organic aerosol (OA) concentration and OA sub-fractions determined by positive matrix factorization (PMF) analysis, c) oxidative stress induction ability (OSIA), d) oxidative potential (OP), and e) the maximum and minimum of the daily number of open burning events during each sampling period in the visual inspection area of + fragment ion (m/z = 60.0211), which is a fragment ion Tsukuba City according to Tomiyama, et al. (2017), the C2H4O2+ ion are averaged over of levoglucosan and tracers of biomass burning, and the wind speed. The OA fraction and C2H4O2the filter sampling period. The x-axis label is the filter ID and indicates the sampling start date. The y axis in c) and d) for OSIA and OP indicate air volume basis (OSIAv, OPv), while the size of the plots indicates the relative magnitude of induction fold change relative to control per PM mass (OSIArc) or OP of PM mass (OPm). Error bars in PM2.5, OA and + represent standard deviation of the continuous measurement 1-hr averaged data during the sampling period. EC: C2H4O2elemental carbon; OC: organic carbon; DMSO: dimethyl sulfoxide; MO-OOA: more-oxygenated oxygenated OA; LO-OOA: less-oxygenated OOA; HOA: hydrocarbon-like OA; BBOA: biomass-burning OA. 2.3 Protocols for HO-1 and DTT Assays 3.1 State of Open Burning of Agricultural Residue and its Contribution to PM in Tsukuba Figure 1 shows the PM2.5 concentration and major chemical composition of the PM mass in the filter samples, organic aerosol-PMF fraction to organic aerosol, OSIA and OP. The PM2.5 concentrations were below the Japanese environmental standard (24-hour value of 35 µg m-3) for all samples. Note, however, that the 1-hr PM2.5 concentration (according to continuous measurement) increased to a maximum of 100 μg m-3 on October 10 due to intensive BB and then declined sharply 3. Results and Discussion means of the IMPROVE protocol and the two analyses were averaged. Pyrolysis of the organic carbon during the analysis was corrected using reflected light. OSIA and OP were determined by means of HO-1 assay and DTT assay according to the protocol of Fujitani, et al. (2023). Briefly, for the HO-1 assay, a rat alveolar epithelial cell line (SV40-T2 cells) was used. The samples were extracted in two separate processes using distilled deionized water (DDW) and dichloromethane (DCM) for water-soluble and water-insoluble components of particles, respectively, and the cells were exposed for three hours to 1 ml of the solution at PM concentrations of 50 µg-PM ml-1 for each solution. For OP, the samples were extracted by Tris-HCl buffer at pH 8.9 or DCM, and the solution concentration was then adjusted to 30 µg-PM ml-1 and Oxidative Stress Induction Ability of Particles Emitting from Agricultural Open Burning in Japan 250 µg-PM ml-1, respectively. DTT was added to each sample at a concentration of 160 μM. The solution was incubated at 37°C for 15 min to complete the reaction. 29

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