30 late in the evening of October 11 due to rain (Fujitani, et al., 2020). As a result, the three-day average value was lower than the environmental standard; the larger standard deviations of PM mass and organic aerosol concentrations in ID 09Oct.2015 compared to the other samples resulted from this variability. associated with an emission source following Fujitani, et al. (2020): freshly emitted BBOA (fresh BBOA), aged BBOA, hydrocarbon-like organic aerosol (HOA), more-oxygenated oxygenated organic aerosol (MO-OOA), and aerosol (LO-OOA). The difference between fresh BBOA and aged BBOA is a feature of the difference in intensity of + (m/z = 60.0211) which is a the fragment ion of C2H4O2fragment ion of levoglucosan. Aged BBOA has a lower + than fresh BBOA due to signal intensity for C2H4O2atmospheric aging through atmospheric evolution due to OH radical reactions occurring either in the aqueous phase or gas phase, or heterogeneous reactions at the gas-particle interface. It is estimated that the aged BBOA had been aged for several hours in the atmosphere. HOA is an tailpipe emissions, and MO-OOA and LO-OOA are indicators of secondary organic aerosols. For organic aerosol-PMF, each of the five factors is less-oxygenated indicator of vehicle Cases of open burning of agricultural residues were observed by visual inspection around the observation site, with 10 October having the highest number of cases and 26 October having the second-highest number of cases (Tomiyama, et al., 2017). On these days, concentrations of PM2.5, fresh and aged BBOA, and the fragment ion of + which is BB tracer were relatively high. A high C2H4O2correlation coefficient (0.82) was found between the number of burning events and the fresh BBOA fraction with respect to organic aerosol for the entire observation period data. These results indicate that BB had an impact on PM mass and organic aerosol, and that BB caused significant air quality perturbation. Averaged over the observation period, the sum of fresh and aged BBOA accounted for about 50% of organic aerosol (OA) (0.5 = BBOA/ OAenvironment). Considering that organic aerosol accounted for about 29% of PM mass (0.29 = OAenvironment/PMenvironment) for the observation period, and that organic aerosol from rice straw burning accounted for about 57% of the PM mass in the emission source experiment (0.57 = BBOA/PMBB) (Fujitani, et al., 2020), which resulted in a contribution of BB aerosols (including organic aerosol, elemental carbon and other species) to PM mass during the observation period, which was estimated to be about 25% (0.25 = PMBB/ PMenvironment = OAenvironment/ PMenvironment ×BBOA/ OAenvironment/ BBOA/ PMBB = 0.29 × 0.5/ 0.57). Metals constituted less than 6% of PM mass in most samples, and their fraction was smallest among the major components, but some metals exhibit OP and OSIA and are thus noteworthy components. oxygenated organic Y. FUJITANI et al. 3.2 Comparison with Other Seasonal Samples with Different BB Impacts on OSIA and OP The impacts of BB on OSIA and OP are discussed by comparing the results of other seasons with different degrees of BB impact. Fig. 2 shows comparisons of three seasons; January 2017 and August 2017 where the periods of observations were conducted in winter and summer, respectively, at the same sampling site. Each observation was conducted for about three weeks and the observation protocols were mostly identical to those in this study. According to the results of a year-round survey of BB frequency in Tsukuba in 2016, BB was also observed in January and August (Fig. 2b). In those months, however, monthly BB frequency was low and the contribution of fresh BBOA to organic aerosol was as small as less than 10%. Comparing the three periods, not only fresh and aged BBOA but also PM2.5 and organic aerosol were highest during October 2015. In particular, the increase in organic aerosol concentration during October 2015 was mostly identical to the increase in fresh and aged BBOA concentration. OSIA can be represented as the induction fold change relative to the control per unit air volume (denoted as OSIAv: [fold change m-3] = [fold change μg-PM -1] × [μg-PM m-3]) and OP can be represented as the DTT consumption rate per unit air volume (OPv: [nmol-DTT min-1 m-3] = [nmol-DTT min-1 μg-PM -1] × [μg-PM m-3]). The size of the plots in Figs. 1c and 1d indicates the relative magnitude induction fold change relative to the control per unit PM mass [OSIArc: fold change μg-1] or DTT consumption rate per unit PM mass [OPm: nmol-DTT min-1 μg-1]. Sample ID 26Oct.2015 exhibited the highest values for all indices among the samples; OSIA and OP were highest per unit air volume as well as per unit PM mass for both solutions. Boxplots of PM components exhibiting OSIA or OP are shown in Fig. 2c. Since the predominant source varies with the season and the chemical compositions of each source differ, the concentration of each component varies with the season and is characteristic. In addition to fresh and aged BBOA, Cu, Pb and Zn tended to be higher in October 2015 than other periods, but the origins of these metals are not clear (see the next section). January 2017 (winter) was the period with the highest contributions of vehicle tailpipe emissions (HOA and elemental carbon) and soil (Fe and Mn). Fe and Mn are considered to be predominantly of soil origin at this location, given the behavior of Al, an indicator of soil composition, in combination with their high abundance in the soil particles. Winter in Tsukuba has drier weather compared to the other seasons, and the wind speeds are higher, so soil particles to aerosolize. This may have contributed to the high winter concentrations of Fe and Mn. August 2017 (summer) was the period with the highest contribution of heavy oil combustion and ship tend

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