Comparative Analysis of PM2.5-Bound Polycyclic Aromatic Hydrocarbons (PAHs), Nitro-PAHs (NPAHs), and Water-Soluble Inorganic Ions (WSIIs) at Two Background Sites in Japan

Daily PM2.5 (particulate matter with aerodynamic diameter ≤2.5 μm) samples were simultaneously collected at two background sites (Wajima Air Monitoring Station (WAMS) and Fukue-Jima Atmosphere and Aerosol Monitoring Station (FAMS)) in Japan in the East Asian winter and summer monsoon periods of 2017 and 2019, to compare the characteristics of air pollutants among different regions and to determine the possible variation during the long-range transport process. Polycyclic aromatic hydrocarbons (PAHs), nitro-PAHs (NPAHs), and water-soluble inorganic ions (WSIIs) were analyzed. Despite the PM2.5 concentrations at FAMS (8.90–78.5 µg/m3) being higher than those at WAMS (2.33–21.2 µg/m3) in the winter monsoon period, the average concentrations of ∑PAHs, ∑NPAHs, and ∑WSIIs were similar between the two sites. Diagnostic ratios indicated PAHs mainly originated from traffic emissions and mostly aged, whereas NPAHs were mostly secondarily formed during long-range transport. WSIIs at WAMS were mainly formed via the combustion process and secondary reactions, whereas those at FAMS mainly originated from sea salt and dust. Backward trajectories revealed the air masses could not only come from Asian continental coastal regions but also distant landlocked areas in the winter monsoon period, whereas most came from the ocean in the summer monsoon period. These findings can provide basic data for the establishment of prediction models of transboundary air pollutants in East Asia.

Text S4. Calculated methods of cation equivalent (CE) and anion equivalent (AE).

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Text S1. Quartz fiber Filter treatment

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To ensure the accuracy of the calculation of PM2.5 concentration, quartz fiber filters were put into 51 the desiccator with constant room temperature (21.5 ± 1.5 °C) and humidity (50 ± 5%) for 48 h. Before 52 sampling, each filter was weighed and take the average of twice, then packaged by aluminum foil.

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After sampling, the filters were put into the desiccator under the same condition as before sampling, 54 also weighed and take the average of twice. After weighing, the filters were packaged by aluminum 55 foil and kept sealed in plastic bags and stored at -25 °C until analyzed. The concentration of each 56 PM2.5 sample was calculated as: PM2.5 concentration = (weigh after -weigh before)/ total volume.

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(BaP-d12, and 2-fluoro-7-nitrofluorene (FNF)) were added to the filter. Then, benzene: ethanol (3: 1, 61 v/v) was added for extracting. After ultrasonic extraction twice, the solution was filtered, and the 62 extracts were washed successively with 5% sodium hydroxide solution, 20% sulfuric acid solution, 63 and distilled water. The solution was concentrated with a rotary evaporator to 100 µL after adding dimethyl sulfoxide. Then, adding ethanol to make the residue up to 1 mL. Finally, the solution was filtered into a vial by a 0.45 µm membrane filter (HLC-DISK13, Kanto Chemical CO., Inc., Tokyo, NPAHs were determined by the fluorescence detection system.

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For WSIIs, one-third of each PM2.5 filter (about 130 cm 2 ) was cut into small pieces and placed in 69 polypropylene tubes. Ultrapure water was added to each tube and put at room temperature for 10  To ensure the accuracy of chemicals detection, quartz fiber filters were roasted at 600 °C for 4 h.

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To checking the background contamination during transportation, blank filters were also analyzed, 77 and no target chemicals were determined, indicating there was no contamination during the 78 transport. To check the HPLC and IC analysis methods, PAHs, NPAHs, and WSIIs standard solution 79 had injected into the systems before actual samples detecting, respectively. The calibration curves of 80 all PAHs, NPAHs, and WSIIs had good linearity (r > 0.998). Pyr-d10 was used to quantify the 4-ring 81 (FR, Pyr, BaA, and Chr) PAHs, BaP-d12 was used to quantify the 5-(BbF, BkF, and BaP) and 6-ring 82 (BgPe and IDP) PAHs and FNF was used to quantify the three NPAHs (1-, 2-NPs and 2-NFR). The 83 recoveries of internal standards of PAHs and NPAHs ranged from 80% to 95% in this study. The limit 84 of determination (LOD) of each PAH, NPAH, and WSII are shown in Table S1.

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Text S4. Calculated methods of cation equivalent (CE) and anion equivalent (AE)

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The value of AE and CE were calculated as followed: