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+ concentrations of SO4(7.16 ± 6.66 μg m-3) were observed during haze events 2-: 2.24 ± compared to those during pre-haze periods (SO4+: 0.41 ± 0.35 μg m-3) and post-haze 1.22 μg m-3, NH4+: 0.16 ± periods (SO40.12 μg m-3) in Kuala Lumpur, Malaysia71. Similar results were also obtained at other locations in Malaysia such as Petaling Jaya55 and Bangi56. In Singapore, significantly + higher concentrations of SO4(6 ± 2 μg m-3) were observed107 in smoke-dominant samples compared to those in non-smoke dominant +: 1.6 ± 1.3 μg m-3). At samples (SO4other locations in Singapore, similar results were also obtained16, 25, 30, 41, 43, 48, 53, 89. In Thailand, Chaisongkaew et al.122 reported that the concentrations of SO4+ during non-haze periods, which represented clean background air, at Thepha beach were 0.658 and 0.198 μg m-3, respectively, whereas during a haze period, those species’ 2-: 2.168 μg m-3 (partial concentrations increased (SO4haze) and 2.248 μg m-3 (strong haze); NH4+: 0.825 μg m-3 (partial haze) and 0.834 μg m-3 (strong haze)). Similar results were also obtained in Hat Yai, Thailand120. - (1200 ± 365 mg kg-fuel-1) and SO4 -) and SO4- and SO43.3 Elemental Components In general, potassium (K+) has been used as a biomass burning indicator (Chuang et al., 2013). Thus, high K+ concentrations during IPF-induced haze events are expected at the receptor sites. In most cases of haze induced by IPFs, however, partial overlapping of the variation ranges of K+ concentrations were observed between samples collected during haze and non-haze periods as shown in the Supplementary Information (the “Water-soluble Ions” sheet in the dataset and Fig. S1). Fujii et al.49 mentioned K+ was not appropriate as an indicator of IPFs due to an extremely low K+ fraction at source (0.0423 ± 0.0400 wt.% of PM2.5), and a lower fraction of 0.026 wt.% was obtained at a site near another fire source77, as shown in Fig. S2 in the Supplementary Information. Based on laboratory Chemical Properties of the Southeast Asian Haze from Indonesian Peatland Fires 2- (20.4 ± 18.4 μg m-3) and NH42-: 1.79 ± 0.98 μg m-3, NH42-: 4 ± 4 μg m-3, NH42- (15 ± 3 μg m-3) and NH42- and NH4chamber combustion experiments on Indonesian or Malaysian peat, chloride (Cl-), nitrate (NO32- were determined as the main ions in most cases26, 36, 83, 90. The EFs of Cl-, NO3- 2- for fresh particles were reported as 25.5 – 48, and SO418.6 –28.6 and 15 –37.2 mg kg-fuel-1, respectively26, 90, which were much smaller than the EFs of OC (6,020 – 18,500 mg kg-fuel-1) and WSOC (3,100–3,840 mg kg-fuel-1) + was one of the main (see Section 3.1). Although NH4water-soluble ions in PM collected at receptor sites during + based on laboratory haze periods, the EF of NH4chamber combustion experiments26, 90 only came to 0.56 – 0.608 mg kg-fuel-1. Chow et al.83 and Watson et al.90, respectively, reported species abundances in PM2.5 mass and EFs for water-soluble ions based on fresh and aged aerosol data from laboratory chamber combustion of Malaysian peat, separated by two- and seven-day photochemical aging times simulated with an OFR. The 2- for fresh particles were 18.6 ± 6.63 EFs of NO3and 37.2 ± 3.18 mg kg-fuel-1, respectively, and those for particles after two days of aging increased significantly to 171 ± 14.0 and 105 ± 6.58 mg kg-fuel-1, respectively. After seven days of aging, further increases in the EFs of NO332.2 mg kg-fuel-1) were revealed. Similar patterns were + (EF: 0.411 – 0.608 mg kg-fuel-1 in also observed for NH4fresh particles, 159 ± 34.7 mg kg-fuel-1 after two days aging and 1,210 ± 219 mg kg-fuel-1 after seven days -) (EF: ~0.00 mg kg-fuel-1 in fresh aging) and nitrite (NO2particles, 0.148 ± 5.48 mg kg-fuel-1 after two days aging and 3.89 ± 7.07 mg kg-fuel-1 after seven days aging). In contrast to the above ions, the EF of Cl- in fresh particles was 25.5 – 34.9 mg kg-fuel-1 and that in aged particles decreased to 19.1–26.6 mg kg-fuel-1. There are many reports on elements in PM at receptor sites (especially in Singapore) during periods of IPF-induced haze. The number of detected elements in those reports, however, ranged from 3 to 48, showing considerable (see Supplementary Information (“Elements” sheet in the dataset)). Based on field observations in Singapore, significant increases in the concentration of Al25, 37, 42, 43, 58, K9, 25, 42, 58, Cr37, 47, 58, Mn37, 47, 58, Fe25, 37, 42, 43, 58, Co37, 47, 58, Cu25, 37, 43, 58, Zn25, 37, 58, Cd37, 47, 58 and Pb37, 58, 118 were reported by some researchers also reported higher concentrations of other elements such as Ca25, 42 and Ni37, 58 during haze events in Singapore, the number of such reports is quite limited. In Kuala Lumpur, Malaysia, Sulong et al.71 analyzed 18 elements in PM2.5 during pre-haze, haze and post-haze periods. They then showed a significant increase of K during haze (604 ± 301 ng m-3) compared to pre-haze (208 ± 118 ng m-3) and post-haze (195 ± 73.7 ng m-3) periods. In Hat Yai, Thailand, Promsiri et al.120 analyzed 11 elements in PM2.5 from transboundary haze samples (only two samples) and background samples. There, the highest concentrations of heavy metals (Cd, Co, Cr, Mn, Ni and Pb) were found during haze events, but neither As nor Cd were detected. They also mentioned K was about three times higher than background levels during the haze events. A similar pattern with K was also observed at Thepha Beach, Songkhla Community College, the Wang Yai Sub-district and Pattani City in Thailand122. At an IPF source, Betha et al.34 showed higher concentrations of Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in IPF samples than in background air samples collected at the same sampling point in the absence of IPFs. The EFs of several elements based on laboratory chamber combustion experiments of Malaysian peat were reported by Watson et al.90. They showed, for example, that the EFs of Al, K and Fe in fresh PM2.5 were 8.79 – 9.94, 6.13 –10.4 and 10.3 –17.0 mg kg-fuel-1, respectively. Detailed information on other elements is provided in the among differences several researchers. Although reports 2- (502 ± 41

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