Journal Pre-proof Soluble trace metals associated with atmospheric fine particulate matter in the two most populous cities in Vietnam

16 Hanoi and Ho Chi Minh City (HCM), the most populous cities in Vietnam, have received 17 increasing global attention because of their poor air pollution status. As part of the recent UK-18 Vietnam 2-Cities project, the concentrations of trace metals in fine particulate matter have been 19 characterized. 24-hour samples of PM 2 were collected at 2 sites in Hanoi and 3 sites in HCM 20 during two 4-week periods in September / October 2018 and March 2019. The soluble fraction 21 of 15 trace metal(oid)s (Fe, Al, Mn, Ti, Zn, V, Cu, Ni, Co, Cd, Pb, Th, Cr, As, and Sb) bound 22 to PM 2 were analyzed by ICP-MS. The results show that Zn was the most abundant soluble 23 metal in PM 2 in both cities, with very large numbers of road vehicles (e.g. tyre wear) likely 24 contributing in both cities and non-ferrous metal production being a substantial additional 25 source in Hanoi. Fe and Al, derived from crustal sources, were the dominant metals after Zn. 26 Most trace metals concentrations in Hanoi were higher than in HCM, especially toxic metals 27 such as Pb, Cd, Cr and As. V and Ni were the only two metals having higher concentrations in 28 HCM than in Hanoi, likely due to shipping emissions (combustion of heavy fuel oil) that 29 strongly affect the air quality in HCM. Coal-power plants and non-ferrous metal production 30 are likely to be the major sources of trace metals in Hanoi. Health risk assessment shows that 31 a high carcinogenic risk exists for inhalation exposure of soluble trace metals bound to PM 2 in 32 both cities. 33


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Aerosol fine particulate matter (PM) affects the Earth's climate (Adams et al., 2001) and can 37 have negative impacts on ecology (Clark et al., 2013) and human health (Dockery, 2001;38 Dominici et al., 2006). The environmental and social impacts of PM are dependent to varying 39 extents on its inorganic and organic composition and the size of the particles. Many hazardous 40 components derived from anthropogenic activities are concentrated mainly on fine particles 41 (Hien et al., 2007;Hu et al., 2014;Li et al., 2015) and particles smaller than 2.5 m (PM2.5) 42 are of particular concern because they are able to penetrate deep into the lungs. 43 PM soluble components have been shown to be released within minutes in simulated lung 44 interstitial environment solution (Palleschi et al., 2018) and to affect the energy metabolism, 45 protein synthesis, and chromatin modification of human cells (Oeder et al., 2015). Trace metals 46 in PM are known to have negative health effects (Okuda et al., 2008;Qi et al., 2016), such as impacts on human attention, executive function, mental flexibility and cognitive efficiency 48 (Jiang et al., 2014;Rafiee et al., 2020;Soetrisno and Delgado-Saborit, 2020), lung damage 49 (Palleschi et al., 2018) and cancer (IARC, 2014;Wang et al., 2015;Galon-Negru et al., 2019). 50 The toxicity of trace metals in PM has been shown to be largely due to the soluble fraction of 51 the metals (Adamson et al., 1999). 52 The trace metals in PM may be derived from natural and anthropogenic sources (Pacyna and 53 Pacyna, 2001;Zhang et al., 2015). Natural sources of trace metals are dominated by the 54 generation of mineral dust through the wind erosion of soils in arid regions. Artificial sources 55 include metallurgical industries, fossil fuel use, and transport activities. The size distribution 56 and soluble fractions of metals in PM vary considerably between natural and anthropogenic 57 sources. Many crustal elements occur predominantly in larger size fractions and have relatively 58 low solubility, while anthropogenic metals tend to be associated with finer particles and have 59 high soluble fractions (Mahowald et al., 2018;Baker et al., 2020). 60 Fine PM has been investigated because of its potentially harmful effects and the increase of  and Certified Reference Materials (CRMs) before analysis by inductively coupled plasma -145 mass spectrometry ICP-MS (Thermo Scientific, iCAP-TQ). Fe, V and As were determined in 146 triple quad mode as their MO + ions, after reaction with O2 in a collision cell. All other elements 147 were determined as M + in single quad mode. Samples were introduced into the instrument using 148 a micromist nebulizer at a flow rate of 1.08 mL min -1 . Instrument RF power was 1.55kW. Cu, Ni, Pb and As (all ≤ 5% of values). Where sample concentrations were below the LOD, 166 they were substituted with 75% of the relevant LOD (e.g. in Fig. 3 & 4). Field blanks for each 167 of the 7 sampling periods were subtracted from their respective sample concentrations, unless 168 the field blank was below the analytical LOD (i.e. for Fe, Ni, Co, Pb: see Table 2) in which 169 case there was no blank subtraction. value was judged to be caused by contamination and has been excluded from the data presented 174 here.

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Significance testing of the dataset was performed using parametric tests (one-and two-sided t-   Table 3. 3 and 4, respectively, and equivalent plots for the background sites can be found in Figure S1. 245 The results obtained at HCM and Hanoi indicate that there were substantial differences in the 246 concentrations of the trace metals studied at the regional level (e.g. Zn, Pb and Cd were 247 substantially higher in Hanoi than in HCM) and over time, both seasonally (e.g. As in HCM) 248 and on a daily basis (Table 4, Fig. 3 & 4). These patterns indicate that a variety of sources 249 influence trace metals in PM2 at the two cities, as discussed below.  each study. Direct comparison between the results from the different studies shown in Table 5 255 is not straightforward because of the different aerosol size fractions collected (e.g. PM10 may 256 contain higher concentrations of a given trace metal than PM2, depending on the element's

Seasonal differences 271
In HCM there were substantial differences between the concentrations of several trace metals 272 observed in the October and March sampling periods (Fig. 3, Table 4 to lead to higher aerosol concentrations in general, due to decreased rates of particle rain-out.

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In some cases, soluble metal concentrations may be enhanced by atmospheric processing of 298 aerosol during transport, independently of total metal concentration changes (Baker and Croot, 299 2010). We did not measure total metal concentrations and are therefore not able to assess the

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There were fewer significant differences between seasons at Hanoi, and all these concentrations 308 differed by less than a factor of 2. Concentrations of Al, Co, Cr and V were significantly higher, 309 and Mn, Zn and Pb significantly lower, respectively, in March than in October. Seasonal 310 differences in atmospheric transport were less marked at Hanoi than at HCM, with a general 311 shift in the prevailing wind to the southeast in March (Fig. 2). 10% of each campaign ( Figure S2). Both air mass back trajectories (Fig. 2c) and local wind 322 data ( Fig. S2a)  Cr (t-test, p = 0.007). Weekend effects can indicate the relative importance of local sources 335 (e.g. due to decreased industrial activity or commuting), so that the small number of significant 336 differences identified here may also point towards regional trace metal sources being 337 significant at HCM and Hanoi. at HCM appear to be associated with transport from the northeast (Fig. 5).

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The CWT plots indicate that sources in northeast Vietnam and southeast China, and possibly  Al and Ti -Al relationships there being very similar to those in HCM (Fig. 7c & d). Both Mn 359 and Co appear to be enriched with respect to Al in several samples in Hanoi, especially in the 360 NE and E air mass types ( Fig. 7a & b). This suggests that anthropogenic sources in northeast  (Fig. 7d). This may be due to the very low solubility of Ti in

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In common with other cities in East Asia (Table 5) ) may also contribute, due to combustion of Zn-396 enriched lubricating oil in these engines. In Hanoi, additional sources of Zn appear to be 397 associated with atmospheric transport from southeast China (Fig. 6). These may well be 398 associated with Zn smelting activities in Guangxi and Hunan provinces (Zhang et al., 2011).
elements associated with mineral dust. Biogeosciences 12, 5771-5792.      J o u r n a l P r e -p r o o f  where available).