Multiple sulfur isotope constraints on sources and formation processes

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Multiple sulfur isotope constraints on sources and formation processes of sulfate in Beijing PM2.5 aerosol Xiaokun Han, Qingjun Guo, Harald Strauss, Cong-Qiang Liu, Jian Hu, Zhaobing Guo, Rongfei Wei, Marc Peters, Liyan Tian, and Jing Kong Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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Environmental Science & Technology

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Multiple sulfur isotope constraints on sources and formation processes of sulfate

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in Beijing PM2.5 aerosol

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Xiaokun Han1, Qingjun Guo1,2*, Harald Strauss3, Congqiang Liu4, Jian Hu4, Zhaobing

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Guo5, Rongfei Wei1, Marc Peters1, Liyan Tian1, Jing Kong1

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1 Center for Environmental Remediation, Institute of Geographic Sciences and Natural Resources

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Research, Chinese Academy of Sciences, Beijing 100101, China.

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2 College of Resources and Environment, University of Chinese Academy of Sciences, Beijing

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100049, China

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3 Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster,

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Corrensstrasse 24, 48149 Münster, Germany

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4 State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese

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Academy of Sciences, Guiyang Guizhou 550002, China

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5 School of Environmental Science and Engineering, Nanjing University of Information Science

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and Technology, Nanjing 210044, China

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*Corresponding author. Email: [email protected]

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Address: 11A Datun Road, Chaoyang, Beijing 100101, China

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Fax: +86-01-64889455

Tel: +86-10-64889455

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ABSTRACT

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Recently air pollution is seriously threatening the health of millions of people in

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China. Multiple sulfur isotopic compositions of sulfate in PM2.5 samples collected in

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Beijing are used to better constraining potential sources and formation processes of

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sulfate aerosol. Here we found the ∆33S values of sulfate in PM2.5 show a pronounced 1

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seasonality with positive values in spring, summer and autumn and negative values in

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winter. Positive ∆33S anomalies are interpreted to result from SO2 photolysis with

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self-shielding, and may reflect air mass transport between the troposphere and the

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stratosphere. The negative ∆33S signature (-0.300‰ Na+ > F- > Mg2+. NO3-, SO42- and NH4+ are the predominant

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ions, on average, collectively accounting for ~85 % (neq %) of WSII. The nitrate,

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sulfate and ammonium equivalent concentrations range from 72 to 1552 neq/m3

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(mean=470±313 neq/m3, n=46), from 90 to 1486 neq/m3 (mean=459±280 neq/m3,

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n=46), and from 131 to 2430 neq/m3 (mean=872±500 neq/m3, n=46), respectively.

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The equivalent concentration ratio of NH4+/[SO42-+NO3-] varies from 0.69 to 1.36

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with a mean value of 0.94±0.14(n=46). Cl- equivalent concentration shows an obvious

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seasonality, i.e. high in winter (mean=252±93 neq/m3, n=16) and low in summer

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(mean=15±14 neq/m3, n=10). The average equivalent concentrations of K+, Na+, F-

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and Mg2+ are lower than 50 neq/m3.

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3.2 Multiple sulfur isotopes

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Multiple sulfur isotopic compositions of sulfate (δ34S and ∆33S) in Beijing PM2.5

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are listed in Table S1 and shown in Fig. 2. δ34S values vary from 3.3 ‰ to 11.4 ‰

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(mean=6.0±2.0 ‰, n=46) and exhibit a significant seasonality, i.e. low values in

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summer and high values in winter. During spring, summer, autumn and winter, the

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mean values of δ34S in sulfate are 5.1±1.2 ‰ (n=10), 4.7±0.9 ‰ (n=10), 4.6±0.5 ‰

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(n=10) and 8.3±1.5 ‰ (n=16), respectively. ∆33S values of sulfate in PM2.5 range from

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-0.664 ‰ to 0.480 ‰ (mean=0.075±0.253 ‰), and also show a pronounced

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seasonality. During spring, summer and autumn, ∆33S values are clearly positive

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around a mean value of 0.227±0.110 ‰ (n=30). In contrast, during winter negative

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∆33S values ranging from -0.664 ‰ to 0.035 ‰ (mean=-0.211±0.188 ‰, n=18) were

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measured for PM2.5. To the best of our knowledge, this is the first report of negative 10


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∆33S signatures for sulfate aerosol in the modern atmosphere.

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4. DISCUSSION

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4.1 δ34S in PM2.5 implying atmospheric sulfur sources

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Sulfur isotopes from sulfate aerosols have been used to distinguish

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anthropogenic and natural sources of atmospheric sulfur based on their distinct δ34S

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values.10,

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indicate that sulfate in PM2.5 is reflecting a complex mixture of different sulfur

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sources having high and low δ34S values. Based on the data from Smithsonian

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database of global volcanic eruptions (www.volcano.si.edu), no volcanic activities

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occurred in China, Mongolia, North Korea and South Korea during 2014-2016.

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Consequently, volcanic SO2 as a source of atmospheric sulfur in Beijing can be ruled

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out. A contribution of sea-salt sulfate is also rather unlikely due to the low

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concentration of Na+ (0.41±0.23 µg/m3, n=30) in spring, summer and autumn. The

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concentration of Na+ is higher in winter (1.71±1.38 µg/m3, n=16). Moreover, back

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trajectory analyses show that the air masses in winter originate from northwest and

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south of Beijing (Fig. S2), which supports the assumption of a very low to negligible

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contribution of sea-salt sulfate. The higher Na+ concentration in winter was likely

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caused by the enhanced contribution from coal combustion and the lower boundary

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layer height.

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The δ34S values, plotted against sulfate concentration (Fig. 3),

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Sulfate in aerosols can be derived from a terrigenous source. Ca2+ has been

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considered as an indicator of this.38 The contribution of sulfate from a terrigenous 11



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source (fts) can be estimated by the following equation: fts= (SO42-/Ca2+)soil / (SO42-/Ca2+)aerosol

(3)

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where the ratio of (SO42-/Ca2+)soil in China ranges from 0.29 to 0.43.39, 40 Results

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indicate that the average contribution of sulfate from a terrigenous source varies from

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3.83±4.40 % to 5.69±6.52 %. However, Ca2+ in PM2.5 could originate from fly ash

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emitted from coal combustion followed by solubilization during atmospheric aging

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process, suggesting that Ca2+ in PM2.5 does not all come from a terrigenous source.

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Therefore, the estimation of terrigenous contribution to sulfate in PM2.5 is higher than

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the actual contribution, which implies that sulfate from soil or mineral dusts account

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for a rather small proportion of sulfate in Beijing PM2.5. This is further confirmed by a

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weak correlation between SO42- and Ca2+ (r=-0.37, Table 1).

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SO2 can be emitted from oil combustion. It was reported that the sulfur content

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and its δ34S values in oil used in North China varied from 0.1 % to 0.6 % and from

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13.7 % and 24.2 ‰, respectively.41 In North China, hundreds of million tons of

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petroleum products are being consumed by a wide range of industries. Consequently,

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oil combustion can be identified as a sulfur source having a relatively high δ34S value.

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Biogenic sulfur from soils and wetlands, which is a natural source of

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atmospheric sulfur, has a low δ34S value of -10 ‰ to -2 ‰.42-44 However, the mean

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values of δ34S in sulfate of PM2.5 for spring, summer, autumn and winter are

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5.1±1.2 ‰ (n=10), 4.7±0.9 ‰ (n=10), 4.6±0.5 ‰ (n=10) and 8.3±1.5 ‰ (n=16),

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respectively. These values for PM2.5 are distinctly different from values for sulfur

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from oil and biogenic origin, suggesting sulfate in PM2.5 is a mixture of different 12


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sulfur sources.

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Previous reports indicated that sulfur emissions from coal combustion make a

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significant contribution to the atmospheric sulfur pool in China.10, 13, 26 Sulfur isotopic

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fractionation occurs during the process of coal combustion.45 The released SO2 is

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depleted in 34S while the SO42- in the solid ash is enriched in 34S relative to the total

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sulfur in the coal.45 Despite of these, Mukai et al. observed that the mean δ34S values

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of atmospheric sulfur in China were close to the sulfur isotope compositions of the

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coal used in the respective area.10 Furthermore, Novák et al. reported that the

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increasing coal consumption in winter lead to a change in δ34S of SO2 in

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atmosphere.46 In consideration of coal as a dominant energy source in China, a direct

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comparison between δ34S values of sulfate in PM2.5 and those of coals in a local

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region appears most logical.

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Here, the mean δ34S value of sulfate in PM2.5 (6.0±2.0 ‰, n=46) is close to

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those of coals used in North China (mean = +6.6 ‰).41,45 Considering sulfate from oil

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and biogenic origin, it suggests that coal combustion is one of the major sources of

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sulfate in PM2.5 aerosols in Beijing (Fig.3). The oxygen isotopic composition of

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sulfate can reflect oxidation processes.11The δ18O value of primary sulfate (~40 ‰)

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formed in the emission source is much heavier than that of secondary sulfate

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(-10~20‰) formed by SO2 oxidation47. The mean δ18O value of sulfate in PM2.5 is

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12.8±4.4 ‰ (n=46, from an unpublished paper). The proportion of primary and

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secondary sulfate in PM2.5 can be estimated based on equations developed by

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Holt(1991)47 and the mean δ18O value of precipitation in Beijing (-7.3‰).48 It shows 13



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that most of the sulfate in PM2.5 is formed through oxidation of SO2, which is

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consistent with the results reported by Guo et al.49 Moreover, the mean δ34S value of

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coal used for heating in the Beijing suburbs (8.0 ± 3.0 ‰, n=14), determined in this

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study, agrees well with the mean value of δ34S in sulfate of Beijing PM2.5 in winter

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(8.3±1.5 ‰, n=16). This indicates that, in winter, the contribution of secondary sulfate

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from coal combustion is considerable to sulfate in PM2.5. This has been confirmed by

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our recent study, which shows that the contribution of coal combustion during the

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winter season in Beijing can account for up to 80.9 % of sulfate in total suspended

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particulates (TSP).13 In addition, the mean δ34S value of sulfate in TSP (6.6±1.8 ‰,

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n=70)13 is similar to that in PM2.5 (6.0±2.0 ‰, n=46), suggesting that sources of

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sulfate were basically identical to TSP and PM2.5.

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4.2 ∆33S signatures of sulfate in PM2.5

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Positive ∆33S values as high as 0.480‰ were measured in sulfate of PM2.5 for

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spring, summer and autumn, while negative ∆33S values as large as -0.664‰ were

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recorded in winter (Fig. 2). Obviously, some samples reflect truly mass independent

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fractionation of sulfur isotopes (S-MIF). In the δ34S-∆33S diagram (Fig. 4A), the

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sulfur isotopic compositions of PM2.5 display a negative slope (δ34S-∆33S slope =

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-0.10) different from the Archean reference array (ARA) (δ34S-∆33S slope = 0.9)50 and

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Felsic volcanic array (FVA) (δ34S-∆33S slope = -0.45).24 The ∆33S and δ34S values of

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Beijing PM2.5 in spring, summer and autumn (-0.011 ‰ ≤ ∆33S ≤ 0.480 ‰ and 3.3 ‰

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≤ δ34S ≤ 7.4 ‰) are within the range reported in Xianghe aerosols (-0.056 ‰ ≤ ∆33S ≤

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0.525 ‰ and 1.4 ‰ ≤ δ34S ≤ 9.2 ‰), which were collected in spring and summer 14


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(Fig.4B)

, reflecting similar processes of sulfate formation. However, strongly

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negative ∆33S values of PM2.5 in winter have not been reported before and are

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distinctly different from California aerosols collected in winter.25

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Mass independent isotopic compositions can be caused by several factors,

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including nuclear field shift effects, photochemical reaction and surface reaction

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effects, etc.25,26,31 However, nuclear field shift effects, surface reaction effects,

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symmetry and magnetic isotope effects have been excluded for mass-independent

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fractionation of sulfur isotopes (S-MIF).26 This leaves SO2 photolysis at short

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wavelengths as the most likely candidate for S-MIF production.29-31,51 Experimental

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studies have shown that SO2 photolysis in the wavelength region of 190-220nm can

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produce large S-MIF signatures influenced by gas pressure and gas composition.29,30

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Different causes responsible for sizeable S-MIF effects related to SO2 photolysis

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have been discussed: the spectrum of radiation available for photolysis (mutual

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shielding and self-shielding); isotopologue-specific probability of photoexcitation

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prior to dissociation; kinetic isotope effects related to state-to-state transitions for

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isotopologues.29,30,50 Farquhar et al. 29 suggested the S-MIF signatures to result from

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simple photodissociation, which did not consider the effect of self-shielding. Recent

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experimental studies, however, have shown that SO2 photolysis with self-shielding

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produces elemental sulfur with large S-MIF signals (positive ∆33S values) and a

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δ34S-∆33S slope of 0.086±0.035.50 This would be fully consistent with sulfur isotope

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anomalies (∆33S) observed for sulfate in Beijing PM2.5 in spring, summer and autumn

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(Fig. 4A). However, SO2 photolysis can not occur in the troposphere because the 15



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radiation in the 190-220nm region is absorbed by O2 and O3 in the modern

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atmosphere.29,31 In contrast, respective processes would be possible at an altitude of

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20-25km, i.e. in the stratosphere.31 Thus, we propose that the positive ∆33S values

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measured in the Beijing PM2.5 sulfate may reflect SO2 photolysis with self-shielding,

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possibly due to an air mass transport between the troposphere and the stratosphere.25

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Ono et al. reported that SO2 photolysis with self-shielding can produce a ∆33S value of

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11.79‰ and a δ34S value of 118.67‰ under the condition of pSO2=0.89mbar

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pN2=0.22bar, and a flow rate=25sccm.50 Based on these values and the maximal ∆33S

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value of 0.480‰ observed in PM2.5, it is estimated that at most 1.57% of the sulfate

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in PM2.5 can originate from the stratosphere, which may cause an increase of less than

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1.86‰ in the δ34S value of sulfate in PM2.5.

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A different source of S-MIF is SO2 photoexcitation in the 250-320nm region,

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recently proposed based on numerical modeling.52 The model results of SO2

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photoexcitation agree well with S-MIF signatures of volcanic sulfate preserved in

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polar ice and snowpacks.52 However, respective photochemical experiments, i.e. SO2

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photoexcitation in the 250-330nm, resulted in sulfate lacking S-MIF and elemental

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sulfur characterized by positive ∆36S/∆33S ratios.51 Considering the negative

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∆36S/∆33S ratios observed in modern sulfate aerosols,25, 26 it may indicate that SO2

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photoexcitation is not a likely source for the sulfur isotope anomalies observed in this

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study.

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Negative ∆33S values of sulfate in Beijing PM2.5 have been measured for aerosol

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samples collected in winter (Fig. 4B). This is different from winter aerosols in 16


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Bakersfield, California (0.00‰

Multiple sulfur isotope constraints on sources and formation processes

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