Optimizing isolation protocol of organic carbon and elemental carbon for 14C analysis using fine particulate samples

Atmospheric Environment 154 (2017) 9e19

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Optimizing isolation protocol of organic carbon and elemental carbon for 14C analysis using fine particulate samples Junwen Liu a, c, Jun Li a, *, Ping Ding b, Yanlin Zhang d, Di Liu a, Chengde Shen b, Gan Zhang a a

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Science, Guangzhou, 510640, China State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Science, Guangzhou, 510640, China Institue for Environmental and Climate Research, Jinan University, Guangzhou, 510632, China d Yale-NUIST Center on Atmospheric Environment, Nanjing University of Information Science and Technology, Nanjing, 210044, China b c

h i g h l i g h t s
A high-vacuum setup was established to collect carbon fractions.
Both OC and EC fractions were isolated from the Sunset carbon analyzer.
Simple protocols for isolating OC and EC for 14C analysis were proposed.
Seasonal 14C levels of OC and EC in a heavily polluted city were reported.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 November 2016 Received in revised form 7 January 2017 Accepted 14 January 2017 Available online 18 January 2017

Radiocarbon (14C) analysis is a unique tool that can be used to directly apportion organic carbon (OC) and elemental carbon (EC) into fossil and non-fossil fractions. In this study, a coupled carbon analyzer and high-vacuum setup was established to collect atmospheric OC and EC. We thoroughly investigated the correlations between 14C levels and mass recoveries of OC and EC using urban PM2.5 samples collected from a city in central China and found that: (1) the 14C signal of the OC fraction collected in the helium phase of the EUSSAR_2 protocol (200  C for 120 s, 300  C for 150 s, 450  C for 180 s, and 650  C for 180 s) was representative of the entire OC fraction, with a relative error of approximately 6%, and (2) after thermal treatments of 120 s at 200  C, 150 s at 300  C, and 180 s at 475  C in an oxidative atmosphere (10% oxygen, 90% helium) and 180 s at 650  C in helium, the remaining EC fraction sufficiently represented the 14C level of the entire EC, with a relative error of <10%. The average recovery of the OC and EC fractions for 14C analysis was 64± 7% (n ¼ 5) and 87 ± 5% (n ¼ 5), respectively. The fraction of modern carbon in the OC and EC of reference material (RM) 8785 was 0.564 ± 0.013 and 0.238 ± 0.006, respectively. Analysis of 14C levels in four selected PM2.5 samples in Xinxiang, China revealed that the relative contribution of fossil sources in OC and EC in the PM2.5 samples were 50.5± 5.8% and 81.4± 2.6%, respectively, which are comparable to findings in previous studies conducted in other Chinese cities. We confirmed that most urban EC derives from fossil fuel combustion processes, whereas both fossil and non-fossil sources have comparable and important impacts on OC. Our results suggested that watersoluble organic carbon (WSOC) and its pyrolytic carbon can be completely removed before EC collection via the method employed in this study. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Radiocarbon Organic carbon Elemental carbon Aerosols RM 8785

1. Introduction Fine carbonaceous aerosols, which are subdivided into organic

* Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.atmosenv.2017.01.027 1352-2310/© 2017 Elsevier Ltd. All rights reserved.

carbon (OC) and elemental carbon (EC), play many adverse roles in air quality, human health, and climate system (Menon et al., 2002; Ramanathan and Carmichael, 2008; Tie et al., 2009; Kulmala et al., 2011; Ding et al., 2016; Wilcox et al., 2016). EC aerosol is the byproduct of poor combustion of carbon-containing fuels used in industry, motor vehicle, power plant, forest fire, and crop burning.

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OC aerosol is produced by combustion processes and is also formed through aerosol aging and the oxidation of anthropogenic and biogenic volatile organic compounds (Claeys et al., 2004; Kleindienst et al., 2007; Robinson et al., 2007). Traditionally, these two carbon fractions' source apportionment are estimated by emission inventory (EI) models and source-specific tracers; however, these methods are associated with large uncertainty. The uncertainties of EC and OC calculations based on EI models were reported from 25 to þ136% and 40 to þ121% in a recent study, respectively (Zhao et al., 2011). Levoglucosan (Lev) is frequently used to estimate the fraction of biomass-burning OC through the Lev-to-OC ratio of biomass-burning plumes in chambers or field experiments. However, it is well known that reported Lev-to-OC
r ratios (approximately 0.08e0.3) are poorly constrained (Gelencse et al., 2007; Zhang et al., 2015) because they can be influenced by biomass types and combustion conditions (Fine et al., 2002; Iinuma et al., 2007). Furthermore, Lev is used to quantify biomass-burning EC (Cheng et al., 2013; Jung et al., 2014), which is questionable in situations where sampling sites are far from emission sources because Lev can reacts with hydroxyl radicals (Hennigan et al., 2010). Therefore, a direct technique for the accurate determination of the source apportionment of OC and EC is urgently needed to guide air pollution control, and to further constrain climate prediction modeling and protect public health. Radiocarbon (14C) analysis has the unique potential to unambiguously distinguish fossil fuel sources from biomass-burning and biogenic emissions (Lewis et al., 2004; Szidat et al., 2004a; Szidat, 2009). The fraction of modern carbon (fm), represents the ratio of 14 12 C/ C relative to the reference year (1950), is 0 in fossil fuels and is slightly higher than 1 in the present-day living biosphere and atmospheric CO2 due to the nuclear weapon tests conducted in the early 1960s (Levin and Kromer, 2004; Levin et al., 2010). Thermal treatment is the most common method used to separate OC from EC for 14C measurement. Using the Two-step Heating system for the EC/OC Determination Of Radiocarbon in the Environment (THEODORE), Szidat et al. (2004a) investigated the correlation between OC fm values and combustion temperatures, and confirmed that the 14 C level in carbon fraction obtained from a reaction at 340  C for 10 min in pure oxygen (O2) can be representative of the 14C level of the entire OC. This OC isolation protocol and similar ones have been intensively employed in many laboratories around the world (Szidat et al., 2004b, 2006, 2009; Zhang et al., 2010b, Zhang et al. 2012; Dusek et al., 2013; Liu et al., 2013, 2016a; Mouteva et al., 2015; Liu et al., 2016a). However, as suggested by Szidat et al. (2004a), this method results in incomplete OC separation, and the remaining carbon materials are therefore not suitable to represent the EC. Several methods have been developed with the aim of completely removing OC and obtaining a pure EC fraction for 14 C analysis. Recent OC elimination methods include: 1) the chemothermal oxidation method (CTO-375), in which aerosols are combusted at 375  C for  4 h in active air using a muffle furnace (Szidat et al., 2004a; Zencak et al., 2007; Liu et al., 2013); 2) the Swiss_4S protocol, in which material is extracted by ultrapure water, then combusted at 375  C for 150 s, at 475  C for 180 s in pure O2 and in a helium (He) atmosphere for two 180 s periods at 450  C and at 650  C (Zhang et al., 2012; Mouteva et al., 2015); 3) samples are extracted by ultrapure water and then combusted in O2 at 360  C for 15 min and at 450  C for 2 min (Dusek et al., 2014); 4) the thermal-optical protocol of the National Institute of Occupational Safety and Health (NIOSH 5 040), in which samples are combusted in He gas and then in a mixture of He and O2 with a steppedtemperature program (Gustafsson et al., 2009; Chen et al., 2013). However, challenges remain regarding the 14C measurements of OC and EC as neither OC nor EC is a discrete chemical species or an entity with unique chemical and/or physical properties, but rather a

chemical continuum. A part of the non-refractory EC (char-EC) can evaporate along with OC in the oxidative atmosphere. The CTO-375 method is thought to harvest only the refractory part of EC (sootEC) (Hammes et al., 2007; Zencak et al., 2007). Although thermaleoptical methodology is most widely applied in the determination of carbonaceous aerosols, this method defines OC and EC operationally rather than chemically. Therefore, the carbon fraction defined by the thermal-optical methodology is neither pure OC nor pure EC, and its isotopic signals probably are not acceptable surrogates for those of either carbon fraction. It has long been recognized that a fraction of OC in ambient aerosols will char to pyrolytic carbon (PyC) during heating process, which disturbs the following EC analysis and collection. Water-extraction step and the addition of pure O2 before the EC collection phase can reduce PyC formation (Yu et al., 2002; Zhang et al., 2012), yet some EC will also be lost. For example, the typical EC recovery using a water-extraction step and pure O2 (e.g., SWISS_4S protocol) is 78± 10%, implying that approximately 22% of EC is lost (Zhang et al., 2015). In practice, the SWISS_4S-derived EC fm value can be extrapolated to 100% EC recovery, which is calculated using the change in laser attenuation assuming a linear relationship and a slope of 0.31 ± 0.1 (Zhang et al., 2012). Noted that this slope is not convergent, and has a high relative standard deviation (RSD) of approximately 30%, reflecting the nonlinearly heterogeneous changes of light-absorbing ability of the entire EC during the thermal treatment and also implying that the relationship between EC fm and recovery is not linear. However, the uncertainty of the fm value estimation for the entire EC fraction due to SWISS_4S is only approximately 5% (Zotter et al., 2014), mainly because this protocol is able to harvest a large proportion of EC (typically > 60%). In this study, we thoroughly investigated correlations between fm values and the mass recoveries of OC and EC in urban PM2.5 samples with the aim of obtaining new knowledge to describe 14C signals in carbonaceous aerosols using simple protocols without the water-extraction step and pure O2. In addition, OC and EC 14C signals in reference material (RM) 8785 (Gaithersburg, MD, USA), produced from standard reference material (SRM) 1649a using a PM2.5 inlet (Klouda et al., 2005), were also measured to enable a direct comparison with other studies (Szidat et al., 2013). We used a carbon analyzer with a low dead volume to separate the OC and EC fractions of the ambient aerosols, which is consistent with the general trend of studies examining 14C signals in atmospheric science. To the best of our knowledge, this is the first study on this aspect of 14C analysis to be conducted in China, following those at University of Bern, Switzerland (Zhang et al., 2012), Stockholm University, Sweden (Gustafsson et al., 2009), and University of California, Irvine, USA (Mouteva et al., 2015). 2. Materials and methods 2.1. PM2.5 samples used in this study PM2.5 samples were collected on prebaked quartz fiber filters (QFF, 8  10 in., Pall Corp.) through a high-volume sampler with a PM2.5 cutting head (Shanghai XinTuo Analytical Instruments Co., Ltd.) at a sampling rate of 300 L/min. The sampler was placed on the roof of a building at the campus of Henan Normal University (35190 N, 113 540 E), Xinxiang, China (Fig. 1). A total of 124 samples were collected in autumn (October 13th, e November 13th, 2013), winter (December 25th, 2013eJanuary 25th, 2014), spring (March 20th, e April 20th, 2014), and summer (July 17th, e September 16th, 2014); their mass concentrations have been published previously (Shen et al., 2016). Located in central China, approximately 67 km northeast of Zhengzhou, the capital of Henan province, Xinxiang has a

J. Liu et al. / Atmospheric Environment 154 (2017) 9e19

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Fig. 1. Geographic position of Xinxiang in China and the 24 h back trajectories of air masses at 100 m for selected samples using HYSPLIT model,ARL,NOAA. Back trajectory was modeled every 2 h.

population of approximately 6 million and experiences severe particulate air pollution. The mass concentration of PM2.5 was 41.0e666 mg/m3 during 2013e2014, with an average of 238 ± 123 mg/m3 (Shen et al., 2016), which was 6.8 and 24 times that of ambient air quality standards in China (35 mg/m3) and of the World Health Organization (10 mg/m3). The parameters of all tested protocols for study the 14C signals of carbon fractions are shown in Table 1. To explore seasonal patterns in the 14C signals of carbonaceous aerosols in Xinxiang, we selected four samples collected in different seasons with PM2.5 mass concentration values close to the median values of the seasons they represented (Table 2). Sample #2 and #4 were selected to evaluate these protocols. Given that there have been very few studies of the seasonal patterns of 14C signals in aerosols in China, the 14C-derived source apportionment of OC and EC conducted in Xinxiang in this study will provide essential information to guide the mitigation of air pollution in this and other heavily polluted cities in China. 2.2. OC and EC determinations and the description of Sunsetvacuum system The Sunset OC-EC thermal-optical transmittance (TOT) analyzer (Model 4L, Sunset Laboratory Inc., Portland, OR, USA), with a nondispersive infrared (NDIR) detector, is a commercial instrument that has been extensively used in aerosol studies and is also employed in this study to isolate different carbon fractions. Two distinct heating phases are recommended during the analysis

(Birch and Cary, 1996). In brief, an aerosol sample deposited on the filter is heated in a completely O2-free He atmosphere (He phase), then combusted in an oxidizing atmosphere containing 90% He and 10% O2 (He/O2 phase). In the He phase, most OC will generally evaporate, while a fraction will evolve into PyC, which will be completely emitted with the EC fraction during the He/O2 phase. The evolved carbon is then oxidized to CO2 by a MnO2 catalyst and quantified by the NDIR detector. The OC fraction is defined at the laser transmittance reaches its initial value (i.e., OCeEC split line) and any carbon fraction after this line is considered to be EC. The protocol for determining the OC and EC fractions in this study is EUSSAR_2 (Cavalli et al., 2010), which agrees well (R2 > 0.87) with other tested TOT-based methods (Piazzalunga et al., 2011). EUSSAR_2 protocol is based on the project of European Supersites for Atmospheric Aerosol Research, and its temperature program is set by 200  C for 120 s, 300  C for 150 s, 450  C for 180 s, and 650  C for 180 s in He, and then 500  C for 120 s, 550  C for 120 s, 700 C for 70 s, and 850  C for 80 s in He/O2, respectively. Given that the sources of water-soluble organic carbon (WSOC) are generally very different from water-insoluble organic carbon (WIOC), we extracted carbon from a 50-mm diameter area of filter samples with 20 mL of ultrapure water using a syringe filter (ANPEL, Shanghai, China), to isolate these two subfractions of OC for 14C analysis. After drying in a desiccator, the OC fraction detected by the Sunset OC-EC analyzer in the water-washed filter is defined as WIOC. WSOC filtrates were stored in 40-mL vials in a freezer until analysis. The WSOC mass concentration was calculated by subtracting WIOC

Table 1 Parameters of thermal methods tested in this study to isolate/remove organic carbon for

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a

Method name

He-200

Parameter description: Carrier gas, temperature, duration

He, 200  C, 180 s

C analysis. He-EUSSAR_2b Step Step Step Step

1: 2: 3: 4:

He, He, He, He,

200 300 450 650

He/O2-300c 

C, C,  C,  C, 

120 150 180 180

s s s s

Step Step Step Step

1: 2: 3: 4:

He/O2, 200  C, 120 s He/O2, 300  C, 150 s He, 450  C, 180 s He, 650  C, 180 s

Note: He ¼ Helium; He/O2 ¼ 10% O2 þ 90% He. a Temperatures of 200  C (He-200), 300  C (He-300), and 450  C (He-450) are tested in this mode. Only He gas are used in these protocols. b This program, i.e., He-EUSSAR_2, is adopted from the study of Cavalli et al. (2010). This protocol covers 4 steps and is the same as the stepped temperature program of EUSSAR_2 during He phase. c Only carrier gas and the temperature in step 3 are changed in these protocols (4 steps in total) comparing with He-EUSSAAR_2. He/O2-300 means the carrier gas in the steps of 1 and 2 is He/O2. By this analogy, He/O2-475 means the temperature in step 3 is 475  C and the carrier gas is He/O2 in the step of 1, 2, and 3. (1) For the isolation of OC for 14 C analysis, protocols of He-200, He-300, He-450, He-EUSSAR_2, He/O2-300, He/O2-450, He/O2-475, and He/O2-500 are tested. (2) For the 14C analysis of EC, He-EUSSAR_2, He/O2-450, He/O2-475, He/O2-500, He/O2-525, and He/O2-550 are tested to heat the filter samples, respectively, then the remaining carbon fraction on the filter will be collected in He/O2 condition by using the stepped temperature program (500  C, 120 s; 550  C, 120 s; 700  C, 80 s; 850  C, 80 s) recommended by EUSSAR_2.

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Table 2 Information for the selected samples collected from Xinxiang city in this study. I.D.

Season

Sampling timea

PM2.5b

OCc

WIOCc

WSOCc

ECc

TCc

TC/PM2.5d

OC/EC

WSOC/OC

WIOC/OC

#1 #2 #3 #4

Autumn Winter Spring Summer

Oct. 26, 2013 Dec. 30, 2013 Apr. 01, 2014 Jul. 25, 2014

266 353 237 174

27.5 26.3 16.7 14.5

17.0 19.9 8.35 7.58

10.5 6.40 8.39 6.93

6.58 12.4 4.69 4.46

34.1 38.6 21.4 19.0

12.8 10.9 9.04 10.9

4.18 2.12 3.57 3.25

0.38 0.24 0.50 0.48

0.62 0.76 0.50 0.52

a b c d

Starting date for the 24-h sampling time. mg/m3. mg C/m3. %.

Fig. 2. A coupled systems of the Sunset OC-EC analyser with the high-vacuum unit.

Fig. 3. OC fm value versus OC mass recovery for sample #2. Total OC mass concentration is defined by the protocol of EUSSAR_2 with a PyC correction using TOT. Colored markers around filled circles represent protocols for isolating OC fraction from filter samples. Mass recovery that higher than 100% are due to the contamination from the emission of EC fraction. The error bars are not shown as they are smaller than the symbol sizes. The average uncertainty was 1.18 ± 0.54 %. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

from OC. This water-extraction procedure has been described in detail by Zhang et al. (2012). The field blank carbon amounts [OC: 0.87 ± 0.29 mg C/cm2 (n ¼ 3), WIOC: 0.64 ± 0.12 mg C/cm2 (n ¼ 3)] were subtracted from the filter samples. EC was undetectable in the field blank sample. The uncertainties of carbon mass determination (n ¼ 6) in this study were 5.7% for OC and 6.5% for EC. Inspired by University of Bern (Zhang et al., 2012) setup, we established a similar high-vacuum system to collect pure CO2 from the Sunset OC-EC analyzer (Fig. 2). All tubes in this high-vacuum system are made of stainless steel or Teflon to prevent CO2 diffusion and contamination from the ambient air. Prior to the collection of the carbon fraction (OC, WIOC and EC) isolated from the Sunset OC-EC analyzer, the high-vacuum system was vacuumed to approximately 107e106 mbar using a molecular pump (Pfeiffer, Asslar, Germany), and then slowly flushed with high-purity He

from a cylinder for 10 min. Gaseous H2O and other possible contaminants from filter samples were removed using a dry ice/ ethanol trap, and the CO2 was trapped by liquid nitrogen and transferred to a glass/quartz ampule in sequence. Glass vials containing the WSOC solution were freeze-dried, then transferred to precombusted quartz tubes, and converted to CO2 in an oven (Liu et al., 2014). 2.3. Graphitization and radiocarbon measurement After purification, the CO2 sample was then cryo-transferred and sealed in a reaction tube [length: 60 mm, outer diameter (OD): 6 mm, inner diameter (ID): 4 mm] with an inner liner (length: 20 mm, OD: 3 mm, ID: 2 mm). The external and liner tubes were loaded with 10e11 mg Zn (Aldrich, #324930) and 3e5 mg TiH2

J. Liu et al. / Atmospheric Environment 154 (2017) 9e19

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Table 3 A comparison of measured carbon amount and fm value of RM 8785 in this study with other labs.

This study Szidat et al. (2013) Zencak et al. (2007) Klouda et al. (2005) d Klouda et al. (2005) e

OC (mg C/cm2)

OC (g/g)

EC (mg C/cm2)

EC (g/g)

EC-to-TC

fm (OC)

fm (EC)

16.1 18 n.r. n.r. n.r.

0.21 n.r.a n.r. 0.112 0.169

6.72 (3, 6, 7, 39)b n.r. n.r. n.r.

0.09 n.r. 0.054 0.111 0.067

0.29 (0.12, 0.18, 0.19, 0.43)b 0.242 0.490 0.279

0.564 ± 0.013 0.621 n.r. n.r. n.r.

0.238 ± 0.006 (0.120, 0.168, 0.191, 0.305 0.567)c n.r. n.r. n.r.

Note: concentrations of WSOC and WIOC are not measured successfully due to the loss of sample when performing the water-extraction. a n.r. ¼ not reported. b Data were performed by 4 independent labs. c All measured data are shown including the data of 0.305 that was thought to be an outlier. d IMPORVE. e STN-NIOSH.

Fig. 4. Thermograms of the sucrose using the protocols of EUSSAR_2 (top) and He/O2-450 (down).

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the AMS-14C preparation laboratory of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences and at the Institute of Heavy Ion Physics, Peking University (NEC, 0.5 MeV), respectively. At least 30 mg carbon amount were collected for 14C analysis from samples in this study. Each isolation process was based on one 1.5 cm2 filter sample. For the samples with low carbon amount, two or more isolation processes were performed and the corresponding CO2 samples would be combined to one. The average uncertainty of the fm values for both OC and EC in this study was 1.9± 1.3%. All reported fm values for OC and WIOC, with the exception of those from RM 8785, were corrected using the field blank according to the isotopic mass balance equation, as follows:

fm; corr ¼ ðCsam  fsam  Cbk  fbk Þ = ðCsam  Cbk Þ where fm, corr, fsam, and fbk are blank corrected fm, AMS-based fm of the samples, and AMS-based fm of the field blank, respectively. Csam and Cbk are the carbon amounts in samples and the field blank, respectively. The field blank fm values were 0.531 ± 0.006 for OC and 0.450 ± 0.008 for WIOC. Because the amount of carbon in the blank sample compared to that in the samples was insignificant, the relative difference between fm, corr and fm was only approximately 1% on average. To compensate for excess 14C produced by nuclear bomb testing, fm values were converted to fractions of contemporary carbon (fc) by normalization, with a factor of 1.10 for EC and 1.06 for OC (Liu et al., 2014). Therefore, the percentage contribution of non-fossil and fossil sources to the carbon fractions was calculated as (fc  100) and [(1-fc)  100], respectively. 3. Results and discussion 3.1. OC fm values versus OC mass recoveries

Fig. 5. Thermograms of water-soluble organic carbon (WSOC) extracted from the sample #2 using the protocol of EUSSAR_2 (top), He/O2-450 (middle), and He/O2-475 (down). The WSOC solution was dropped on pre-baked filters.

(Alfa Aesar, #12857) and 4e5 mg Fe catalyst (Alfa Aesar, 350 mesh, #39813), respectively. The temperature ramp for graphitization was optimized to 4 h at 500  C and then 3 h at 550  C. Graphitization and determination of the isotopic ratio were performed at

It is generally recognized that carbon fractions released during the He phase are a part of the total pure OC and that a fraction of OC will char to PyC; thus, pure OC cannot be entirely isolated using only He as the carrier gas. Harvesting OC in oxidative air results in suppressed PyC formation and, consequently, a higher OC mass recovery; however, there is a risk of contamination by less refractory EC. We thoroughly investigated the effects of OC mass collection under both conditions, in He and in He/O2, on OC fm values in sample #2 (Fig. 3). It is clear that the OC 14C level is not thermally homogeneous. The OC fm of sample #2 was 0.389 ± 0.006, while its mass recovery was only 11%. Under these O2-free conditions, the OC fm increased to 0.563 ± 0.005 at 300  C and to 0.652 ± 0.012 at 450  C, but dropped to 0.495 ± 0.008 with He-EUSSAR_2 (i.e., the He phase of EUSSAR_2), with corresponding mass recoveries of 23, 41 and 65%, respectively. This result indicates that OC fm is mass recovery-dependent, and that the resulting correlation is nonlinear. To understand the fm value of entire OC, He/O2 was introduced within a temperature interval of 300e500  C. We found that the OC fm with He/O2-300 was 0.510 ± 0.008, which is very similar to that from He-EUSSAR_2. However, OC fm decreased slightly and systematically to 0.472 ± 0.003 at He/O2-450 and then remained very stable with He/ O2-475 (0.478 ± 0.003) and He/O2-500 (0.464 ± 0.002), although their mass recoveries were found to be higher than 100% due to the undesired emission of char-EC. The mass recovery of OC reached 122% with He/O2-500, suggesting that although the entire OC fraction was evaporated using this heating protocol, approximately 20% of the OC harvested for 14C measurement was actually derived from EC. Unfortunately, there is not yet a thermal method that can completely separate the entire OC fraction from EC in ambient aerosols. However, we observed that the absolute difference of OC fm between the He-EUSSAR_2 protocol, in which isolated carbon is

J. Liu et al. / Atmospheric Environment 154 (2017) 9e19

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Fig. 6. EC fm value versus EC mass recovery for sample #2 (A) and sample #4 (B). Total EC mass concentration is defined by the protocol of EUSSAR_2 with a PyC correction using TOT. Colored markers around filled circles represent protocols (Table 1) for removing OC fraction from filter samples. Mass recovery that higher than 100% are due to the contamination from PyC. The error bars are not shown as they are smaller than the symbol sizes. The average uncertainty was 2.11 ± 1.57 %. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

guaranteed to be pure OC, and the He/O2-500 protocol, in which isolated carbon surely contains some EC, was only 0.03, with a relative difference of 6%. This small difference implies that undesired EC emissions derived with He/O2-450, He/O2-475, and He/O2500 had little effect on the measurement of the OC fm and negligible impacts on the source apportionment of the entire OC fraction. This finding is largely consistent with a previous study performed by Szidat et al. (2004a), who found that the SRM 1649a OC fm value remained constant with temperatures increasing from 340 to 440  C in a pure O2 atmosphere. In addition, the RM 8785 OC fm, based on He-EUSSAR_2 protocol, was found to be 0.564 ± 0.013 in this study (Table 3), which is 0.057 lower than the only published value (0.621 ± 0.012; pure O2 at 340  C) (Szidat et al., 2013). Therefore, the absolute difference of the relative contribution (%) of fossil/non-fossil sources in RM 8785 calculated by these two methods is only approximately 6%, and this small difference can probably be partly attributed to inhomogeneous filter loadings for RM 8785. The fraction of carbon isolated through He-EUSSAR_2 therefore is acceptable for 14C measurement of the entire OC, based on our results from both field and reference samples.

3.2. EC fm values versus EC mass recoveries Because the EC fraction in ambient aerosols is much lower than the OC fraction, the collection of EC should be performed carefully to avoid contamination arising from OC. Ideally, the method employed should completely remove the OC fraction and guarantee no EC emission. This can't be achieved only via thermal treatments, due to the fact that the charring phenomenon (i.e., PyC formation) is unavoidable during the process of OC elimination, and the PyC, which mainly originates from WSOC matter, has a similar thermal stability to a part of the EC fraction (Yu et al., 2002). Based on this consideration, we focus in this section on seeking a protocol that can both extricate as much WSOC as possible without waterextraction and also allow a high EC mass recovery, such that isolated EC will be perfectly representative of the 14C signal of the entire EC fraction in ambient aerosols. The He-EUSSAR_2 protocol was incapable of removing the entire OC fraction (Figs. 4 and 5). We observed that 93% of the sucrose carbon in solution and 77% of the WSOC extracted from sample #2 appeared in the He-EUSSAR_2 phase. Similarly, Yu et al. (2002) found that 71e88% of the ambient WSOC fraction in PM2.5 would evolve in a He atmosphere during heating, with the rest appearing during the He/O2 step. We found that all sucrose carbon

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and ambient WSOC, including the corresponding PyC fraction, evolved with He/O2-450, indicating that the He/O2-450 protocol and those with higher heating temperatures (e.g. He/O2-475, He/ O2-500, and He/O2-525) are able to remove WSOC completely, and therefore the EC harvested following these thermal treatment protocols will be free from contamination by PyC formed by WSOC. This was reflected in the EC fm plot and the mass recovery (Fig. 6A). The EC fm of sample #2 from the He-EUSSAR_2 protocol had the highest value (0.315 ± 0.002) among the protocols we evaluated, with a mass recovery reaching 170%, suggesting that 70% of the carbon collected for EC 14C determination stemmed from OCderived PyC, probably mainly derived from WSOC, with a higher fm value than that of EC. Clearly, the isotopic signals of the remaining carbon fraction on the filter treated by the He-EUSSAR_2 method are not representative of the entire EC fraction. Overall, EC fm values in sample #2 displayed a positive, nonlinear correlation with mass recovery (Fig. 6A), but a plateau was reached in the correlation of the He/O2-450 and He/O2-475 protocols. The absolute difference of EC fm with He/O2-450, in which isolated carbon accounted for 117% of EC, and with He/O2-475, in which isolated carbon accounted for 85% of EC, was insignificant (0.002), with a relative error of 0.80%, suggesting that both the PyC formed with He/O2-450 and the EC losses with He/O2-475 are very likely to have the same 14C level as native EC, and thus exert very little influence on the entire EC fm value. Nearly all PyC fraction was removed with He/O2-475, due to all of the WSOC, including its PyC, being evaporated (Figs. 4 and 5) and the OC-EC split line appearing before EC harvesting (Fig. 7). Therefore, He/O2-475 was selected to remove OC for the subsequent EC collection for its 14C analysis. We found the EC fm of RM 8785 to be 0.238 ± 0.006 using the He/ O2-475 protocol (Table 3). Although this value is within the reported range (0.120e0.567), it is clear that the 14C measurement of EC in RM 8785 remains poorly constrained due to the heterogeneity of this reference material and the unconformity of the available methods. Given that 14C measurements of OC, but not EC, generally agree well between laboratories (Szidat et al., 2013), 14C EC signals from the summer season (sample #4) were explored in this section to reevaluate the He/O2-475 protocol (Fig. 6B). Although the absolute difference (0.016) and the relative error (9%) of fm values between He/O2-475 and He/O2-450 were higher than those in winter (sample #2), the absolute difference of the results of source apportionment based on these two methods was only 1.6%, which is acceptable in this research field. In addition, we found that the fm value of sample #4 was much less sensitive to mass recovery than sample #2. A similar phenomenon was observed in European samples by Zotter et al. (2014) who observed that the slope of the correlation between EC fm and EC yield for the winter sample was approximately five times higher than for the summer sample. This may be because summer EC has a higher ratio of soot-to-char than winter EC, with the refractory ability of soot-EC being much stronger than that of char-EC (Han et al., 2007). Altogether, our results suggest that He/O2-475 can be employed to remove the OC fraction for 14C analysis of the entire EC.

Fig. 7. Thermograms of the sample #2 using the protocols of EUSSAR_2 (top), He/O2450 (middle) and He/O2-475 (down).

Table 4 Relative contribution (%) of fossil and non-fossil sources basing on OC

41.0 53.3 51.2 56.6 50.5

C analysis.

WIOC

Fossil Autumn Winter Spring Summer Average

14

± ± ± ± ±

2.8 2.3 2.3 2.1 5.8

Non-fossil

Fossil

± ± ± ± ±

44.5 72.4 55.0 58.6 57.6

59.0 46.7 48.8 43.4 49.5

2.8 2.3 2.3 2.1 5.8

± ± ± ± ±

WSOC

2.7 1.8 2.2 2.0 10.0

Non-fossil

Fossil

± ± ± ± ±

34.2 32.6 34.4 53.2 38.6

55.5 27.6 45.0 41.4 42.4

2.7 1.8 2.2 2.0 10.0

± ± ± ± ±

EC

3.1 3.2 4.9 2.2 8.5

Non-fossil

Fossil

± ± ± ± ±

81.1 80.0 78.7 85.6 81.4

65.8 67.4 65.6 46.8 61.4

3.1 3.2 4.9 2.2 8.5

± ± ± ± ±

Non-fossil 0.8 0.9 1.0 0.7 2.6

18.9 20.0 21.3 14.4 18.7

± ± ± ± ±

0.8 0.9 1.0 0.7 2.6

J. Liu et al. / Atmospheric Environment 154 (2017) 9e19

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Fig. 8. Fire spot map in central China using the dataset of MODIS NRT C5 (http://firms.modaps.eosdis.nasa.gov/firemap/). The circle represent the position of Xinxiang city.

3.3. A case study in Xinxiang, central China 3.3.1. Concentrations and ratios Table 3 shows the concentrations of carbon species and ratios in the selected samples of this study. The total carbon concentrations (TC, sum of OC and EC) were 34.1, 38.6, 21.4, and 19.0 mg C/cm3, for autumn, winter, spring, and summer, respectively, with corresponding percentages of 12.8, 10.9, 9.04, and 10.9% in PM2.5, respectively. The OC concentration showed similar levels in autumn (27.5 mg C/cm3) and in winter (26.3 mg C/cm3), followed by spring (16.7 mg C/cm3) and summer (14.5 mg C/cm3). However, the EC concentrations in winter (12.4 mg C/cm3) were 1.8, 2.6 and 2.8 times larger than in autumn (6.58 mg C/cm3), spring (4.69 mg C/cm3), and summer (4.46 mg C/cm3), respectively. Correspondingly, the OC/EC ratio (2.12) in winter was found to be only 50e65% of that in other seasons (3.25e4.18), suggesting that the oxidation level of volatile organic compounds and the formation of secondary organic carbon (SOC) were relatively poor in winter due to lower temperatures and radicals. This was further confirmed by the lower WSOC/OC ratio (0.24) in winter compared to the other seasons (0.38e0.50), because WSOC species are good SOC surrogates in urban air (Weber et al., 2007; Zhang et al., 2010a). 3.3.2. Relative contributions of fossil and non-fossil sources The 14C-derived source apportionments of OC, WIOC, WSOC, and EC are listed in Table 4. For these four samples, the proportion of isolated mass of OC, WIOC, and EC were 62, 72, and 87%, respectively. The average contribution of non-fossil sources (NFS)

to OC was 49.5± 5.8%, with the remainder (50.5± 5.8%) from fossil sources (FS), implying that either NSF or FS are important contributors to urban OC. This result is consistent with those of previous studies conducted in other Chinese cities. It has been reported that the NFS contribution to OC of Guangzhou (winter), Guangzhou (spring), Wuhan (winter), Beijing (spring), Beijing (winter), Xi'an (winter), Shanghai (winter), and Lanzhou (winter) were 63± 4% (Liu et al., 2014), 54± 6% (Liu et al., 2016c), 62± 5% (Liu et al., 2016b), 59± 4% (Liu et al., 2016c), 42% ± 5% (Zhang et al., 2015), 63± 3% (Zhang et al., 2015), 51± 2% (Zhang et al., 2015), and 55± 3% (Xu et al., 2016). In our study, a relatively higher NFS contribution observed in autumn (59.0± 2.8%) than in spring (48.8± 2.3%), winter (46.7± 2.3%), and summer (43.4± 2.1%), because the air masses of the autumn sample were derived from southern Xinxiang (Fig. 1), where open burning of crop/straw was more extensive and severe than in other areas (Fig. 8). Despite active open fires in southern Xinxiang during the summer season, the corresponding contribution of NFS to OC was relatively low (i.e., a higher FS contribution). This is probably because the summer sample was strongly impacted by air masses from the Bohai Sea coast and Beijing-Tianjin-Hebei (Fig. 1), which is a key urban agglomeration region in China. This also results in all carbon species except WIOC species in the summer sample having the largest FS contribution. Due to intensive coal-derived central heating between November 15 and March 15 in Xinxiang, the contribution of winter WIOC arising from FS was high (72.4± 1.8%), while the corresponding values for autumn, spring, and summer were 44.5± 2.7%, 55.0± 2.2%, and 58.6± 2.0%, respectively. No obvious

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J. Liu et al. / Atmospheric Environment 154 (2017) 9e19

difference was observed in the contribution of FS (32.6e34.4%) to WSOC between seasons except for the summer sample (53.2± 2.2%). Hence, coal burning boosts the FS contribution to WIOC; conversely, open burning mitigates this contribution and promotes the NFS contribution. Both coal and biomass burning exerted little influence on 14C WSOC signals, showing that most of the OC directly emitted from these two key emission sources in central China is likely water-insoluble, while most WSOC probably are not directly emitted by combustion processes. Not surprisingly, the EC 14C level was much lower than that of other carbon species. FS and NFC accounted on average for 81.4± 2.6% and 18.6± 2.6% of EC, respectively. The proportion of FS in EC was very stable (78.7e81.1%) between autumn, winter, and spring, with a higher value in summer (85.6± 0.7%), indicating that most EC had probably originated from sources (e.g., traffic emissions) in areas surrounding the sampling site, and EC transferred from other areas (e.g., biomass burning from southern Xinxiang) may have had little influence on the EC sampled in this study. Although only four samples were selected to measure 14C, their FS contribution to EC corresponded with those reported in previous studies conducted in Chinese cities such as Beijing (73e85%) (Chen et al., 2013; Andersson et al., 2015), Guangzhou (52e91%) (Liu et al., 2014, 2016c; Zhang et al., 2015; Liu et al., 2016c), Shanghai (79± 4%) (Zhang et al., 2015), but clearly higher than in remote sites such as Hainan Island, China (25e56%) (Zhang et al., 2014), Hanimaadhoo, Maldives (47± 9%) (Budhavant et al., 2015), Sinhagad, India (49± 8%) (Budhavant et al., 2015), and Svalbard (48± 15%) (Winiger et al., 2015), suggesting that urban fossil fuel use has significantly enhanced the contribution of FS to EC. These key findings should be taken into accounted by governments when creating informed policies to mitigate air pollution, and taking action toward reducing carbon emissions following the 2015 United Nations Climate Change Conference (COP 21).

4. Conclusion 14

C analysis is a unique and powerful tool for the source apportionment of OC and EC, although great challenges remain because no available method can completely separate these two carbon fractions for the harvesting of OC and/or EC with 100% recovery. In this study, we thoroughly and systematically investigated the thermal dynamics of 14C signals in the OC and EC of ambient PM2.5 using a high-vacuum system coupled with a Sunset OCeEC carbon analyzer. After a series of 14C measurements, we confirmed that: (1) the 14C level both in OC and EC is not linearly homogeneous; (2) the 14C level of the OC fraction separated by the HeEUSSAR_2 protocol is representative of that of the entire OC; and (3) the WSOC fraction including newly formed PyC can be emitted with the He/O2-475 method, with an insignificant difference in 14C levels between the EC remainder and the entire EC. The fm values of OC (0.564 ± 0.013) and EC (0.238 ± 0.006) detected in RM 8785 using the methods evaluated in this study were comparable to published values. The case study conducted in Xinxiang, China showed that the contribution of fossil fuel sources to EC, OC, WSOC, and WIOC were 81.4± 2.6%, 50.5± 5.8%, 38.6± 8.5%, and 57.6± 10%, respectively, with the remainder derived from non-fossil sources. Compared to other source apportionment methodologies, few 14Crelated studies have been performed and our study presents a relatively simple and effective separation/collection method for 14C determination of both OC and EC, which will promote the refinement of 14C analysis techniques and thus improve our knowledge and understanding of the mitigation of air pollution and the prediction of weather and climate change.

Acknowledgements This work was supported by “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (Grant Nos. XDB05040503), Natural Science Foundation of China (Grant Nos. 41473101 and 41603096), Guangzhou Science and Technology Plan Projects (Grant Nos. 201504010002), and China Postdoctoral Science Foundation (Grant Nos. 2015M572377 and 2016T90804). This is contribution No. IS-2338 from GIGCAS. The authors gratefully acknowledge the National Oceanic and Atmospheric Air Resources Laboratory for the provision of the HYSPLIT transport and dispersion model. References € €ld, M., Orjan, Andersson, A., Deng, J., Du, K., Zheng, M., Yan, C., Sko G., 2015. Regionally-varying combustion sources of the January 2013 severe haze events over eastern China. Environ. Sci. Technol. 49 (4), 2038e2043. Birch, M.E., Cary, R.A., 1996. Elemental carbon-based method for monitoring occupational exposures to particulate diesel exhaust. Aerosol Sci. Technol. 25, 221e241. Budhavant, K., Andersson, A., Bosch, C., Kruså, M., Kirillova, E.N., Sheesley, R.J., € 2015. Radiocarbon-based source apporSafai, P.D., Rao, P.S.P., Gustafsson, O., tionment of elemental carbon aerosols at two South Asian receptor observatories over a full annual cycle. Environ. Res. Lett. 10, 064004. Cavalli, F., Viana, M., Yttri, K., Genberg, J., Putaud, J.-P., 2010. Toward a standardised thermal-optical protocol for measuring atmospheric organic and elemental carbon: the EUSAAR protocol. Atmos. Meas. Tech. 3 (1), 79e89. Chen, B., Andersson, A., Lee, M., Kirillova, E.N., Xiao, Q., Kruså, M., Shi, M., Hu, K., € 2013. Source forensics of black carbon Lu, Z., Streets, D.G., Du, K., Gustafsson, O., aerosols from China. Environ. Sci. Technol. 47 (16), 9102e9108. Cheng, Y., Engling, G., He, K.-B., Duan, F.-K., Ma, Y.-L., Du, Z.-Y., Liu, J.-M., Zheng, M., Weber, R.J., 2013. Biomass burning contribution to Beijing aerosol. Atmos. Chem. Phys. 13 (15), 7765e7781. Claeys, M., Graham, B., Vas, G., Wang, W., Vermeylen, R., Pashynska, V., Cafmeyer, J., Guyon, P., Andreae, M.O., Artaxo, P., Maenhaut, W., 2004. Formation of secondary organic aerosols through photooxidation of isoprene. Science 303 (5661), 1173e1176. €ja €, T., Su, H., Cheng, Y.F., Ding, A.J., Huang, X., Nie, W., Sun, J.N., Kerminen, V.-M., Peta Yang, X.-Q., Wang, M.H., Chi, X.G., Wang, J.P., Virkkula, A., Guo, W.D., Yuan, J., Wang, S.Y., Zhang, R.J., Wu, Y.F., Song, Y., Zhu, T., Zilitinkevich, S., Kulmala, M., Fu, C.B., 2016. Enhanced haze pollution by black carbon in megacities in China. Geophys. Res. Lett. 43, 2873e2879. €ckmanna, T., Dusek, U., Brink, H.M., Meijer, H.A.J., Kos, G., Mrozek, D., Ro Holzinger, R., Weijers, E.P., 2013. The contribution of fossil sources to the organic aerosols in The Netherlands. Atmos. Environ. 74, 169e176. Dusek, U., Monaco, M., Prokopiou, M., Gongriep, F., Hitzenberger, R., Meijer, H.A.J., €ckmann, T., 2014. Evaluation of a two-step thermal method for separating Ro organic and elemental carbon for radiocarbon analysis. Atmos. Meas. Tech. 7, 1943e1955. Fine, P.M., Cass, G.R., Simoneit, B.R., 2002. Chemical characterization of fine particle emissions from the fireplace combustion of woods grown in the southern United States. Environ. Sci. Technol. 36 (7), 1442e1451.
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