Observation of Fullerene Soot in Eastern ChinaJunfeng Wang,† Timothy B. Onasch,‡ Xinlei Ge,*,† Sonya Collier,§ Qi Zhang,§,† Yele Sun,∥ Huan Yu,†

Mindong Chen,*,† Andre S. H. Prevo t,⊥,†,# and Douglas R. Worsnop‡

†Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Collaborative InnovationCenter of Atmospheric Environment and Equipment Technology (AEET), School of Environmental Science and Engineering,Nanjing University of Information Science & Technology, Nanjing 210044, China‡Aerodyne Research Inc., Billerica, Massachusetts 01821, United States§Department of Environmental Toxicology, University of California at Davis, Davis, California 95616, United States∥State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100029, China⊥Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen 5232, Switzerland#Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075,China

*S Supporting Information

ABSTRACT: This work reports the observation of a series offullerene ions, indicating the occurrence of fullerene soot (FS)in ambient air for the first time using an Aerodyne sootparticle-aerosol mass spectrometer (SP-AMS) deployed ineastern China. We found the distribution of these ions showeda pattern almost identical with that of an Alfa Aesar FSstandard. Although the SP-AMS may provide only asemiquantitative measurement of the FS, the measuredconcentrations can still reflect the temporal variations ofairborne fullerenes. Combining results from factor analysesand meteorological data, we identified the petrochemicalplants situated northeast of the site as the major sourceresponsible for the FS-like ions. Our findings indicate thegeneral presence of FS in ambient air, especially in oil and gas production regions. The SP-AMS technique may offer new insightsinto characterizing fullerene-related species in other environmental samples, as well.

■ INTRODUCTION

Refractory black carbon (rBC), or soot, is a distinct componentof atmospheric particles1 that strongly absorbs solar andterrestrial radiation; it can accelerate glacial melting, interactwith other atmospheric pollutants to serve as cloudcondensation nuclei (CCN),2 and affect cloud albedo andprecipitation. It plays a critical role in earth’s climate3 withsome studies considering it as the second largest positiveradiative forcing contributor after CO2.

4 rBC is also detrimentalto human health and the ecosystem.3 Fullerenes, discoveredfirst by Kroto et al.,5 refer to a series of high-molecular weightcarbon clusters (C60, C70, C80, etc.) with unique cagelike hollowstructures and have been studied extensively, including theirenvironmental impacts.6 As fullerenes today are often used inthe pharmaceutical industry and cosmetic products, there arealso concerns regarding their toxicities after prolonged contactwith the human body.7

Fullerene soot (FS), as a rBC subcomponent, can beproduced by various combustion processes, including domesticpropane and natural gas combustion,8 fuel-gas burning,9

ethylene flames,10 candle emissions,11 and coal12 and biomassburning.13 It has also been found in spark-ignited,14 dieselengine,15 and jet-fuel emissions.16 One study reports thatpolycyclic aromatic hydrocarbons can transform into sootcontaining fullerenes during combustion.17 Some airbornefullerenes may undergo further oxidation to form a variety ofoxygen-containing species (like C60O and C60O2).

18 Currentanalytical methods for particle-phase fullerenes includescanning electron microscopy (SEM), X-ray photoelectronspectroscopy (XPS), and transmission electron microscopy(TEM).9,11,19 Quantitative analyses of a few specific fullerenesand their derivatives (like C60, C70, N-methyl fuller pyrrolidine,etc.) were achieved via liquid chromatography/mass spectrom-etry (LC/MS) in Mediterranean Sea20 and Arizona State15

aerosols, as well as other samples like wastewater effluents,21

Received: February 3, 2016Revised: February 27, 2016Accepted: March 2, 2016

Letter

pubs.acs.org/journal/estlcu

© XXXX American Chemical Society A DOI: 10.1021/acs.estlett.6b00044Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX

municipal water,22 river sediment, and surface soils.23 Laserdesorption ionization aerosol mass spectrometry was also usedto detect fullerenes from flame-generated soot17 and othercarbon clusters (C14−C19) in urban aerosols.24

Despite recent studies, the atmospheric behaviors andenvironmental consequences of FS remain poorly understood,mainly because of the lack of robust techniques that are able toquickly characterize the FS in air. The aforementioned studiesare mostly for samples collected directly from emission sourcesrather than under ambient conditions, or for ones collected ona coarse time resolution. On the other hand, current methodscapable of measuring rBC particles, for example, a single-particle soot photometer (SP2, DMT Inc., Boulder, CO),cannot differentiate FS from other rBC species, although it usesFS as the calibration material.25 In this work, we deployed anAerodyne soot particle-aerosol mass spectrometer (SP-AMS)26,27 and achieved a real-time and fast characterizationof FS in ambient air for the first time. The ion distributionpattern, temporal variations, sources, and relevant atmosphericimplications are also discussed.

■ EXPERIMENTAL METHODSSP-AMS Measurement. The details of the SP-AMS and its

initial application were described by Onasch et al.26 In brief, theSP-AMS incorporates the SP2 into the Aerodyne high-resolution AMS,28 with an intracavity Nd:YAG laser vaporizer(1064 nm) allowing it to vaporize refractory aerosolcomponents, particularly rBC, in addition to the nonrefractory(NR) species (ammonium, sulfate, nitrate, chloride, andorganics) that evaporate via contact with a thermal tungstenvaporizer normally at 600 °C. The instrument uses 70 eVelectron impact (EI) ionization and a high-resolution (HR)(m/Δm = 5000−6000) time-of-flight mass spectrometer torecord signals of all vaporized species according to their mass-to-charge (m/z) ratios. Various rBC species can be detected inthe form of Cn

+ ions.

From February 20 to March 23, 2015, we deployed the SP-AMS at a suburban site of Nanjing, a megacity of the YangtzeRiver Delta (YRD) in eastern China, for online characterizationof rBC and other NR components in ambient fine particles.Details of the sampling site and operation of the SP-AMS areprovided in the Supporting Information. In particular, weoperated the instrument at an m/z range of up to ∼2000 andfor the first time observed a series of high-molecular weightcarbon ions with m/z ratios equivalent to those of variousfullerenes (C32

+−C160+, termed “FS-like ions” below) in eastern

China air.Data Analyses and Source Identification. The spectral

data were postprocessed using standard Analysis ToolkitSQUIRREL version 1.56D and PIKA version 1.15D.29 Notethe mass concentrations of rBC (C1

+−C31+) in this study were

calculated on the basis of the HR fitting, while concentrationsof FS-like ions were calculated using the unit mass resolution(UMR) method. This is due to the fact that HR fittingparameters that are suitable for small m/z ions (m/z <190)cannot reproduce the signal distribution pattern of the FS-likeions, while the UMR method can integrate all signals for theseions (see the details in the Supporting Information).Positive matrix factorization (PMF)30 was applied for source

apportionment of the HR mass spectra of organics acquiredwith both laser and tungsten vaporizers on, using PET version2.06.31 We identified six organic aerosol (OA) factors, includingtraffic (HOA), industry (IOA), cooking (COA), a semivolatileoxygenated OA (SVOOA), a low-volatility oxygenated OA(LVOOA), and a local secondary OA (LSOA) (see theSupporting Information). Correlations between the time-dependent mass concentrations of FS-like ions and these OAfactors were calculated and integrated with meteorological datato elucidate the origins of the observed FS-like ions.

Figure 1. (a) Campaign-averaged pattern of ambient fullerene soot-like ions (FS-like ions) and (b) laboratory-determined spectral pattern of thefullerene soot standard (Alfa Aesar, stock no. 40971, lot no. E23Q17, AAFS standard). The inset of panel a shows the ion distributions of the C60cluster. The inset of panel b shows the correlation of the ion patterns between the ambient data and AAFS standard for the ≥C50

+ ions. The pieshows the fractional contribution of ≥C50

+ and <C50+ ions.

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B

■ RESULTS AND DISCUSSION

Distribution of the FS-like Ions. Throughout thesampling period, we consistently observed a series of fullerenecarbon ions standing out above the background in the large m/z range (e.g., >600; see a real-time display in Figure S6) onlywhen the laser was on. The campaign-averaged pattern of theseions is presented in Figure 1a, which was fairly constant acrossdifferent mass loadings (Figures S6 and S7). Each carboncluster typically includes four m/z values, with one parent peakand three isotope ions. For example, the C60 cluster containsC60

+, 13CC59+, 13C2C58

+, and 13C3C57+ (inset of Figure 1a), and

the signal intensity ratios among the four ions match theisotopic ratios of the buckminsterfullerene (C60) spectrum inthe NIST database.32 Dominant carbon ions are distributed at24 amu (i.e., two C atoms), with much weaker peaks spaced at12 amu (i.e., one C atom), in accordance with the hollow three-dimensional or closed structure and reactivity of fullerenes.33,34

Previously, Onasch et al.35 investigated the carbon iondistributions for 12 different types of rBC particles using theSP-AMS and found that only the rBC materials with a highcontent of fullerenes can yield such FS-like ions. Thus, ourobserved FS-like ions unequivocally indicate the occurrence offullerenes in ambient air.For comparison, the mass spectrum of the Alfa Aesar

fullerene soot (AAFS) standard (stock no. 40971, lot no.E23Q17) was also measured under the same conditions and isshown in Figure 1b. Besides some smaller FS-like ions in Figure1a, whose signals might be significantly affected by otherorganics (Figure S8), the ion patterns of ambient data and theAAFS standard are strikingly similar [r2 = 0.96 for the ≥C50portion (inset of Figure 1b)]. Note the fullerenes in the AAFSstandard actually contain mainly C60 and C70, with only a smallportion of larger fullerenes (2.6%), yet the ions with m/z valueslarger than those of C70 account for ∼77% of the total spectralsignal (Figure 1b). This mismatch implies that the observed FS-like ions have multiple formation pathways, and they cannot beused for molecular identification and quantification of specific

fullerenes. This limitation is in fact an inherent feature of allAMS,36 as 70 ev EI ionization typically breaks the parentmolecule into fragments and complicates the speciationanalyses. However, the large fullerene ions were unlikely fromfragmentation of C60 and C70. Instead, Onasch et al.35 foundthat in addition to the EI ionization mechanism, the SP-AMSlaser itself can produce the FS-like ions, especially the large m/zions.

Quantification and Sources of the FS-like Ions. As aportion of the detected FS-like ions was generated due to laservaporization rather than EI ionization, it is difficult to quantifythe exact amount of airborne fullerenes via the SP-AMS.However, because these FS-like ions were measured underconsistent operating conditions and the pattern of iondistributions was fairly stable throughout the campaign, onemay expect the contributions of EI and laser-induced processesto these ions to be relatively stable, as well. Thus, the temporalvariations of FS-like ions as an ensemble, although not a directmeasure of real airborne fullerenes, should still reflect thedynamic changes of the preexisting fullerene content in theambient particles.Because the AAFS standard presents the same pattern as the

observed FS-like ions, it can be used as the calibration materialfor quantifying the FS-like ions. We performed suchcalibrations in the middle and at the end of the campaignand determined a relative ionization efficiency (RIE, relative tonitrate) of 5.0 (details in the Supporting Information). Thedetection limit of FS-like ions was determined to be 3 times thestandard deviation (3σ) of its measured values in particle-freeambient air sampled through a high-efficiency particulate air(HEPA) filter, which is 0.63 ng m−3, lower than the observedmass loadings in all cases.As mentioned earlier, because some small FS-like ions were

greatly affected by other organics, we calculated the massloading of the FS-like ions (mFS) according to the equation mFS

= mFS(≥C50,ambient)/f FS(≥C50,standard), where mFS(≥C50,ambient) is themass concentration of ambient FS-like ions taking into account

Figure 2. (a) Time series of FS-like ions and rBC (C1+−C31

+). (b) Diurnal variations of the mass concentrations of FS-like ions and rBC (C1+−

C31+). Box plots are for the FS-like ions; the whiskers above and below the boxes are the 90th and 10th percentiles, respectively, and the upper and

lower boundaries of the boxes indicate the 75th and 25th percentiles, respectively. The lines in the boxes indicate the median values and the filledsymbols the mean values. (c) Campaign-averaged mass-based size distributions of FS-like ions and rBC (C1

+−C31+).

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only fragments larger than C50+ (≥C50

+) and f FS(≥C50,standard) isthe fractional contribution of ≥C50

+ ions [97.3% (pie chart inFigure 1b)] to the total signal in the standard spectrum.Temporal variations of concentrations of the FS-like ions acrossthe campaign are shown in Figure 2a, ranging from 0.8 to 83.2ng m−3, with an average concentration of 14.0 ng m−3.Figure 2a also shows the time series of mass concentrations

of rBC (C1+−C31

+), which correlate moderately with FS-likeions [r2 = 0.56 (Figure S9)]. The average diurnal pattern of FS-like ions (Figure 2b) displays a bimodal distribution with oneprominent peak at 9:00 local time (LT) and another peak at21:00 LT but overall remains elevated during daytimecompared to late evening and early morning. This behavior isdifferent from that of rBC (C1

+−C31+), which has more obvious

morning and evening rush hour peaks. The mass-based sizedistribution of FS-like ions (Figure 2c) appears to be broadcentering on the 250−600 nm range, but still narrower thanthat of rBC (C1

+−C31+). These differences imply some diversity

of sources for FS compared to rBC (C1+−C31

+), whichassociates more significantly with traffic activities than the FSdoes: the r2 of rBC (C1

+−C31+) with HOA is 0.36, while the r2

of FS-like ions with HOA is only 0.12 (Table S1).Figure 3a depicts the distribution of industrial plants in

Nanjing. A large number of petrochemical plants are situatednortheast of the site, and during the campaign, wind mostlyblew from this direction (Figure S10). Figure 3b furtherdemonstrates the relationship between FS-like ions concen-trations and wind directions, indicating very clearly that it isstrongly linked with petrochemical industrial emissions. Indeed,the FS-like ions correlate poorly with all secondary species,including nitrate (r2 = 0.08), sulfate (r2 = 0.15), chloride (r2 =0.07), SVOOA (r2 = 0.07), LVOOA (r2 = 0.06), and LSOA (r2

= 0.07), and even primary species such as COA (r2 = 0.06) andHOA (r2 = 0.12) but correlate much better with IOA (r2 =0.45). Furthermore, we applied a multilinear decompositionalgorithm37,38 to estimate contributions from traffic andindustry to these FS-like ions: tsFS = a × tsHOA + b × tsIOA,

where tsFS, tsHOA, and tsIOA are time series of FS-like ions,HOA, and IOA, respectively, and a and b are the fittingparameters. The reconstructed FS-like ion concentrationscorrelate reasonably well with the measured values [r2 = 0.52and a slope of 0.92 (Figure S11)], validating the effectiveness ofthis analysis, which yields a relative contribution of 70% fromindustry and 30% from traffic for the observed FS-like ions(Figure 3c); a similar analysis estimates 55% from traffic and45% from industry for rBC (C1

+−C31+).

Perspective and Future Work. Our observation of FS in alocation heavily influenced by petrochemical industry likelysuggests the general presence of this specific rBC subtype insimilar oil and gas production regions throughout the world.Because the SP-AMS measured FS-like ions are partially fromthe laser vaporization process, the instrument is limited inquantifying molecular fullerenes. However, preliminary inves-tigation35 reveals that some doubly charged ions like C60

2+,13CC59

2+, etc., can be generated only by the EI process (forinstance, 13CC59

2+ was indeed observed in this study, as shownin Figure S12); therefore, quantification of C60 and/or otherfullerenes may be possible, which will be the subject of ourfuture work.Furthermore, preliminary data show both diesel engines and

biomass burning emit fullerenes, as well (we have recentlyobserved the FS-like ions in the air affected by bothconstruction vehicles and urban traffic in Nanjing, andintermittently in Tibet likely due to local yak dung burningor biomass burning emissions transported from south Asia),further highlighting the importance of characterizing FS toevaluate its environmental, climatic, and health influences onregional and global scales. Moreover, although the SP-AMS isdesigned as a field deployable instrument, it can be used toanalyze offline liquid samples using nebulization;39,40 thus, theSP-AMS measurement results can be validated by otherapproaches, such as LC/MS techniques. The techniquedemonstrated here may be useful for characterizing fullerenes

Figure 3. (a) Distributions of industrial point sources around the sampling site. (b) Rose plot of the observed mass loadings of FS-like ions withwind direction. (c) Estimated contributions of industry and traffic to the FS-like ions.

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in other environmental samples such as sewage water, sedimentand soil, etc.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.estlett.6b00044.

Details of sampling site, SP-AMS operations, identi-fication and quantification of the FS-like ions, PMFresults (Figures S3−S5), ion pattern (Figures S6 and S7),and selected m/z (Figures S8 and S12), concentration(Figure S9), and source analyses of rBC and FS-like ions(Figures S10 and S11) and correlation coefficients of FS-like ions and rBC with other components (Table S1)(PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: caxinra@163.com. Phone: +86-25-58731394.*E-mail: chenmdnuist@163.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Natural Science Foundation ofChina (21407079, 21577065, 91543115, and 91544220), theInternational ST Cooperation Program of China(2014DFA90780), the Priority Academic Program Develop-ment of Jiangsu Higher Education Institutions, the JiangsuNatural Science Foundation (BK20150042), the JiangsuProvincial Specially-Appointed Professors Foundation, theStartup Foundation for Introducing Talent of NUIST(2014r064), and the LAPC Open Fund (LAPC-KF-2014-06).We thank the Environmental Science Research Institute ofNanjing for the supporting data.

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Environmental Science & Technology Letters Letter

DOI: 10.1021/acs.estlett.6b00044Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX

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