Role of elemental carbon in the photochemical agingof sootMeng Lia,b, Fengxia Baoa,b, Yue Zhanga,b, Wenjing Songa,b, Chuncheng Chena,b,1, and Jincai Zhaoa,b

aKey Laboratory of Photochemistry, Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy ofSciences, 100190 Beijing, People’s Republic of China; and bSchool of Chemical Sciences, University of Chinese Academy of Sciences, 100049 Beijing,People’s Republic of China

Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved June 22, 2018 (received for review March 14, 2018)

Soot, which consists of organic carbon (OC) and elemental carbon(EC), is a significant component of the total aerosol mass in theatmosphere. Photochemical oxidation is an important agingpathway for soot. It is commonly believed that OC is photoactivebut EC, albeit its strong light absorption, is photochemically inert.Here, by taking advantage of the different light absorptionproperties of OC and EC, we provide direct experimental evidencethat EC also plays an important role in the photochemical aging ofsoot by initiating the oxidation of OC, even under red lightirradiation. We show that nascent soot, in addition to undergoingphotochemical oxidation under blue light with a wavelength of440 nm, undergoes similar oxidation under red light irradiation ofλ = 648 nm (L648). However, separated OC (extracted from soot byn-hexane) and EC exhibit little reactivity under L648. These obser-vations indicate that EC plays a pivotal role in photoaging of sootby adsorbing light to initiate the oxidation of OC. Comparison of insitu IR spectra and photoelectrochemical behaviors suggests thatEC-initiated photooxidation of OC proceeds through an electrontransfer pathway, which is distinct from the photoaging inducedby light absorption of OC. Since the absorption spectra of EC havea much larger overlap with the solar spectra than those of OC, ourresults provide insight into the chemical mechanism leading torapid soot aging by organic species observed from atmosphericfield measurements.

soot | photochemistry | aging | elemental carbon

Soot particles produced by the incomplete combustion ofbiomass and fossil fuels are emitted in large quantities to the

atmosphere (1, 2). These particles have important impacts onglobal radiative balance and climate, directly by absorbing solarenergy and indirectly by acting as cloud condensation nuclei (2–4). In the atmosphere, soot particles undergo transformations intheir structure, hygroscopicity, and optical properties by inter-acting with other atmospheric chemical constituents, includingreactive inorganic and organic gases (5–11). Such aging signifi-cantly impacts their atmospheric fate, lifetimes, and effects. Forexample, the light absorption and direct radiative forcing of sootare markedly enhanced during atmospheric aging (12). Underdark conditions, aging usually rapidly ceases because of the de-pletion of reactive sites (13–19). However, aging of the sootbecomes time-independent upon exposure to light. For example,persistence of NO2 uptake on soot extends from 1 to 2 min in thedark to almost 70 h under illumination, which is comparable tothe lifetime of soot in the atmosphere (20). In addition, irradi-ation was reported to enable the oxidation of soot by O2 in theair. The apparent rate constants for loss or formation of specieson soot in O2 under sunlight irradiation were larger by factors of1.5–3.5 than those in 100 parts per billion O3 (21). Therefore,photochemical aging of soot has recently attracted increasingattention in the field of atmospheric chemistry (22, 23).Soot particles are primarily composed of elemental carbon

(EC) and organic carbon (OC). EC has a graphite-like mi-crocrystalline structure and is refractory and strongly light-absorptive. The OC on soot is primarily composed of saturated

and unsaturated hydrocarbons, polycyclic aromatic hydrocarbons(PAHs), and partially oxidized organics that condense onto sootparticles during their emission (21, 24, 25). In the ambient at-mosphere, the soot would adsorb other OC species from at-mosphere; consequently, plenty of proteinaceous substances,lipopolysaccharidic substances, lignans, cellulose, pollens, bac-teria, and humic-like substances can also be included in the OCcomponent (26). Recent studies found that enhancement of thephotoaging of soot by sunlight is caused by the promoted oxi-dation of the extractable OC, especially PAHs (21, 23), whichhas been proposed to be initiated by the excitation of OC. Such amechanism is analogous to the self-sensitized photodegradationof PAHs on inert supports such as silica (27–29), as supported bythe similar photochemical reactions of OC on soot and the OCextracted by n-hexane, as well as by the inactiveness of EC underirradiation (21, 23).The light absorption ability of EC is much greater than that of

OC, particularly for long-wavelength radiation (24, 25, 30). Thestrong light absorption of EC was predicted to inhibit the pho-tolysis of PAHs on soot by screening the incoming light, as thephotolytic half-lives for PAHs on silica and alumina are muchshorter than those of PAHs on carbon black (29, 31, 32). On theother hand, carbon-based materials (e.g., carbon dot) were re-cently found to exhibit promising photocatalytic activity for theoxidation and reduction of organic species under visible irradi-ation (33–36). Whether the strong visible light absorption of ECin soot, which has graphite-like microcrystalline structures simi-lar to those of carbon dots, can induce photoreactions is of great

Significance

Photochemical oxidation is an important aging pathway forsoot. The organic carbon (OC) component in soot is believedto be photoactive, while the elemental carbon (EC) part withstrong light absorption is photochemically inert. By consid-ering the distinct light absorption properties of OC and EC, wehave provided direct experimental evidence that EC also playsan important role in the photochemical aging of soot by ab-sorbing the solar light. Our work reveals that the photo-chemical aging of soot occurs in an extended active spectrum(up to red light) by the light absorption of EC and has majorenvironmental effects, such as enhancing the hydrophilicityof soot, that would enhance its ability to act as cloudcondensation nuclei.

Author contributions: M.L., W.S., C.C., and J.Z. designed research; M.L., F.B., and Y.Z.performed research; M.L., C.C., and J.Z. analyzed data; and M.L., C.C., and J.Z. wrotethe paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the PNAS license.1To whom correspondence should be addressed. Email: ccchen@iccas.ac.cn.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1804481115/-/DCSupplemental.

Published online July 9, 2018.

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environmental significance, since visible light is very abundant inthe incident solar light near the Earth’s surface.In the present work, we show that irradiation by red light at a

wavelength of 648 nm, which cannot excite OC, induces efficientphotochemical aging of soot by O2. By comparing in situ IRspectra taken during the photochemical oxidation of soot andOC under blue light with 440-nm irradiation (L440) and underred light with 648-nm irradiation (L648), we reveal that the visiblelight absorption of EC in air initiates the oxidation of OC in soot.The mechanism underlying the photochemical activity of EC isfurther examined by the photoelectrochemical method and byelectron paramagnetic resonance (EPR) spectroscopy, and anelectron transfer pathway is identified to explain the photo-sensitizing activity of EC.

Results and DiscussionThe n-hexane soot, which has been widely used to model theadsorption and reaction properties of soot in atmosphericaerosols (21, 23, 37–40), was produced by combustion of n-hexane in a coflow diffusion burner (21, 23). Consistent withthe earlier studies (21, 24, 25), measurement using diffuse re-flectance UV-visible (UV-vis) spectroscopy indicates that thenascent soot absorbs radiation over a broad wavelength range,from 200 to 800 nm (Fig. 1, black solid line). Soot particles areprimarily composed of EC and variable fractions of OC, and theOC was extracted by solvent (21, 23–25). To distinguish theabsorption characteristics of OC and EC, we abstracted the sootwith n-hexane to separate the extractable OC from EC, duringwhich almost all of the EC was left on the support (SI Appendix,Fig. S1). The absorption spectrum measurements indicate thatthe absorption characteristics of the residual EC (Fig. 1, red solidline) are similar to those of the nascent soot. However, theextracted OC absorbs light at wavelengths ranging from 200–600 nm but is relatively transparent at 600–800 nm (Fig. 1, bluesolid line). The different light absorption properties of OC andEC provide an excellent opportunity to distinguish the roles ofthese two components in the photochemical aging of soot.Light at the wavelengths of 440 nm, which is absorbed by both

components of soot, and 648 nm, which excites only the ECmoiety, was employed to excite specific parts of soot. The in situattenuated total internal reflection IR (ATR-IR) method wasused to monitor the photochemical reactions of soot. As shownin SI Appendix, Fig. S2, after irradiation by L440 (blue line,

denoted Soot440) and L648 (red line, denoted Soot648), the in-tensities of the IR bands at 3,286 cm−1 and 3,038 cm−1, which areassigned to the stretching vibrations of alkyne C-H (≡C-H) andaromatic C-H (Ar-H), respectively (41–44), significantly de-crease, indicative of the loss of the C-H species under irradiationwith either wavelength. At the same time, bands in the rangefrom 1,800–1,500 cm−1 prominently increase. The peaks in thisregion relate to the stretching vibrations of carbonyl C = Ospecies (41, 42, 45, 46). The increase in the intensities of thesepeaks indicates the formation of carbonyl-containing species byphotoinduced reactions of soot. No reactions are observedwithout irradiation or in the absence of O2 (in Ar atmosphere).These results suggest that the spectral changes under irradiationare caused by heterogeneous photochemical reactions of sootwith O2.A closer inspection of the IR spectra, which bear helpful in-

formation about the photoaging process of soot, reveals a com-plex profile for the distribution of the formed carbonyl-containing species. Fig. 2 shows the temporal changes of theIR spectra in the range from 1,800–1,500 cm−1 for soot samplesduring photooxidation by O2. Specifically, the peaks at 1,590 cm−1

are related to chelated carbonyl species or aromatic stretch-enhanced by carbonyls conjugated to the aromatic structure(43). The bands at 1,685 cm−1 originate from the C = O stretchof unsaturated ketones/aldehydes, and the peak at 1,716 cm−1 isfrom C = O groups in saturated ketones/aldehydes (43, 45). Thepeak at ∼1,762 cm−1 is assigned to the C = O stretch of lactoneor anhydride species (43, 45). All these carbonyl-containingspecies are observed by irradiating nascent soot with both L440

Fig. 1. Normalized diffuse reflectance UV-vis spectroscopy of nascent soot(black solid line), OC extracted by hexane (blue solid line), residual EC afterextraction (red solid line), and the solar spectrum air mass 1.5 (AM1.5) in therange of 200–800 nm (pink dashed line). The data on the solar spectrum arefrom the International Electrotechnical Commission (59).

Fig. 2. Temporal changes in ATR-IR spectra in the range from 1,800 to1,500 cm−1 for nascent soot under L440 (A), nascent soot under L648 (B),extracted OC under L440 (C), and extracted OC under L648 (D). Residual ECunder L648 (E) and OC reloaded on EC under L648 (F) are shown. The IRspectra were collected by using corresponding samples untreated by radia-tion on ZnSe crystals as references. The black lines represent the IR spectra ofsamples before irradiation. The other lines represent the IR spectra of sam-ples undergoing different times of irradiation, and the time interval of eachline is 1 h.

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(Fig. 2A) and L648 (Fig. 2B). For the extracted OC, C =O speciesare obviously formed upon L440 (Fig. 2C). However, extractedOC does not react under L648 (Fig. 2D) because of its lack ofabsorption at this wavelength (Fig. 1). Residual EC exhibits littlephotoreactivity under L648 either (Fig. 2E). Interestingly, whenwe reloaded the extracted OC onto the EC, the photooxidationof OC under L648 is restored (Fig. 2F). Since the separated OCand EC do not exhibit any photoreaction activity under L648, thephotoactivity of soot that consists of OC and EC under thisirradiation (Fig. 2 B and F) is somewhat unexpected. These resultsclearly indicate the pivotal role of EC in the photochemical oxi-dation of OC in the soot.Although the irradiation leads to a general increase in C = O

species content under both L440 (Fig. 2A) and L648 (Fig. 2B), therelative intensities of different C = O species are notably dif-ferent in samples treated with these two wavelengths. For ex-ample, the relative intensity of the unsaturated ketone/aldehyde(at 1,685 cm−1) in Soot648 is more significant than that in Soot440.To better quantify this intensity difference, we compared theratio in intensity of the different C = O peaks under differentirradiation. SI Appendix, Fig. S3 shows that the ratio of the un-saturated (at 1,685 cm−1)-to-saturated (at 1,716 cm−1) ketone/aldehyde band intensities (Iunsat/Isat) for Soot648 is much higherthan that for Soot440. However, the Iunsat/Isat value of extractedOC that received L440 (OC440) is lower than that of Soot440.These results indicate that a greater number of unsaturatedketones/aldehydes are produced under L648, while the formationof saturated ketones/aldehydes is more significant under L440(particularly in OC440).The different intermediate distribution suggests that different

reaction mechanisms are responsible for the photooxidation ofOC under L440 and L648. Two mechanisms are usually invoked toexplain photochemical oxidation reactions of organic com-pounds: an electron transfer pathway and an energy transferpathway (47). In the electron transfer pathway, the radical re-actions, which are initiated by the photoinduced electron trans-fer, dominate the oxidation, as in photocatalytic degradation oforganic pollutants on TiO2 (48). The oxidative cleavage of aro-matic rings by radical reactions would lead to the formation ofunsaturated ketones/aldehydes, such as unsaturated muconaldehyde(SI Appendix, Scheme S1A) and 2-formylcinnamaldehyde (49–53)(SI Appendix, Scheme S1B). The enrichment of unsaturatedketones/aldehydes in the photoproducts of soot under L648 suggeststhat the oxidation of OC is dominated by radical reactions andinitiated by the electron transfer pathway under these conditions.Because of the lack of absorption of OC itself under L648, thereaction should be induced by the excitation of EC. Excited ECabstracts electrons from the OC and donates electrons to

adsorbed O2. As a result, an OC radical cation and reactiveoxygen species, such as superoxide anion radicals (O2

•−) andhydroxyl radicals (•OH), would be formed. This pathway isfurther verified by the detection of radicals and photo-electrochemical measurements as discussed below.The excitation of OC, especially PAHs, forms singlet oxygen

(1O2) by energy transfer (21, 29). Addition reactions between1O2 and unsaturated and aromatic compounds, such as ole-fins and PAHs, are very rapid. Compared with the radicalreactions, these reactions are quite mild and result in the for-mation of dioxetanes. Further cleavage of unstable dioxetanesproduces a substitution-dependent range of ketones/aldehydes(SI Appendix, Scheme S1C). If the substituent groups in theolefins are saturated alkyls, the formed ketones/aldehydes aresaturated (54). The energy transfer pathway therefore producesa significant portion of saturated ketones/aldehydes. Accord-ingly, the significance of saturated ketones/aldehydes in OCoxidation under L440 (particularly for OC; Fig. 2C) is an in-dication of the important role of the energy transfer pathway inphotooxidation of OC by exciting OC itself. To confirm theprevalence of the energy transfer pathway in these reactions, weexamined the formation of 1O2 by using furfuryl alcohol (FFA)as a probe (55, 56). Significant amounts of 1O2 are observed inboth the soot and OC systems under L440, as shown by the rapiddegradation of FFA (SI Appendix, Fig. S4). By contrast, the lackof FFA loss under L648 indicates that little 1O2 is generatedunder these conditions. These results support the above con-clusion that the photochemical oxidation of OC under L440 isinduced dominantly by exciting OC itself and the following 1O2oxidation. Both the photooxidation of OC itself (energytransfer pathway) and the oxidation of OC induced by EC(electron transfer pathway) should contribute to the photo-aging of the nascent soot under L440, since the significance ofsaturated ketones/aldehydes in Soot440 falls between that ofSoot648 and OC440. The key processes in the EC-initiatedphotooxidation of OC and the direct photolysis of OC aresummarized in Fig. 3.By loading the sample of interest (soot, EC, and OC) on

fluorine-doped tin oxide (FTO) as a working electrode, the light-induced events were further probed by open circuit voltage (Vo)measurements, which allow one to probe a photopotentialcaused by steady-state accumulation of a photogenerated hole/electron. Fig. 4 shows the typical Vo profile in air-equilibratedelectrolytes. Upon L648 and L440, both the soot and EC elec-trodes show significant positive Vo (Fig. 4), indicating the ac-cumulation of a positive charge (photoinduced holes) on theelectrodes due to efficient charge separation by the capture ofphotoinduced electrons by dissolved O2. The Vo decay after ir-radiation represents charge recombination and/or transfer toavailable acceptors (OH−) in the electrolyte. The Vo of sootduring irradiation shows a slight increase probably owing to thephotochemical reaction, as shown in Fig. 2, while this is not

Fig. 3. Reaction scheme for the EC-initiated photooxidation of OC and thedirect photolysis of OC.

Fig. 4. Vo responses of soot (blue line), EC (red line), and OC (black line)under L648 (A) and L440 (B) in air-equilibrated 1 M KCl solutions.

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observed for EC, consistent with the lack of photochemicalconversion of EC (despite its efficient charge separation). Incontrast to soot and EC, OC electrodes do not show responses toL648 (Fig. 4A, black line). Even under 440-nm irradiation, underwhich condition the photochemical reaction occurs, the Vo of theOC electrode is quite small at the beginning stage of irradiation,indicative of the poor charge separation feature of OC. Thegradual Vo increase of OC with irradiation time could be attrib-uted to the enhanced charge separation ability of the photooxi-dation product of OC, as in the case of irradiated soot. Moreover,it is interesting to note that the Vo of soot is higher than that of ECunder L648 (Fig. 4A), suggesting that more holes are accumulatedin the soot electrodes due to the presence of OC. Since L648cannot excite OC in soot, the higher Vo of soot should not beattributed to the light absorption of OC. Reasonably, because ofits electron-donating properties, the OC in soot traps the holegenerated by excitation of the EC moiety, which would enhancethe accumulation of holes in the soot, and consequently increasethe Vo. In other words, the higher Vo of the soot electrode sug-gests that the photoinduced hole in the EC moiety transfers to theOC part, which leads to the photooxidation of OC.In Ar-saturated electrolytes (SI Appendix, Fig. S5), the Vo

generally changes according to two trends: (i) the Vo of elec-trodes in air-equilibrated electrolytes is higher than that ofelectrodes in Ar-saturated electrolytes, which implies that O2can trap the photogenerated electron and thus enhances theaccumulation of a positive charge on the electrodes, and (ii) theincrease in the Vo with irradiation time is largely eliminated inAr-saturated electrolytes, indicative of depression of the pho-tochemical reaction in the absence of O2. These observationssuggest that the O2 can trap the photogenerated electron.The short-circuit photocurrents, which reflect the type and

rate of the interfacial photochemical redox reactions, of the sootelectrodes under different conditions are shown in SI Appendix,Fig. S6. Pronounced negative photocurrents are observed for thesoot electrode under L648 and L440 in the air and Ar-saturatedelectrolytes, indicating that the soot electrode abstracts electronsfrom the external circuit, which is in agreement with the positiveVo. The photocurrent of the OC electrode, compared with thenascent soot, is quite low, even under L440. The small short-circuit photocurrent of extracted OC confirms that OC oxida-tion under L440 mainly occurs via the energy transfer pathway.Under identical levels (20 mW·cm−2) of L648 and L440, the short-circuit photocurrent of EC is much larger than that of soot in air-saturated electrolytes (Fig. 5), demonstrating the excellent abilityof EC to realize photoinduced charge separation. The lowerphotocurrent of soot suggests that the presence of OC in sootcan inhibit the photocurrent. This inhibition might be caused bythe consumption of photoinduced holes by OC, since OC candonate electrons to the exited EC and become oxidized, which

will competitively depress hole transfer from the excited EC tothe FTO electrode.Fig. 5 shows that the short-circuit photocurrents in air-equilibrated

electrolytes are much larger than those in Ar-saturated electrolytes,particularly for the EC electrode, suggesting that dissolved O2 canenhance the photocurrent. Moreover, changes in the transient pho-tocurrent of the EC electrode after the light is turned on largelydepend on the atmosphere. In the presence of O2, the change in thetransient photocurrent is insignificant, whereas it rapidly decays in theabsence of O2. Such a photocurrent decay is attributed to the re-combination of photogenerated charges on the surface of the elec-trode. The electron is thus removed from the electrode by reactingwith the O2, inhibiting the recombination of photogenerated chargesand resulting in a more sustainable photocurrent. However, the ac-cumulation of electrons on the electrode surface upon irradiation inthe absence of O2 leads to faster charge recombination, which de-creases the photocurrent. As proposed from the product analysis andVo measurements, the reaction of O2 with the photoinduced elec-trons on EC would generate reactive oxygen species such as O2

•− and•OH, which play an important role in the aging of soot.

Environmental ImplicationPhotooxidation of OC on soot largely determines the environ-mental effects and fate of soot. EC-initiated photooxidation ofOC on soot occurs even under red light irradiation. Such anextension of active spectra caused by EC has important effectson the photoaging of soot since red light is abundant in sunlight(Fig. 1), particularly during a small solar zenith angle or cloudyweather. Under these situations, EC-initiated photooxidationlikely dominates the photoaging of soot. Moreover, the photo-activity of EC may have important implications for the persistentuptake of environmental species, such as NO2, by soot underirradiation (20) since the light absorption and photoactivity ofEC should persist even after labile OC sites are consumed.We show that the photooxidation of soot initiated by light

absorption of EC transforms the olefins and PAHs in OC intovarious carbonyl C = O species. These oxygen-containing speciesare more polar than the original olefins and PAHs. As a result,the hydrophilicity of soot can be largely increased. To verify thischange, the hydrophilicity of soot after irradiation was examinedby ATR-IR spectroscopy. The dashed lines in Fig. 6 representthe IR spectra for samples equilibrated with a stream of water-saturated Ar before irradiation; the IR bands at 3,415 and

Fig. 5. Short-circuit photocurrent responses of soot (black line) and EC (redline) in air-equilibrated electrolytes and soot (blue line) and EC (pink line) inAr-saturated electrolytes under L648 (A) and L440 (B).

Fig. 6. ATR-IR spectra for soot equilibrated with a stream of water-saturatedAr before (dashed line) and after (solid line) irradiation with 648 nm and 440 nmand in the dark. The absorbance intensities of IR bands at 3,415 cm−1 representthe amount of adsorbed water, which is related to the water affinity of soot.

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1,623 cm−1 are attributed to the stretching vibration of O-H andthe H-O-H bending vibration of adsorbed water molecules.Thus, the absorbance intensities of IR bands at 3,415 cm−1

represent the amount of adsorbed water, which is related to thewater affinity of soot. After the samples were irradiated witheither L648 (Fig. 6, red solid line) or L440 (Fig. 6, blue solid line),the absorbance intensities of these adsorbed water peaks signif-icantly increase, indicating the enhanced hydrophilicity of sootafter irradiation. By contrast, the water affinity of soot is barelychanged after 12 h in the dark (Fig. 6, black solid line). Theincreased hydrophilicity of soot would enhance its ability to actas cloud condensation nuclei or ice nuclei (5, 57).Our experimental results also show that the EC-initiated

oxidation proceeds mainly via an electron transfer pathway inwhich many radical species can be formed. Reactive oxygenradicals are important environmental species that can causediverse reactions in addition to oxidizing OC. To show theformation of reactive oxygen radicals during the photochemicaloxidation of soot, we examined radical formation by the spin-trapping EPR technique, using 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) as a spin-trapping agent (58). Fig.7 shows that significant amounts of BMPO-OH adducts aredetected in the suspensions of soot and EC under both L648 andL440, indicative of the formation of •OH. When these systemsare purged with Ar to remove O2, no BMPO-OH adduct signalsare observed any longer (SI Appendix, Fig. S7H), indicating thatthe generation of •OH is closely related to the presence of O2.However, we did not detect the superoxide radical with BMPO,probably because of the rapid disproportionation of superoxidein aqueous solution.

ConclusionsWe show that, in addition to the direct photolysis of OC, theabsorption of sunlight by the unreactive EC initiates efficientphotooxidation of surface OC. This process occurs not only withblue light (λ = 440 nm), which photolyzes OC directly as well, butalso with red light (λ = 648 nm). Unlike the photochemical ox-idation of OC itself, which is dominated by an energy transfer-based 1O2 mechanism, EC-initiated oxidation of OC proceedsthrough an electron transfer pathway, where reactive radicalspecies such as •OH are formed. The identification of EC-induced

photoaging of soot may have important environmental andatmospheric implications.

Materials and MethodsSoot Production. Soot was produced by the combustion of n-hexane (HPLCgrade) in a coflow diffusion burner, as described by Han et al. (21, 23). Theburner consisted of a diffusion flame maintained by an airflow that could beexactly controlled using mass flowmeters. The airflow was a mixture of high-purity O2 and N2, and the O2 content was 21.3%. The fuel/oxygen ratio was0.18. Fuel transport was achieved using a cotton wick extending into theliquid fuel reservoir. Soot particles from the diffusion flame were directlydeposited on supports, such as quartz plates, ZnSe crystals, FTO, and quartzfiber filters, for further reactions and characterization by several methods,including UV-vis absorption (U-3900; Hitachi), in situ ATR-IR spectroscopy,photoelectrochemical measurements, and EPR.

Preparation of Residual EC and Extracted OC Samples. Residual EC wasobtained by soaking the soot samples (deposited on quartz plates, ZnSecrystals, FTO, or quartz fiber filters) four times in 50mL of n-hexane for 10 min,and the n-hexane on the residue was subsequently evaporated in air. Carewas taken not to disturb the sample to minimize mechanical removal of in-soluble particles. Most OC, especially PAHs, can be removed from the sootparticles, and the residue is mainly EC.

The n-hexane extractions were first concentrated and then added drop-wise to quartz plates, ZnSe crystals, FTO, or quartz fiber filters, as they wereused for soot collection. After the solvent was evaporated in air, the extractedOC was left on the supports. To reload the extracted OC onto the EC, then-hexane extractions were added dropwise to ZnSe crystals with EC. Afterevaporating n-hexane off in the air, the photoaging was examined by ATR-IRspectroscopy. An inspection using ATR-IR spectroscopy indicates that such areloading process can recover the OC onto the EC (SI Appendix, Fig. S8).

Aging Experiments. The in situ ATR-IR spectrawere recorded using an IS50 FTIRspectrometer, which is equipped with a high-sensitivity mercury-cadmium-telluride detector cooled by liquid N2. A Lumencor solid-state light enginewith switchable wavelength control and tunable intensity was used as thelight source throughout the whole study. Soot EC or OC samples on the ZnSecrystals were put into the ATR-IR cell. The ATR-IR cell was sealed with quartzglass, through which samples were irradiated by light with wavelengthsof 440 ± 25 nm and 648 ± 25 nm. The light intensities were adjusted to20 mW·cm−2. Before the reactions, the cell was purged with 20 mL·min−1 Aruntil the IR spectrum was constant. A mixture of high-purity O2 (4 mL·min−1)and Ar (16 mL·min−1) was then introduced into the ATR-IR cell. The spectraof samples were recorded (100 scans, 4-cm−1 resolution) using the ZnSe orZnSe loaded with a sample as a reference.

Photoelectrochemical Analysis. Vo values and short-circuit photocurrentswere examined in a conventional three-electrode electrochemical cell with asample/FTO electrode as the working electrode, a platinum wire as thecounterelectrode, and Ag/AgCl (saturated KCl) as the reference electrode inan aqueous solution of 1 M KCl. All of the measurements were carried out on aCHI760e electrochemical workstation. For comparing the photoelectrochemicalproperties between EC and soot, the EC electrode was prepared by washing thesoot/FTO electrode with n-hexane after the photoelectrochemical measurementshad been completed on this soot/FTO electrode. Care was taken to not disturbthe sample to minimize mechanical removal of EC.

EPR Measurements. A BMPO (a freshly prepared 0.1 M solution in deionizedwater) spin trap was used for capturing hydroxyl radicals (•OH) in aqueoussolutions. These samples, including soot, OC, and EC, were loaded on a quartzfiber filter (3 mm × 30mm) and continuously monitored for free radical signals inan EPR spectrometer (ELEXSYS E500 EPR; Bruker) with a modulation of 100 kHzand a microwave frequency of 9.5 GHz. The typical parameters for EPR mea-surement were as follows: the sweep width was 100 G, the modulation ampli-tude was 2.00 G, and the x axis had 1,024 points. The EPR microwave power wasset specifically to 13 dB, and the sweep time was 81.92 ms.

Water Affinity Test. Soot samples were first dehydrated by a pure Ar flow untilthe IR spectrumwas constant, and subsequently flushed by a water-saturatedAr flow (20 mL·min−1) for 3 h to ensure that water vapor adsorption to thesoot reached equilibrium. The ATR-IR spectra of the water-saturated sootwere recorded by using the dehydrated soot surface as the referencebackground spectrum (100 scans, 4-cm−1 resolution). Soot particles werethen flushed with a dry Ar flow (100 mL·min−1) for 3 h to remove the water

Fig. 7. EPR signals for typical BMPO-OH adducts. Soot under L648 (greenline), soot under L440 (pink line), EC under L648 (blue line), EC under L440 (redline), and samples in the dark (black line) are shown.

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from ATR-IR cell until the spectrum showed no further changes. A mixture ofhigh-purity O2 and Ar was then introduced into the ATR-IR cell, and thesamples were irradiated by light with wavelengths of 440 nm or 648 nm for12 h. The cell was subsequently flushed with a water-saturated Ar flowagain, and the IR spectra were recorded. The control experiment was per-formed using the same procedure in dark.

ACKNOWLEDGMENTS. We thank Yan Zhao [Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (CAS)] for her instruc-tive suggestions. This research was financially supported by Strategic PriorityResearch Program of the CAS Grant XDA09030200; National Natural ScienceFoundation of China Grants 21525729, 21590811, 21521062, and 2177168;Key Research Program of Frontier Sciences Grant QYZDY-SSW-SLH028 of theCAS; and the CAS Interdisciplinary Innovation Team Program.

1. Bond TC, et al. (2004) A technology-based global inventory of black and organiccarbon emissions from combustion. J Geophys Res 109:D14203.

2. Bond TC, et al. (2013) Bounding the role of black carbon in the climate system: Ascientific assessment. J Geophys Res Atmos 118:5380–5552.

3. Gustafsson Ö, Ramanathan V (2016) Convergence on climate warming by black car-bon aerosols. Proc Natl Acad Sci USA 113:4243–4245.

4. Ramanathan V, Carmichael G (2008) Global and regional climate changes due to blackcarbon. Nat Geosci 1:221–227.

5. Liu Y, Liu C, Ma J, Ma Q, He H (2010) Structural and hygroscopic changes of sootduring heterogeneous reaction with O(3). Phys Chem Chem Phys 12:10896–10903.

6. Knauer M, et al. (2009) Soot structure and reactivity analysis by Raman micro-spectroscopy, temperature-programmed oxidation, and high-resolution transmissionelectron microscopy. J Phys Chem A 113:13871–13880.

7. Xue H, Khalizov AF, Wang L, Zheng J, Zhang R (2009) Effects of dicarboxylic acidcoating on the optical properties of soot. Phys Chem Chem Phys 11:7869–7875.

8. Zhang R, et al. (2008) Variability in morphology, hygroscopicity, and optical propertiesof soot aerosols during atmospheric processing. Proc Natl Acad Sci USA 105:10291–10296.

9. Qiu C, Khalizov AF, Zhang R (2012) Soot aging from OH-initiated oxidation of tolu-ene. Environ Sci Technol 46:9464–9472.

10. Khalizov AF, et al. (2013) Role of OH-initiated oxidation of isoprene in aging ofcombustion soot. Environ Sci Technol 47:2254–2263.

11. Peng J, et al. (2017) Ageing and hygroscopicity variation of black carbon particles inBeijing measured by a quasi-atmospheric aerosol evolution study (QUALITY) cham-ber. Atmos Chem Phys 17:10333–10348.

12. Peng J, et al. (2016) Markedly enhanced absorption and direct radiative forcing ofblack carbon under polluted urban environments. Proc Natl Acad Sci USA 113:4266–4271.

13. Lelievre S, et al. (2004) Heterogeneous reaction of ozone with hydrocarbon flamesoot. Phys Chem Chem Phys 6:1181–1191.

14. Arens F, Gutzwiller L, Baltensperger U, Gäggeler HW, Ammann M (2001) Heteroge-neous reaction of NO2 on diesel soot particles. Environ Sci Technol 35:2191–2199.

15. Kleffmann J, Wiesen P (2005) Heterogeneous conversion of NO2 and NO on HNO3

treated soot surfaces: Atmospheric implications. Atmos Chem Phys 5:77–83.16. Aubin DG, Abbatt JPD (2007) Interaction of NO2 with hydrocarbon soot: Focus on

HONO yield, surface modification, and mechanism. J Phys Chem A 111:6263–6273.17. Prince AP, et al. (2002) Heterogeneous reactions of soot aerosols with nitrogen di-

oxide and nitric acid: Atmospheric chamber and Knudsen cell studies. Atmos Environ36:5729–5740.

18. Lelievre S, Bedjanian Y, Laverdet G, Le Bras G (2004) Heterogeneous reaction of NO2

with hydrocarbon flame soot. J Phys Chem A 108:10807–10817.19. Aumont B, et al. (1999) On the NO2 plus soot reaction in the atmosphere. J Geophys

Res Atmos 104:1729–1736.20. Monge ME, et al. (2010) Light changes the atmospheric reactivity of soot. Proc Natl

Acad Sci USA 107:6605–6609.21. Han C, Liu Y, Ma J, He H (2012) Key role of organic carbon in the sunlight-enhanced

atmospheric aging of soot by O2. Proc Natl Acad Sci USA 109:21250–21255.22. Han C, Liu Y, He H (2013) Heterogeneous photochemical aging of soot by NO2 under

simulated sunlight. Atmos Environ 64:270–276.23. Han C, Liu Y, He H (2016) The photoenhanced aging process of soot by the hetero-

geneous ozonization reaction. Phys Chem Chem Phys 18:24401–24407.24. Kirchstetter TW, Novakov T, Hobbs PV (2004) Evidence that the spectral dependence

of light absorption by aerosols is affected by organic carbon. J Geophys Res Atmos109:D21208.

25. Adler G, Riziq AA, Erlick C, Rudich Y (2010) Effect of intrinsic organic carbon on theoptical properties of fresh diesel soot. Proc Natl Acad Sci USA 107:6699–6704.

26. Pan Y-L, et al. (2007) Single-particle laser-induced-fluorescence spectra of biologicaland other organic-carbon aerosols in the atmosphere: Measurements at New Haven,Connecticut, and Las Cruces, New Mexico. J Geophys Res 112:D24S19.

27. Barbas JT, Dabestani R, Sigman ME (1994) A mechanistic study of photodecomposi-tion of acenaphthylene on a dry silica surface. J Photoch Photobio A 80:103–111.

28. Barbas JT, Sigman ME, Arce R, Dabestani R (1997) Spectroscopy and photochemistryof fluorene at a silica gel/air interface. J Photoch Photobio A 109:229–236.

29. Barbas JT, Sigman ME, Dabestani R (1996) Photochemical oxidation of phenanthrenesorbed on silica gel. Environ Sci Technol 30:1776–1780.

30. Bahadur R, Praveen PS, Xu Y, Ramanathan V (2012) Solar absorption by elementaland brown carbon determined from spectral observations. Proc Natl Acad Sci USA109:17366–17371.

31. Behymer TD, Hites RA (1985) Photolysis of polycyclic aromatichydrocarbons adsorbedon simulated atmospheric particulates. Environ Sci Technol 19:1004–1006.

32. Behymer TD, Hites RA (1988) Photolysis of polycyclic aromatic-hydrocarbons adsorbedon fly-ash. Environ Sci Technol 22:1311–1319.

33. Yu H, et al. (2016) Smart utilization of carbon dots in semiconductor photocatalysis.Adv Mater 28:9454–9477.

34. Lim SY, Shen W, Gao Z (2015) Carbon quantum dots and their applications. Chem SocRev 44:362–381.

35. Zhang Z, et al. (2016) Progress of carbon quantum dots in photocatalysis applications.Part Part Syst Charact 33:457–472.

36. Li H, et al. (2013) Near-infrared light controlled photocatalytic activity of carbonquantum dots for highly selective oxidation reaction. Nanoscale 5:3289–3297.

37. Aubin DG, Abbatt JP (2003) Adsorption of gas-phase nitric acid to n-hexane soot:Thermodynamics and mechanism. J Phys Chem A 107:11030–11037.

38. Han C, Liu Y, He H (2013) Role of organic carbon in heterogeneous reaction of NO2with soot. Environ Sci Technol 47:3174–3181.

39. Al-Abadleh HA, Grassian VH (2000) Heterogeneous reaction of NO2 on hexane soot: AKnudsen cell and FT-IR study. J Phys Chem A 104:11926–11933.

40. Zhao Y, et al. (2017) Heterogeneous reaction of SO2 with soot: The roles of relativehumidity and surface composition of soot in surface sulfate formation. Atmos Environ152:465–476.

41. Kirchner U, Scheer V, Vogt R (2000) FTIR spectroscopic investigation of the mechanismand kinetics of the heterogeneous reactions of NO2 and HNO3 with soot. J Phys ChemA 104:8908–8915.

42. Cain JP, Gassman PL, Wang H, Laskin A (2010) Micro-FTIR study of soot chemicalcomposition-evidence of aliphatic hydrocarbons on nascent soot surfaces. Phys ChemChem Phys 12:5206–5218.

43. Akhter MS, Chughtai AR, Smith DM (1985) The structure of hexane soot I: Spectro-scopic studies. Appl Spectrosc 39:143–153.

44. Cain JP, et al. (2011) Evidence of aliphatics in nascent soot particles in premixedethylene flames. Proc Combust Inst 33:533–540.

45. Daly HM, Horn AB (2009) Heterogeneous chemistry of toluene, kerosene and dieselsoots. Phys Chem Chem Phys 11:1069–1076.

46. Nieto-Gligorovski L, et al. (2008) Interactions of ozone with organic surface films inthe presence of simulated sunlight: Impact on wettability of aerosols. Phys ChemChem Phys 10:2964–2971.

47. Foote CS (1991) Definition of type I and type II photosensitized oxidation. PhotochemPhotobiol 54:659.

48. Chen C, Ma W, Zhao J (2010) Semiconductor-mediated photodegradation of pollut-ants under visible-light irradiation. Chem Soc Rev 39:4206–4219.

49. Wu R, Pan S, Li Y, Wang L (2014) Atmospheric oxidation mechanism of toluene. J PhysChem A 118:4533–4547.

50. Bui TD, et al. (2011) Two routes for mineralizing benzene by TiO2-photocatalyzedreaction. Appl Catal B 107:119–127.

51. Nishino N, Arey J, Atkinson R (2012) 2-Formylcinnamaldehyde formation yield fromthe OH radical-initiated reaction of naphthalene: Effect of NO(2) concentration.Environ Sci Technol 46:8198–8204.

52. Vialaton D, Richard C, Baglio D, Paya-Perez AB (1999) Mechanism of the photo-chemical transformation of naphthalene in water. J Photoch Photobio A 123:15–19.

53. Ohno T, Tokieda K, Higashida S, Matsumura M (2003) Synergism between rutile andanatase TiO2 particles in photocatalytic oxidation of naphthalene. Appl Catal A Gen244:383–391.

54. Frimer AA (1979) The reaction of singlet oxygen with olefins: The question ofmechanism. Chem Rev 79:359–387.

55. Fu H, et al. (2016) Photochemistry of dissolved black carbon released from biochar:Reactive oxygen species generation and phototransformation. Environ Sci Technol 50:1218–1226.

56. Qu X, Alvarez PJJ, Li Q (2013) Photochemical transformation of carboxylated multi-walled carbon nanotubes: Role of reactive oxygen species. Environ Sci Technol 47:14080–14088.

57. Han C, Liu Y, Liu C, Ma J, He H (2012) Influence of combustion conditions on hy-drophilic properties and microstructure of flame soot. J Phys Chem A 116:4129–4136.

58. Zhao H, Joseph J, Zhang H, Karoui H, Kalyanaraman B (2001) Synthesis and bio-chemical applications of a solid cyclic nitrone spin trap: A relatively superior trap fordetecting superoxide anions and glutathiyl radicals. Free Radic Biol Med 31:599–606.

59. International Electrotechnical Commission (2016) Photovoltaic devices-Part 3: Mea-surement principles for terrestrial photovoltaic (PV) solar devices with referencespectral irradiance data (International Electrotechnical Commission, Geneva), IEC60904-3:2016 RLV.

7722 | www.pnas.org/cgi/doi/10.1073/pnas.1804481115 Li et al.

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