Contribution of Charge-Transfer Complexes to Absorptivity of Primary

Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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Contribution of Charge-Transfer Complexes to Absorptivity of Primary Brown Carbon Aerosol Alina Trofimova, Rachel F. Hems, Tengyu Liu, Jonathan P. D. Abbatt,* and Elijah G. Schnitzler* Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada

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S Supporting Information *

ABSTRACT: Light-absorbing organic aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate. A large fraction of the absorptivity of ambient BrC is unassigned, and organic charge-transfer (CT) complexes have the potential to contribute to this fraction. Here, the contributions of CT complexes to the absorptivity of laboratory-generated BrC and ambient aerosol material influenced by biomass burning have been investigated, using a wide range of chemical, spectroscopic, and physical analyses. Chemical functionalization experiments are inconclusive about the role of CT complexes, whereas fluorescence spectra exhibit distinct spectral features indicative of individual chromophores. Determinations of the concentration and temperature dependences of absorbance are more conclusive. In particular, for laboratory-generated BrC extracted in either water or methanol, absorbance scaled linearly with orders-of-magnitude changes in concentration, indicating that intermolecular complexes do not contribute to the absorptivity. Furthermore, whereas the absorbance of BrC extracts in dimethyl sulfoxide exhibited a slight temperature dependence, consistent with a 15% contribution from intramolecular CT complexes at 15 °C, the complete temperature independence of absorbance of water-soluble extracts from surrogate and ambient BrC indicates a negligible role for CT complexes. Overall, our results find little evidence for CT complexes in the primary BrC studied, suggesting that they do not contribute significantly to the missing absorptivity of ambient BrC. KEYWORDS: Carbonaceous aerosol, biomass burning, brown carbon, water-soluble organic compounds, charge-transfer complexes

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INTRODUCTION Carbonaceous aerosol has significant but poorly constrained effects on climate,1,2 including direct effects of absorbing and scattering solar radiation and indirect effects of altering cloud properties. Light-absorbing organic aerosol, or brown carbon (BrC),3 is an important contributor to this uncertainty because its optical properties vary widely with source and atmospheric residence time.4 To gain a sense of the complexity, consider low temperature biomass burning, which generates primary BrC particles5,6 and semivolatile phenolic species.7,8 The primary BrC can be chemically modified in the particle phase due to photolysis9 or heterogeneous oxidation,5,10 and its constituent water-soluble organic compounds (WSOCs) can similarly age by photolysis11,12 or oxidation13,14 in the aqueous phase. As well, nonabsorbing phenolic molecules may partition to the aqueous phase15,16 of cloud droplets or aqueous aerosols and react to form secondary BrC composed of light-absorbing functionalization and oligomerization products, including substituted biphenyls.17,18 Consequently, the composition of BrC is very complex, and significant efforts have been made to comprehensively characterize the individual chromophores responsible for its absorptivity.19,20 However, the combined absorbance of the individual chromophores quantified is generally much smaller than the total absorbance of the BrC mixture. For example, the © XXXX American Chemical Society

combined contribution of strongly absorbing nitroaromatics to the total absorbance at 370 nm of ambient BrC during field campaigns in China and Germany was less than 5% under all conditions.21 This contribution was higher (at about 50%) during a field campaign in Israel, but a significant fraction of the total absorbance was still missing.22 It is possible that the missing absorbance is due to incomplete characterization of individual chromophoric molecules. However, another possibility is that organic charge-transfer (CT) complexes contribute to the missing absorptivity of BrC, as recently hypothesized.23−25 For example, organic aerosol particles may contain phenols and quinones,26 and the functional groups in these species make them candidate electron donors and acceptors, respectively.27 Furthermore, ambient BrC has been shown to contain high molecular weight oligomers,28,29 which also may contain phenol and quinone moieties. Consequently, it is conceivable that inter- and/or intramolecular CT complexes form in BrC. Special Issue: New Advances in Organic Aerosol Chemistry Received: Revised: Accepted: Published: A

April 30, 2019 June 15, 2019 June 21, 2019 June 21, 2019 DOI: 10.1021/acsearthspacechem.9b00116 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry

aerosol by thermo-optical transmittance (TOT).37 During the summer months, significant BC was measured,36 consistent with large estimated fire emissions38 to the north of ETL. For each of three filter samples, aerosol was collected at a flow rate of 16.5 L min−1 for 1 week, and the filters were collected over 3 weeks in succession, from July 12 to August 1, 2008. Punches were prepared from the filters and stored in a freezer, in accordance with standard ECCC procedures. Representative punches were analyzed using TOT, indicating an appreciable fraction (13 ± 2%) of the collected mass could be attributed to pyrolytic organic carbon (which is grouped with carbonate carbon).37 Furthermore, back trajectories indicate that the air masses during this period often originated from the northwest, as shown in Figure S1 in the Supporting Information (SI), passing over wildfires in the neighboring boreal forest,39 so the samples were influenced by biomass burning. Extracts from punches of the three filters, each in 0.3 mL of water, were combined to give sufficient absorbance and total volume. The extracts were passed sequentially through PTFE and nylon syringe filters with pore sizes of 0.45 μm (Whatman) and 0.2 μm (Chromspec), respectively. The second filtration was required to completely remove undissolved particles, including elemental carbon. Reactions. BrC extracts were treated using the following procedures for selective reduction, selective acetylation, and addition of model electron acceptor and donor species. The absorbance of the extracts was strongly dependent on pH, consistent with previous observations of ambient BrC,22,40 likely due the equilibrium between the neutral and deprotonated forms of phenolic species. The selective reduction and acetylation procedures caused drastic changes in pH, which inadvertently increased the absorbance at the initial pH slightly; accordingly, nascent aqueous extracts were cycled between pH 2 and 11 before reduction and acetylation so that this effect did not arise during the reactions.40,41 Specifically, BrC extracts in water were adjusted from their initial pH (about pH 4 and 8 for laboratory-generated and ambient BrC, respectively) down to pH 2, from pH 2 up to pH 11, and from pH 11 to the initial pH, using dilute stock solutions of HCl (Sigma-Aldrich, 37% w/w) and NaOH (Caledon, >97%). During a subsequent cycle, absorbance was still strongly dependent on pH, but the absorbance at the initial pH was unchanged, so one preparatory pH cycle was performed prior to each of these reactive assays. The alkalinity of the ambient samples is attributed to carbonate carbon. For reduction, 5.0 mg of NaBH4 (Sigma-Aldrich, 99%) was added to the solution and mixed, as described by Phillips and Smith.23 To facilitate comparisons, the established procedure was followed exactly, and no steps were taken to remove inorganics including carbonate. Carbonate, for example, is not expected to affect the potential CT complexes nor hinder selective reduction.42 For acetylation, the extract was adjusted to pH 11 using NaOH and placed in an ice bath; 30 μL of acetic anhydride (Ac2O; Sigma-Aldrich, ≥99%) was added to the solution and mixed. The procedure for acetylation was also performed with phenol (Sigma-Aldrich, >99%) substituted for BrC. Following both reduction and acetylation, the extracts were adjusted to their initial pH values with a 0.1 M HCl solution. Added volumes were recorded throughout to correct absorbance for dilution. For the addition of model electron acceptor and donor species, BrC extracts in water were mixed 1:1 v/v with 5 mM solutions of 1,2-naphthoquinone (Sigma-

A model of intramolecular CT complexes has been used to explain the absorbance and fluorescence of chromophoric dissolved organic matter (CDOM) in natural waters,30,31 though it has been the subject of recent debate.32−34 Based on selective reduction of carbonyl groups, about 50% of the total absorbance of ambient BrC samples collected in the Southeastern United States has been attributed to CT complexes.23 Still, the role of CT complexes has not been investigated for a wider range of BrC.4 Here, a multifaceted approach is used to investigate the contribution of CT complexes to the absorptivity of primary BrC, including laboratory-generated surrogates and ambient BrC aerosol influenced by biomass burning. Experiments with added reagents were performed in an attempt to either diminish or enhance CT complexation. Additionally, absorbance was measured as a function of concentration, temperature, and solvent polarity, in experiments modeled, to some degree, after recent work in which the role of CT complexes in natural waters has also been assessed.32 Together, the present results provide a comprehensive characterization of the role of CT complexes in the primary BrC considered in this work.

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EXPERIMENTAL METHODS BrC Samples. Surrogates of BrC were generated in the laboratory by heating pine and cedar chips in a quartz flow tube with an inner diameter of 22 mm. Clean air (Linde, grade Zero 0.1) was passed through the flow tube at a flow rate of 2 L min−1. The wood chips were of roughly uniform mass (2.2 ± 0.3 g) and shape (50 × 20 × 7 mm). The velocity of air over the wood was about 14 cm s−1, which is at the center of the optimal range for smoldering previously reported.35 A 305 mm length of the quartz tube was heated to 400 °C in a tube furnace (Thermo, Lindberg Blue M); a 270 mm length of the quartz tube extended downstream of the heated section before being connected by conductive silicon tubing to a polycarbonate filter holder housing 47 mm filters composed of borosilicate glass bonded with PTFE (Pall, Emfab). BrC particles were collected only once the fuel began smoldering, after it had dried and blackened, and exhibited a sharp smoldering front. Flaming did not occur during the particle generation, and black carbon (BC) was not observed on the filters. The laboratory-generated BrC collected on filters, which were visibly yellow-brown, was left to dry. Typically, the mass of BrC was 25−50 mg, which was then extracted in 10 mL of one of the following solvents: deionized water (18.2 MΩ cm), methanol (MeOH; Sigma-Aldrich, ≥99%), acetonitrile (ACN; Sigma-Aldrich, ≥99%), or dimethyl sulfoxide (DMSO; SigmaAldrich, ≥99%). Each extract was passed through a PTFE syringe filter with a pore size of 0.45 μm (Whatman) before spectroscopic analysis or reagent addition. Complementary measurements of the absorption and composition of suspended surrogate particles were performed using a photoacoustic spectrometer (Droplet Measurement Technologies) and high-resolution time-of-flight aerosol mass spectrometer (Aerodyne), respectively. Ambient BrC in fine particulate matter (PM1.0) was extracted from punches of filters collected at the East Trout Lake (ETL) atmospheric observatory in central Saskatchewan, Canada, at latitude 54.35° N, longitude 104.98° W, and an elevation of 493 m a.s.l. The facility has been operated by Environment and Climate Change Canada (ECCC) since 2006; it was one of six sites included in a long-term program (2006−2015)36 to measure the composition of carbonaceous B

DOI: 10.1021/acsearthspacechem.9b00116 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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integrated signals at m/z 44 and 60, respectively, to the total signal in the organic fraction mass spectrum.44 Fresh biomass burning aerosol is distinguished by much higher f60 values than observed for secondary organic aerosol, due to the abundance of levoglucosan-like species, which fragment to give the C2H4O2+ ion.44 With a f60 value of about 0.05, as shown in Figure S3, the surrogate BrC meets this criterion. The current value of f60 is higher than that measured during Flight 18 of the ARCTAS campaign,44 for example, but this observation is not surprising since here the source is smoldering wood (composed primarily of cellulose, the precursor of levoglucosan, and lignin) without other plant tissues. BrC has water-soluble and insoluble components. The mass of the laboratory-generated samples collected on filters was large enough to allow precise measurements of the mass fractions extracted into each solvent using gravimetric analysis. For the extraction procedure followed here, 75 ± 2% of the mass of the laboratory-generated samples was soluble in water, irrespective of the fuel, whereas nearly all the mass (>95%) was soluble in MeOH, ACN, and DMSO. The quantitative extraction in DMSO, for example, further indicates that the samples are solely BrC in the absence of BC. The resulting spectra for BrC from pine extracted in the different solvents are shown in Figure S4, in terms of mass absorption coefficient (MAC). The MAC values at 405 nm of the extracts in water and MeOH were 310 ± 40 and 1100 ± 100 cm2 g−1, respectively, which are lower than the typical range of (2−30) × 103 cm2 g−1 for ambient BrC from biomass burning.4 The MAC values increased drastically at lower wavelengths, typical of BrC, such that the MAC value at 365 nm of the extract in water, 1700 ± 200 cm2 g−1, is near the range observed for WSOCs over East Asia, (2−11) × 103 cm2 g−1.45 Assuming additivity and without adjusting the pH of aqueous solutions, the 21−24% of BrC extracted in MeOH, ACN, and DMSO, but not in water, contributes about 70% of the absorbance at 400 nm. Furthermore, extractions in DMSO were the most efficient (99 ± 2%) and gave the highest MAC values, indicating that the water-insoluble compounds are very strongly absorbing. These compounds may include high molecular polyphenolic oligomers.28,29 Previously, fluorescence spectra of CDOM and BrC have been discussed extensively in the context of CT complexes,24,31 and here, spectra were measured for nascent extracts (without adjustment of pH) of the laboratory-generated BrC derived from pine and cedar. The excitation−emission matrices of BrC from the flow tube, as shown in Figure S5, share similarities with previous measurements of ambient BrC,23 suggesting that it is a suitable surrogate. At excitation wavelengths between 320 and 350 nm, the intensity of the emission increases, and the peak wavelength is roughly steady; at excitation wavelengths between 360 and 450 nm, the intensity of the emission decreases, and the peak wavelength increases, as shown in Figure S6. The latter observation may be attributed to either a manifold of CT complex states or superposition of many individual compounds,31 with steadily decreasing quantum yields, likely in a homologous series of oligomers. In contrast to previous observations, the present emission is not featureless, exhibiting several distinct though overlapping bands, highlighted in Figure S6b. Such distinct bands have been observed for CDOM only at excitation wavelengths below 300 nm,46,47 giving rise to emission that is classified as either tyrosine- or tryptophan-like.46,48 The present features, evident at excitation frequencies between roughly 350 and 450

Aldrich, 97%) and 1,3,5-trihydroxybenzene (dihydrate; SigmaAldrich, 97%). Spectroscopic Analyses. Measurements were predominantly of absorbance in the ultraviolet−visible (UV−vis) range. For the strongly absorbing solutions of laboratorygenerated BrC, absorbance was measured on one of two commercial dual-beam UV−vis spectrometers (PerkinElmer, Lambda 25 or 1050), each with a path length of 1 cm. Absorbance at different temperatures was measured on the Lambda 1050 spectrometer, which is equipped with a water circulator and controller to adjust the temperature of the sample; temperature was varied between 15 to 45 °C in intervals of 5 °C. Typically, a spectrum was measured at 15 °C before each spectrum at higher temperatures. The temperature control was validated using a solution of chloranil (SigmaAldrich, 99%) and mesitylene (Sigma-Aldrich, 98%) in heptane (Sigma-Aldrich, ≥99.5%). Measurements of the absorbance of BrC of the same composition in different solvents were facilitated by diluting BrC water extracts 25:1 v/v with the respective solvents. Measurements of absorbance of BrC across a wide range of concentrations were facilitated by extracting three filter samples in MeOH, concentrating under a stream of nitrogen, dissolving in a minimal volume (1.5 mL) of DMSO, and repeatedly diluting the solution by a factor of 2. The use of DMSO permitted a very high initial concentration. Serial dilution was also performed for BrC extracts in water, but with a lower initial concentration. Before and after the selective reduction and acetylation of weakly absorbing solutions of ambient BrC, absorbance was measured on a modular spectrometer consisting of broadband light source (Ocean Optics, DT-Mini-2), a liquid waveguide capillary cell (World Precision Instruments, LWCC-3050) with a path length of 50 cm, and a UV−vis spectrometer (Ocean Optics, USB2000+). Fluorescence of nascent BrC extracts was measured on a commercial spectrometer (PerkinElmer, LS-55); the excitation wavelength was varied from 250 to 550 nm in intervals of 10 nm. Following the acetylation of phenol, 1H NMR spectra were recorded on a 400 MHz spectrometer (Bruker, AVANCE III) in D2O (Sigma-Aldrich, ≥99.9%).

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RESULTS AND DISCUSSION Characteristics of BrC Surrogates. For the wide range of experiments reported here, it was essential to use samples with reproducible origin and composition, as well as appreciable mass. Accordingly, laboratory surrogates of BrC were used throughout, in addition to select ambient samples. To generate these surrogates, we designed and implemented a flow-tube particle source to allow the conditions that control smoldering, including temperature and flow velocity,35 to be finely tuned. Similar combustion sources have been used in previous laboratory studies of BrC.12,43 Using a photoacoustic spectrometer, the absorption coefficients at 405 and 781 nm of suspended aerosol particles generated from smoldering pine in the source were measured. Negligible absorption at 781 nm, as shown in Figure S2, indicated that the aerosol was solely BrC in the absence of BC, as expected under the current smoldering conditions. Furthermore, the large absorption Angström exponent, about 5.6, is within the range observed for ambient BrC.4 Using a high-resolution time-of-flight aerosol mass spectrometer, the composition of the suspended surrogate particles derived from pine was characterized in terms of the parameters f44 and f60, which are the ratios of the C


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Figure 1. Absorbance of initial, reduced, and acetylated BrC from (a) pine, (b) cedar, and (c) ambient BrC aerosol influenced by biomass burning.

reduction are consistent with earlier work on the contributions of CT complexes. The decrease in absorbance seen in the reduction experiments could also arise via the transformation of individual constituents, such as quinones, that are strongly lightabsorbing on their own, i.e., without participating in CT complexation. For this reason, an alternate approach was taken to avoid bleaching due to the loss of individual quinone chromophores via the reduction procedure with NaBH4. In particular, selective acetylation of hydroxyl groups with Ac2O, adapted from a long-established procedure in organic synthesis,50 was performed. This transformation is also expected to disrupt CT complexation because the ester groups are not as strongly electron-donating as the hydroxyl groups, i.e., the electron donor capabilities of an OH group on an aromatic ring will be reduced. Furthermore, the ester groups are not likely to participate in strong hydrogen bonding. In fact, acetylation of kraft lignin, using Ac2O in pyridine, has been shown to diminish CT complexation.49 The current method was first evaluated by the reaction of phenol to give phenyl acetate. The formation of phenyl acetate was monitored by 1H NMR. After the addition of Ac2O, three new features appeared above 7 ppm, as shown in Figure S7, and the combined integrated intensity of these aromatic protons indicates a yield of roughly 25%. Consequently, for solutions of BrC, this procedure is expected to also result in appreciable conversion of hydroxyl groups into acetyl groups. In contrast to reduction, acetylation of BrC (Figure 1) results in a slight increase in absorbance at 400 nm (and at lower wavelengths for ambient BrC). Incorporating acetyl groups into the BrC is not expected to contribute to the observed increase in absorbance. For comparison, consider the absorption band of phenyl acetate that is closest to the visible region, which is blue-shifted relative to that of phenol.51,52 Though the origin of the increased absorbance is beyond the scope of this study, the observation that acetylation does not decrease the absorbance indicates either that CT complexation does not occur or that it is not sufficiently disrupted by acetylation. To complement the attempts to diminish the proposed CT complexation, attempts were also made to enhance the complexation with the addition of known electron acceptor and donor species. Previously, the addition of a model electron acceptor (3,5-di-tert-butyl-1,2-benzoquinone) to kraft lignin dissolved in 2-methoxyethanol resulted in enhanced absorption, indicating free phenolic groups were in excess of the quinone groups in the lignin.49 Here, a model acceptor (1,2-

nm, do not rule out CT complexes since a broad featureless band could be an underlying component of the superposition, but they are unequivocal evidence that smoldering wood generates individual species capable of absorbing and emitting in the visible region. The absence of distinct bands in the earlier excitation−emission matrices of ambient BrC23,24 may have been due to the presence of many more constituents from a much wider range of sources. Consequently, excitation at a given wavelength may have resulted in many more emission bands that were accordingly more closely packed and unresolved. Altogether, the above results indicate that the BrC extracts from wood pyrolysis are a suitable surrogate for atmospheric samples. As well, there is some evidence that individual chromophores contribute to the observed absorptivity and fluorescence. Reduction, Acetylation, and Addition of Acceptors and Donors. A series of experiments with added reagents was conducted to assess the presence of CT complexes. In particular, attempts were made to either diminish the proposed CT complexation by altering specific functional groups or enhance it by adding effective acceptor and donor species. For the former approach, a well-established method involving selective reduction with NaBH4 was conducted, and a new procedure involving selective acetylation with Ac2O was also attempted. Models of CT complexes in CDOM and BrC involve functionalized aromatic groups participating in noncovalent interactions, either hydrogen bonding or π−π stacking.23,31 A common method to gauge the contribution of CT complexes is selective reduction with NaBH4,30,49 which transforms carbonyl groups into hydroxyl groups. In the proposed CT complexes, carbonyl-substituted aromatic rings are the electron acceptors, so their reduction to hydroxyl-substituted aromatic rings, excellent electron donors, is expected to disrupt the complexation and decrease absorbance.31 Selective reduction with NaBH4 has been used to investigate the role of CT complexes in CDOM,30 BrC,23 and kraft lignin.49 Here, reduction of the pine- and cedar-derived BrC from the flow tube resulted in significantly decreased absorbance (Figure 1), consistent with earlier results of ambient BrC.23 Reduction of WSOCs extracted from ambient BrC aerosol influenced by biomass burning also resulted in decreased absorbance, though not as significant as for the surrogates; for example, the most significant decrease for ambient BrC occurred at 350 nm, where 23% of the initial absorbance may be attributed to CT complexes. To this extent, the present results of the selective D


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sequently, relative absorbance would scale with concentration to the powers of one and two at the respective concentrations. In contrast to this predicted trend, the experimental relative absorbance scales with concentration to the power of one across 4 orders of magnitude; the lowest concentrations are representative of those of organics in cloudwater.56 For more details about the relative absorbance as a function of concentration, refer to Figure S8. To obtain very concentrated solutions, multiple samples of BrC derived from pine and cedar were extracted in MeOH, dried, and combined in DMSO. (Note that the absorbance of extracts of ambient BrC was too small to permit serial dilution experiments of this type.) For samples of pine-derived BrC in water, such high concentrations could not be obtained due to comparatively low solubility, but the relative absorbance did scale linearly with concentration across the smaller range that was examined. Even if CT complexes were to contribute half of the absorbance only at a much higher concentration (e.g., 10 000 mg L−1), the contribution of the complexes would be discernible across this range of concentrations. In a previous study, the contribution of CT complexes to the absorptivity of ambient BrC was estimated to be about 50%,23 likely at concentrations lower than 100 mg L−1, so the predicted contribution of intermolecular CT complexes would predominate across most of the experimental range explored in this work. Based on these considerations, intermolecular CT complexes do not contribute to the absorptivity the laboratory-generated BrC surrogates investigated here. If the CT complexes potentially contributing to the absorptivity of BrC were solely intramolecular, their contribution would depend on temperature and solvent but not concentration. From the van’t Hoff equation, the equilibrium constant of complexation, which is exothermic, would decrease as temperature increased, and complexes would dissociate, as shown for the well-known chloranil−mesitylene complex57,58 in Figure S9. Since the potential intramolecular CT complexes in BrC are expected to involve π−π interactions, the enthalpy of formation may be taken as that of the p-benzoquinone− benzene complex (i.e., −7.5 kJ mol−1 in n-heptane59), as assumed previously for CDOM.32 If all the absorption at a given wavelength were due to CT complexes with this enthalpy of formation, the relative absorption would decrease to 0.74 as temperature increased from 15 to 45 °C. If only 50% of the absorption at 15 °C and a given wavelength were due to these CT complexes, the relative absorption would decrease to 0.87. In contrast to these significant predicted changes, the observed changes as a function of temperature are minimal, as shown in Figure 3. For pine- and cedar-derived BrC, extracts in DMSO at a range of concentrations gave similar results, so they are grouped together. These samples exhibit a small but systematic decrease in absorbance with increasing temperature, consistent with a 15% contribution of CT complexes to the absorbance at 15 °C. Pine-derived BrC and ambient BrC aerosol influenced by biomass burning extracted into water were also investigated, but these samples did not exhibit any temperature dependence. The discrepancy between the water and DMSO extracts is interesting. It could arise from differences in the BrC−solvent interactions,23 favoring CT complexes in DMSO, but it could also be due to the differences in the composition of the BrC extracts; for example, most of the 25% of the BrC mass not extracted in water is extracted in DMSO, and this strongly absorbing fraction is likely composed of high molecular weight polyphenolic compounds that could facilitate intramolecular

naphthoquinone) was added to BrC in water, but no additional absorbance attributable to CT complexes was observed. This observation could indicate that any phenolic groups in the BrC do not participate in CT complexes with the model acceptors or that there is not a significant excess of phenolic groups. However, addition of a model donor (1,3,5-trihydroxybenzene) to BrC in water also did not result in enhanced absorption. Concentration, Temperature, and Solvent Effects. Together, the experiments described above are at best inconclusive regarding the presence of CT complexes, and they are instead more consistent with individual chromophores giving rise to the BrC absorbance. Accordingly, additional studies were conducted. In particular, since CT complexation is expected to depend on concentration (if intermolecular complexes are involved), temperature, and solvent polarity, additional experiments were performed to probe the effects of these parameters on the absorptivity of the BrC extracts. If intermolecular CT complexation contributed significantly to the absorptivity of the BrC extracts, decreasing the concentration of the extracts would lead to a decrease in the absorbance greater than that expected from dilution, as complexes dissociated. For example, some natural pigments, like malvidin 3-glucoside, self-aggregate53 or form colored complexes with other species.54 They exhibit absorbance that does not scale linearly with concentration, with implications on the absorptivity of wine,55 for example. If the potential CT complexation in the BrC extracts is similarly assumed to be bimolecular, decreasing the concentration by a factor of 2 would be expected to decrease the concentration of the complex by a factor of 4. However, the relative change in the absorbance would also depend on the contribution of the complex at a given concentration. As shown in Figure 2, if intermolecular CT complexes were to contribute half of the absorbance of the BrC extracts at a mass concentration of 1000 mg L−1, they would contribute negligibly to the absorbance at much lower concentrations but predominantly at much higher concentrations. Con-

Figure 2. Relative absorbance of laboratory-generated BrC as a function of concentration for MeOH extracts in DMSO and water extracts. The solid line shows a fit to the data in which absorbance scales linearly with concentration. The dashed curves show the expected trends if intermolecular CT complexes were to contribute 50% of the total absorbance at (i) 100, (ii) 1000, and (iii) 10 000 mg L−1. E


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polar aprotic solvents (ACN and DMSO) exhibit deviations from water, perhaps due to drastic differences in solvation, the polar protic solvents (MeOH and water) exhibit the same absorbance, despite a significant difference in relative permittivities. This agreement is consistent with little contribution of CT complexes to the total absorptivity of the water-soluble fraction of the laboratory-generated BrC surrogates. Atmospheric Implications. The potential role of CT complexes in BrC aerosol was investigated using a wide range of chemical, spectroscopic, and physical tests. Consistent with an earlier study that proposed a significant role for CT complexes,23 selective reduction resulted in a drastic decrease in absorbance, which has previously been attributed to the loss of carbonyl groups involved in complexation.23,30 However, other lines of evidence indicate that the role of CT complexes in the BrC considered here is minimal. Acetylation resulted in a small increase in absorbance, rather than the decrease expected of CT complexes,49 and acceptor and donor addition resulted in no change in absorbance. Fluorescence spectra of aqueous solutions of laboratory-generated BrC exhibited discrete bands, demonstrating that individual compounds absorb and emit at visible wavelengths. Importantly, the absorbance of extracts of ambient and laboratory-generated BrC in water did not change with temperature, indicating that CT complexes may be unlikely to form among WSOCs from these types of primary BrC; the absorbance of extracts in DMSO decreased slightly as temperature increased, indicating CT complexes may account for a small fraction (about 15%) of the total absorptivity of BrC at 15 °C. Finally, since the absorbance of extracts scaled linearly with concentration, this potential contribution must arise from intramolecular, rather than intermolecular, CT complexes. The atmospheric implications of these results are significant. To constrain the climate effects of BrC, it is vital to understand how its absorptivity varies with composition and processing in the atmosphere. Consider primary aerosol particles freshly emitted from biomass burning. These particles may subsequently undergo one or more processes, including uptake of water, ingestion into clouds, and lifting to higher altitudes, before they are removed by deposition. Here, the only physical evidence for CT complexation was observed for DMSO extracts, indicating that CT complexes, if present, likely contribute more to the absorptivity of the organic phase of biomass burning aerosol than the aqueous phase resulting from uptake of water or ingestion into clouds. In aerosol particles and cloud droplets, organic compounds are more concentrated than CDOM in natural waters, with the potential to enhance intermolecular complexation. As discussed above, intermolecular CT complexes did not contribute to the absorptivity of the surrogates of primary BrC considered here. Previously, intermolecular CT complexes were found to contribute to the absorptivity of oligomeric products of aqueous pyruvic acid,25 a surrogate of secondary BrC. From these observations, it may be that significant processing in the atmosphere, resulting in appreciable secondary BrC, is required before intermolecular CT complexation plays an important role, even in concentrated aerosols. Since aerosols may be lifted to higher altitudes, the effects of ambient temperature must also be considered. If the aerosol were lifted from the surface to 5 km, the temperature could decrease from about 15 °C to −20 °C, and the contribution of intramolecular CT complexes to the absorptivity of the organic phase would increase from 15% to

Figure 3. Relative absorbance of laboratory-generated and ambient BrC as a function of temperature, averaged across 365−405 and 600− 650 nm for weakly- and strongly absorbing solutions, respectively. The dashed curves show the expected trends if intramolecular CT complexes with an enthalpy of formation of −7.5 kJ mol−1 were to contribute (i) 100, (ii) 50, and (iii) 15% of the absorbance at 15 °C. Error bars indicate one standard deviation for triplicate experiments, except in the case of ambient BrC, for which error bars indicate one standard deviation across the 365−405 nm range for one sample.

CT complexation. The agreement between the water extracts of the laboratory-generated and ambient BrC suggests that even intramolecular CT complexation is unlikely to contribute significantly to the absorptivity of the aqueous phase in aerosols or cloud droplets. Similarly, no temperature dependence has been observed for the absorbance of CDOM in natural waters.32 Finally, an examination of the solvent dependence of the BrC absorbance provides supporting evidence for the minimal contribution of CT complexes. Polar solvents shift CT absorption bands to lower wavelengths as relative permittivity increases; solvent molecules are oriented to stabilize the ground state and, on the time scale of electronic transitions, cannot move to reorient around the excited state, which is consequently destabilized.27 To probe the effect of solvent on BrC of the same composition, a water extract of pine-derived BrC was divided and diluted (25:1) to the same mass concentration in polar solvents of increasing relative permittivities: MeOH, ACN, DMSO, and water. Again, the absorbance of extracts of ambient BrC was too small to permit dilution. The resulting spectra for the pine-derived BrC solutions are shown in Figure S10. The absorbance at 365 nm, relative to that in water, is shown in Figure 4. Though the

Figure 4. Relative absorbance of pine-derived BrC at 365 nm as a function of the polarity of the solvent, in terms of relative permittivity. Error bars indicate one standard deviation for triplicate experiments. F


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ACS Earth and Space Chemistry 21%, again assuming an enthalpy of formation of −7.5 kJ mol−1, as in Figure 3. Even at higher altitudes, then, CT complexes are expected to have a much smaller role than individual constituents of the BrC considered here. The results also have implications for the analysis of BrC samples. The absorptivity and composition of BrC are often characterized following extraction into water or MeOH, as for samples from filter or particle-into-liquid collection, so solvent effects must be considered. For instance, the MeOH extract of ambient BrC is generally much more absorbing at visible wavelengths than the water extract.43,60 The present results suggest that most of this difference arises from more efficient extraction of individual compounds that are highly absorbing and poorly soluble in water.60 Likely, only a small fraction of the additional absorbance in MeOH arises from CT complexes. Future studies of the thermodynamics of CT complexation of BrC of the same composition dissolved in different solvents could further probe whether it is the composition of the BrC or the BrC−solvent interactions23 that facilitate the small contribution of intramolecular CT complexes in these solutions. Directly measuring the absorptivity of suspended BrC particles using photoacoustic spectroscopy,61,62 for example, avoids these complications due to solvents. However, solvent effects cannot be avoided during analytical separations using high-performance liquid chromatography (HPLC).20,22 It is worthwhile noting that even the minimal CT complexes in the primary BrC considered here are not expected to contribute to the missing absorptivity observed in HPLC analyses because the complexes are intramolecular, and the donor and acceptor moieties are not separable. There are many parallels between the models of CT complexes in atmospheric aerosols23,24 and in natural waters,30,31 so the current results must be placed in the context of the ongoing debate regarding CT complexes in CDOM.32−34 Specifically, a model of closely packed contact CT complexes,33 which form instantaneously upon collision and quickly dissociate, has been invoked as an alternate explanation of the lack of temperature dependence for the absorbance of CDOM.32 While it is true that absorbance due to transient contact CT complexes63 is largely independent of temperature, since their enthalpies of formation are negligible,64 it is unlikely that highly substituted aromatic rings in phenolic oligomers would interact so weakly. From this perspective, the absorbance measurements reported here and elsewhere32 across a range of 30 °C are important constraints on the role of the CT complexes in atmospheric aerosols, cloud droplets, and natural waters. Furthermore, since the proposed CT complexes are accessible to selective reagents and sensitive to changes in pH, they must be accessible to the solvent.34 For contact CT complexes, the polarity of the solvent would influence the position of the absorption band,27 and the viscosity of the solvent would influence the intensity.65 (Note that viscosity does not influence the absorbance of bound CT complexes.65) These predictions are in contrast with the agreement between the spectra of water-soluble BrC diluted in MeOH and water reported here (Figure 4) and the agreement between the spectra of CDOM in mixtures of glycerol and water of different viscosities reported elsewhere.32 Based on these observations, contact CT complexes are unlikely to contribute significantly to the absorptivity of atmospheric BrC. Taken together, the results of this multifaceted study suggest that the role of CT complexes in the primary BrC considered

here is minimal. Consequently, important individual constituents may yet be identified to account for the commonly observed missing fraction of the total absorptivity of ambient BrC.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.9b00116. Back trajectories during collection of the first ambient aerosol filter sample (Figure S1). Relative absorption coefficients at 405 and 781 nm of suspended surrogate aerosol particles (Figure S2). Composition of suspended surrogate aerosol particles in terms of f44 and f60 (Figure S3). Mass absorption coefficients (Figure S4), excitation−emission matrices (Figure S5), and fluorescence spectra (Figure S6) of laboratory-generated BrC. 1H NMR spectrum of phenol solution after acetylation (Figure S7). Absorption spectra of BrC in serial dilution experiments (Figure S8). Validation of temperature control using mesitylene and chloranil (Figure S9). Absorption spectra of BrC in solvent dependent experiments (Figure S10) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.P.D.A) E-mail: [email protected]. *(E.G.S.) E-mail: [email protected]. ORCID

Rachel F. Hems: 0000-0002-4990-1374 Jonathan P. D. Abbatt: 0000-0002-3372-334X Elijah G. Schnitzler: 0000-0001-9477-3992 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Lin Huang and Wendy Zhang (ECCC) for providing the ambient BrC samples and sharing thermaloptical transmittance results and Jared Mudrik (University of Toronto) for assisting with temperature-dependent UV−vis absorbance measurements. We also thank Kristopher McNeill (ETH Zurich) for helpful discussions about early results. This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). A.T. gratefully acknowledges a summer internship from the Centre for Global Change Science. E.G.S. gratefully acknowledges a postdoctoral fellowship from NSERC.

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REFERENCES

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Contribution of Charge-Transfer Complexes to Absorptivity of Primary

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