Special issue review Received: 16 September 2014

Revised: 19 January 2015

Accepted: 27 January 2015

Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/mrc.4227

Unraveling the structural features of organic aerosols by NMR spectroscopy: a review Regina M. B. O. Duarte* and Armando C. Duarte Our limited understanding of the effect of organic aerosols (OAs) on the climate and human health is largely because of the vast array of formation processes and sources that produce a multitude of molecular structures and physical properties. The need to unravel the enormous complexity and heterogeneity of OAs and thus understand their effects on the climate and human health has led to the development of different off-line methods based on the use of advanced analytical techniques. Within this context, nuclear magnetic resonance (NMR) spectroscopy has become essential for acquiring detailed structural characterization of the complex natural organic matter contained in atmospheric aerosols. In this article, we present a critical review on the application of NMR spectroscopy in OAs (primary and secondary) studies, focusing mainly on the water-soluble organic fraction, and how NMR has impacted our knowledge on atmospheric organic matter. A major emphasis is given on the wealth of chemical information that solid-state and multi-dimensional solution-state NMR can provide, including the sources, formation pathways, seasonal, and regional characterization of atmospheric OAs. Finally, major challenges are discussed and recommendations for future research directions are proposed. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: NMR; 1H; 13C; atmospheric aerosols; organic carbon; natural organic matter; water-soluble organic matter; structural composition; source apportionment

Introduction The carbonaceous fraction of atmospheric aerosols constitute more than 40% of the fine particulate matter (PM2.5, particles with aerodynamic diameter less than 2.5 μm) in urban and continental areas.[1–3] This carbonaceous fraction is often characterized according to the elemental (EC) or organic carbon (OC) fraction and water-solubility. As recently reviewed,[4] the growing interest in water-soluble organic matter (WSOM) is fuelled by the realization that this fraction affects the properties that determine the aerosols’ ability to act as cloud condensation nuclei.[5,6] In addition, the WSOM can significantly contribute to the absorption of solar radiation, and thus atmospheric heating and global climate change.[7–9] Furthermore, the wet deposition fluxes of atmospheric WSOM indicates that it may be an important temporal source of OC to surface waters and likely plays a vital role in the global carbon cycle.[10] Our understanding of the fate and impacts of aerosol WSOM on the climate and human health is limited by the uncertainty surrounding their mechanisms of formation, sources as well as source-related properties. Biomass burning and secondary formation (involving both anthropogenic and biogenic volatile organic compounds) are considered to be major sources of aerosol WSOM.[11] To further enhance the complexity of this aerosol fraction, once in the atmosphere, the particulate organics can become increasingly oxidized, less volatile and more hygroscopic and thus more water soluble.[3] Nevertheless, determining the structural composition of aerosol WSOM is needed to increase the current understanding of its role in various atmospheric processes,[12] and as recently reviewed, there is no universal technique for aerosol WSOM analysis.[4] However, several different off-line and on-line analytical methodologies have been developed and applied to simplify the complexity of WSOM by unraveling the structure and composition of this aerosol component.[4] Nuclear magnetic resonance (NMR) spectroscopy fits into the group of off-line techniques

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that allow for analysis of most of the WSOM mass while providing resolution on functional groups and substructural components.[4,12] This spectroscopic technique can be used to obtain information about chemical bonding modes of H, C, N or even P nuclei within complex environmental organic matrices, being more informative that, e.g. infrared spectroscopy. The level of chemical resolution attained by NMR spectroscopy also contrast from those obtained by techniques that provide information at the molecular level (e.g. liquid or gas chromatography coupled to mass spectrometry), which are usually able to account for only a small fraction (10–20%) of the mass of aerosol WSOM.[4] When questioning which method appears to hold the greatest potential for investigating the structure and the composition of aerosol WSOM, the answer lies in the level of information desired. If one is interested in linking the molecular composition of aerosol WSOM to its primary and/or secondary origins, then techniques coupling the molecular speciation with high time resolution may be all that are needed. However, if one aims at understanding the chemistry of aerosol WSOM, then a multifaceted approach for targeting structural average features (e.g. functional groups) should be developed. In this regard, the reader is encouraged to consult the comprehensive review of Duarte and Duarte[4] to obtain a critical assessment and specific details on the advantages and disadvantages of using such panoply of sophisticated analytical tools for the analysis of aerosol WSOM. Undoubtedly, during the past 15 years, NMR spectroscopy has gained widespread popularity as a method for structural

* Correspondence to: Regina M. B. O. Duarte, Department of Chemistry, CESAM, University of Aveiro, 3810-193 Aveiro, Portugal. E-mail: [email protected] Department of Chemistry, CESAM, University of Aveiro, Aveiro, Portugal

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R. M. B. O. Duarte and A. C. Duarte characterization of atmospheric WSOM. Figure 1 highlights seven NMR experiments that are typically applied to study such complex atmospheric organic matrices – both in the solid-state and solutionstate. Specifically, it focuses on how going from 1D to 2D NMR experiments both reduces spectral overlap and provides connectivity information of atoms belonging to the same compound. This review intends to convey a critical assessment of what has been learned about aerosol WSOM using these different NMR techniques. This manuscript highlights the recent review works of Duarte and Duarte[4,12] and Chalbot and Kavouras,[13] as well as papers that were not included in our previous published reviews. After which, 2D solution NMR approaches are discussed in regards to the interpretation of the molecular structural features, primary sources, and secondary formation of atmospheric organic aerosols (OAs). Finally, major challenges are discussed and recommendations for future research directions are proposed.

1D NMR studies of aerosol WSOM Application of 1D solution-state 1H NMR spectroscopy To date, 1D solution-state proton NMR (1H NMR) technique has been the most widely used technique in aerosol WSOM studies.[4,12–26] The reason for this is twofold: (i) it allows a rapid screening and determination of the general structural properties of the complex aerosol WSOM and (ii) it provides a semiquantitative overview of the distribution of different functional groups with C–H bonds.[12] These two major benefits of the 1D solution 1H NMR technique further adds to the possibility of obtaining an adequate 1H NMR spectrum of aerosol WSOM samples at concentrations as low as 2 mg of sample per milliliter of solvent using the standard 5 mm NMR tubes. This amount of sample is much lower than that required for solid-state 13C NMR analysis, discussed latter. As a result, sample integration times can be reduced to a few hours (e.g. 7 h), allowing the collection of more than one aerosol sample per day. The aerosol WSOM sample preparation is also very simple, involving the extraction of soluble materials from the collection substrates by sonication in the solvent (i.e. H2O), drying, and redissolving in the deuterated solvent. On the other hand, in more specific applications using the standard 5 mm NMR probe, e.g. in 2D NMR studies of aerosol WSOM discussed in the succeeding sections, higher sample concentrations are needed (typically more than 20 mg per milliliter of solvent). In such cases, high-volume samplers operating at flow rates up to 1.13 m3 min 1

and long-time integrated samples (lasting at least 24 h) are required to collect sufficient quantities of air particulate matter. These relatively long sample integration periods inhibits investigating changes in aerosol WSOM composition over shorter time scales. Sample alteration during long collection times is also a potential drawback.[27] One possible way of overcoming this scenario is using microcoil technology, which has been recently considered an excellent improvement for the analysis of mass limited environmental samples.[28] Using microcoil NMR tubes (e.g. 1.7 mm), one can run multidimensional NMR experiments, including 1D NMR, of complex environmental organic matrices using 1–2 mg of organic material dissolved in a few microliters of deuterated solvent (~50 μL).[29] Indeed, the use of microcoils has the potential of offering sensitivity gains (on a mass basis) compared with a standard 5 mm coil, and it holds great promise for a more widespread application of multidimensional NMR spectroscopy for aerosol WSOM compositional studies on higher resolved time scales. In most of the aerosol WSOM studies using 1D 1H NMR spectroscopy, deuterium oxide (D2O) is the solvent of choice. One of the limitations of using D2O as a solvent is that the hydrogen atoms of carboxyl and hydroxyl groups rapidly exchange with a deuterium of D2O, rendering them invisible by 1H NMR.[12] Although dimethyl sulfoxide-d6 (DMSO-d6) or methanol-d4 (MeOH-d4) can also be employed as solvents for the 1H NMR analyses, the WSOM samples may not be completely dissolved in these organic solvents, which puts into question whether or not the spectrum is truly representative.[30] In addition, the spectrum is deconvoluted when the sample is not thoroughly dried, as a clear water (HOD) signal at δH = 4.8 ppm overlaps with signals from the WSOM, along with the organic solvent signals (e.g. at δH = 2.5 ppm for DMSO-d6, and at δH = 3.31 ppm for MeOH-d4). Nowadays, however, it is possible to suppress this residual HOD peak by applying water suppression pulse sequences, which are thoroughly addressed in the review work of Simpson and coworkers.[28] With the advent of higher field systems (500–600 MHz), there have been substantial improvements in the NMR signal-to-noise ratio/sensitivity, which determines the number of scans needed (up to 1000 scans, depending on the sample concentration). For example, Tagliavini et al.[31] estimated that 800 scans, corresponding to an overall acquisition time of about 55 min with a 600 MHz instrument, is adequate to detect levoglucosan at a concentration in the air of 0.04 μg m 3. Regardless of the deuterated solvent employed, the typical 1H spectrum of the WSOM matrix is very complex, consisting of broad bands (arising from structural assemblies of organic molecules),

Figure 1. Representation of the range of NMR experiments currently employed for the study of aerosol WSOM in solid and solution-state. Definition of the acronyms is provided in the text. Adapted from the works of Duarte and Duarte.[4,12]

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NMR applications in organic aerosols studies superimposed by a relatively small number of sharp peaks (arising from either small molecules or highly mobile segments of the structural assemblies) dispersed over a small chemical-shift range (0–10 ppm).[12] The end result is that chemical group assignments in aerosol WSOM samples can only be made at the macroscopic level. Typically, four main categories of functional groups carrying C–H bonds are identified in aerosol WSOM: (i) H–C: aliphatic protons in extended alkyl chains (δH = 0.5–1.9 ppm), (ii) H–C–C=: protons bound to aliphatic carbon atoms adjacent to unsaturated groups, such as alkenes, carbonyl, imino or aromatic groups, as well as protons in secondary (H–C=NR2) and tertiary (NR3) amines (δH = 1.9–3.2 ppm),. (iii) H–C–O: protons bound to oxygenated aliphatic carbons atoms, such as aliphatic alcohols, ethers, and esters (δH = 3.3–4.1 ppm), and (iv) Ar-H: aromatic protons (δH = 6.5–8.3 ppm). There are additional studies of aerosol WSOM also reporting the presence of a fifth resonance region assigned to acetalic (O–CH–O) and vinylic (=C–H) protons (δH = 5.0–5.5 ppm).[17,18,31] Individual water-soluble organic compounds have also been identified on the basis of their chemical shift, namely: methanesulfonic acid (MAS, a singlet at δH = 2.81 ppm), dimethylammonium (DMA, a singlet at δH = 2.72 ppm), and diethylammonium (DEA, quartet and triplet at δH = 3.08 ppm and δH = 1.28 ppm), which were identified as secondary organic aerosol (SOA) components in marine aerosols;[15,21,22,32] levoglucosan (an unambiguous tracer of cellulose combustion),[14,21] sucrose, fructose, and glucose (with a biological origin, i.e. pollen),[20,21] with sharp peaks between δH = 3.3 and 5.4 ppm; aliphatic and aromatic amino acids (e.g. isoleucine, valine, arginine, proline, tyrosine, and histidine), and vitamins C (e.g. ascorbic acid) and B (e.g. thiamin B1 and riboflavin) in airborne pollen particles[20]; and phthalic acid (resonance at δH = 7.58 and 7.73 ppm) and its isomer, terephthalic acid (resonance at δH = 8.01 ppm), which were associated with vehicular exhausts from urban areas.[21] Source apportionment of aerosol WSOM using solution-state 1H NMR spectroscopy While the primary motivation for applying solution-state 1H NMR spectroscopy is to acquire valuable information as to the overall composition of aerosol WSOM, spectra interpretation can be further extended by the quantitative integration of each 1H NMR spectral region. After correction for possible baseline drift and calibration to an internal standard (typically 3-trimethylsilylpropane sulfonic acid (DSS), sodium 3-trimethylsilyl-2,2,3,3-d4-propanoate (TSP-d4), or dibromomethane (CH2Br2)),[12] it has been concluded that protons in aliphatic structures are the dominant moieties in aerosol WSOM (30–60% of the total 1H content), followed by oxygenated aliphatic compounds and unsaturated aliphatic groups (5–27%), and only a minor contribution from aromatic groups (typically, below 15%).[12,13] Despite holding similar 1H functional groups, the obtained NMR data allowed seasonal, spatial and temporal characterization of aerosol WSOM.[14–20,22–24,32–35] Functional group distribution has proven useful for molecular modeling,[25] study of SOA formation,[25,32,35] and source apportionment[17,18,21,26] of aerosol WSOM. Probably, the most striking study that shows the robustness of 1 H NMR spectroscopy to differentiate aerosol WSOM sources – on the basis of compositional information – was introduced by Decesari and coworkers.[26] In their study, the authors showed that 1 H NMR spectra could be interpreted as fingerprints for source contribution analysis by evaluating and plotting the carbon-weighted ratios of H–C–C=O/total aliphatics versus O–C–H/total aliphatics.

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The fraction of H–C–C=O (i.e. carbonylic/carboxylic aliphatic groups) was indirectly estimated from the relative intensity/ integration of the 1H signal adjacent to unsaturated carbon atoms (H–C–C=), once the contribution from aromatic 1H signal was subtracted. The fraction of O–C–H was estimated from the integration of the 1H signal bound to oxygenated saturated aliphatic carbons (e.g. hydroxyls). The total aliphatic carbon includes the saturated (H–C–O) and unsaturated (H–C–C=O) oxygenated functional groups, nonoxygenated groups (i.e. the benzylic groups (H–C–Ar)), and the unfunctionalized alkyls (H–C). The obtained fingerprints are schematically illustrated in Fig. 2, where the source boxes in solid black line were delineated on the basis of the data reported by Decesari et al.[26] for well-defined aerosol WSOM samples, i.e. marine OA, SOA, and biomass burning aerosols. Using an identical approach, Ziemba et al.,[16] Cleveland et al.,[17] Shakya et al.,[18] and Chalbot et al.[21] were able to update and/or create new source boundaries for aerosol WSOM samples collected at coastal,[16] urban,[17,21] and semi-rural[18] areas. The source apportionment analysis of Ziemba et al.[16] at a costal location suggested that aerosol WSOM has contributions from SOA/aged OA/marine OA, in addition to a potential biomass burning source. Cleveland et al.[17] reported that aerosol WSOM samples collected at an industrial urban center differ from the three established source regions, especially in the contribution of H–C–C=O, underscoring a lower level of oxidation of the urban aerosol WSOM samples. Also for an urban area, Chalbot et al.[21] concluded that their aerosol WSOM samples have contributions from both urban and biomass burning emissions. Based on the identification of 1H NMR resonances in the δH = 3.30–4.15 ppm range, typically assigned to carbohydrate-like structures, but also using the 1H NMR profile of atmospheric pollen, Chalbot et al.[21] also suggested the possible contribution of biological aerosols to coarse air particles (dp >3.0 μm), yielding low H–C–C=O/total aliphatics ratios (between 0.15 and 0.25) with a clear separation from combustion-related processes (between 0.30 and 0.50). For ambient aerosol samples collected at semi-rural location,[18] there have been reported difficulties in achieving a source attribution on the basis of the 1H NMR data. Although SOA

Figure 2. Schematic representation of functional group distribution of different aerosol WSOM samples. Boxes in solid black line representing specific sources of marine OAs, SOA, and biomass burning aerosols were recreated based on Decesari et al.,[26] while dashed gray line shapes 1 represent H NMR fingerprints retrieved from Cleveland et al.,[17] Shakya et al.,[18] and Chalbot et al.[21]

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R. M. B. O. Duarte and A. C. Duarte and biomass burning emissions could explain some of the 1H NMR signatures, most of the WSOM samples fell outside of these predefined regions, particularly in terms of the H–C–C=O/total aliphatics ratio, whose values were lower than those reported by Decesari et al.[26] Therefore, the usefulness of 1H NMR analysis as a fingerprinting tool for source contribution assessment is seemingly unquestionable. Nevertheless, atmospheric researchers should also be aware that 1H NMR spectroscopy has its own limitations, particularly in terms of its low sensitivity for detecting functional groups that do not carry protons (e.g. substituted aromatic compounds) or contain acidic functions with rapidly exchangeable protons (e.g. carboxylic acids).[36] In this regard, Decesari et al.[26] reported a carbon content of 86% of the total WSOC measured by a total organic carbon analysis method, which indicates that some structures of the WSOM samples remain unidentified through solution-state 1H NMR spectroscopy. Because a highly variable fraction (14–40%) of the aerosol WSOM still eludes 1H NMR detection,[25,25,31] it is questionable whether the obtained results truly represent the chemical nature of these samples.[12] Multivariate data analysis of 1H NMR data Factor analysis has recently emerged as an alternative approach for H NMR data processing, with the purpose of dealing with the inherent complexity and variability of the spectral data for subsequent identification and quantification of aerosol WSOM sources. Decesari et al.[15] employed hierarchical cluster analysis, principal components analysis (PCA), positive matrix factorization (PMF), non-negative matrix factorization (NMF), and multivariate curve resolution (MCR) using alternating least square (MCR-ALS) and weighted alternating least square (MCR-WALS) methods to apportion the sources of aerosol WSOM in the North Atlantic boundary layer air. Factor analysis of a data set of 21 1H NMR samples apportions less than 15% of WSOM to components related to pollution sources, with most of the variability being explained by factors linked to methanesulfonic acid, amines, and to oxygenated aliphatic compounds witnessing formation or transformation processes of OAs in the marine boundary layer.[15] While the first two factors are indicative of background marine OAs, which can be contributed by gas-to-particle conversion sources, the third factor is clearly affected by pollution sources.[15] The same group of authors also applied three different factor analysis algorithms, namely PMF, NMF, and MCR-ALS, on a total of 17 1H NMR spectra to apportion the sources of biogenic SOA in the boreal forest, Finland.[35] NMR factor analysis was able to separate four distinct components, namely: (i) ‘glycols’ factor, characterized by compounds with hydroxyl (or ether) linkages and n-butyl chains, and whose origin was associated with adsorption artifacts on quartz-fiber filters during aerosol sampling; (ii) ‘HULIS-containing’ factor (HULIS = ‘humic-like substances’), considered to be a mix of long-range transported pollution and wood burning products, being characterized by a pronounced band of aromatic protons and signals of levoglucosan; (iii) ‘amines’ factor, exhibiting intense peaks attributable to DEA, DMA, and MSA, thus suggesting that this factor can be impacted by biogenic marine sources; and (iv) ‘terpene-SOA-like’ factor, associated with the formation of terrestrial biogenic SOA and whose 1H NMR spectra is similar to those of laboratorygenerated water-soluble aerosols from the photo-oxidation and ozonolysis of terpene mixtures. Finessi et al.[35] concluded that biogenic OAs in the boreal forest have at least two independent sources, both involving oxidation and condensation mechanisms: reactive terpenes emitted by the boreal forest and compounds of 1

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marine origin, with the latter relatively more important when predominantly polar air masses reach the sampling site. More recently, Paglione et al.[22] employed PMF, NMF, and MCR-ALS, on a total of 25 1H NMR spectra to define the possible nature and prevalent sources of aerosol WSOM collected at Cabauw site (the Netherlands), a location that allow different aerosol types, from polluted to maritime air masses, to be observed. The authors identified a three-factor solution, showing a substantial agreement between all algorithms: (i) ‘MSA-containing’ factor, associated with marine WSOM transported directly from the Atlantic to the sampling site during the ‘marine’ period of the campaigns; (ii) ‘NMR-HULIS’ factor, exhibiting spectral characteristics attributable to branched/cyclic and polysubstituted aliphatic compounds and whose origin was associated with secondary continental sources at the regional scale; and (iii) ‘linear aliphatics’ factor, characterized by compounds rich in linear aliphatic chains and whose presence was assumed to be linked to anthropogenic emissions and may correspond to a ‘fresher’ type of SOA than that of ‘NMR-HULIS’ factor. As recently reviewed by Chalbot and Kavouras,[13] the use of factor analysis algorithms for decomposing 1H NMR data has its own challenges, with the classic small number of samples to predict substantial (ten times) more variables being the most noticeable inference problem. The authors suggest that the ratio of samplesto-variables should be at least 3 : 1, a requisite that seems to be fulfilled and surpassed by the aforementioned exploratory factor analysis studies (samples-to-variables ratio is typically 7 : 1). Nevertheless, it is expected that the aerosol WSOM source apportionment emerging from the 1H NMR factor analysis can become better constrained and more accurate by increasing the number of samples analyzed by 1H NMR spectroscopy. 1

H HR-MAS NMR spectroscopy

1

H high-resolution magic-angle spinning (HR-MAS) NMR spectroscopy provides information on structures hidden in less soluble domains (e.g. hydrophobic moieties) of complex organic mixtures, which become easily accessed through the use of penetrating solvents (e.g. dimethyl sulfoxide-d6 (DMSO-d6)).[37] As recently reviewed,[12] only one study has applied 1H HR-MAS NMR, combined with solution-state and solid-state NMR methods, for studying the organic composition of surface films originated from atmospheric deposition in urban environments.[23] In comparison with solution-state 1H NMR, the use of 1H HR-MAS NMR alone provides less detail on the distribution of all protons in the sample, leading to an incomplete description of its components. Used in combination with other 1D or 2D solution-state NMR methods, 1H HR-MAS NMR is likely to give an enhanced contribution for achieving a detailed structural identification within aerosol WSOM.[23] In our recent review,[12] it argued the possibility of applying 1H HR-MAS NMR to the structural studies of the total suspended matter. It was concluded that the low amount of aerosol organic component (usually less than 35% of the total aerosol mass) and the difficulties in recovering the atmospheric aerosols from the collection media (usually quartz fiber filters) without sample contamination might be the two most important obstacles that prevent the straightforward application of such NMR method to the structural investigation of the whole air particulate matter.[12] Application of 1D solid-state CP-MAS 13C NMR spectroscopy Solid-state cross polarization magic-angle spinning (CP-MAS) 13C NMR spectroscopy has been used less than solution-state 1H NMR

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NMR applications in organic aerosols studies for investigating the structural composition of aerosol WSOM. Nevertheless, 13C NMR spectroscopy is a more suitable tool than 1 H NMR for investigating the distribution of carbon functional groups of such complex organic matrix. Thus far, there has been only five studies reporting the use of solid-state CP-MAS 13C NMR spectroscopy for the characterization of organic matter, including the water-soluble fraction, in atmospheric aerosols[27,38–40] and atmospheric dust.[41] The lack of additional published studies using this technique is most likely because of its insufficient sensitivity for the analysis of WSOM at the concentration levels usually found in air particles.[4,12] Typically, this NMR technique requires about 20–100 mg of sample (depending on the size of the NMR probe, although 4 mm diameter probes have been usually employed),[12,27,38–41] which means that the sample must contain 10–50 mg of carbon in order to acquire a CP-MAS 13C NMR spectrum with an adequate signal-to-noise ratio.[27,39] To achieve the target carbon concentration for atmospheric WSOM samples, high-volume samplers and long-time (24 h or more) integrated samples are required to collect enough material for subsequent analysis. Furthermore, to ensure that a large amount of WSOM material is available, the studies published thus far have also adopted the approach of combining samples according to similar ambient conditions, yielding representative composite samples of a given sampling period and/or seasonal event. For example, Subbalakshmi et al.[38] combined several 24 h total suspended particulate samples collected in a urban site to get particulate quantities of 2.0–5.0 g. The authors used 100 mg of both original air particulate matter and extracted organic matter for the solid-state 13C NMR studies. On the other hand, Duarte et al.[27] used a relatively long sampling time (7 days) to collect PM2.5 samples at an agricultural site for the subsequent solid-state 13C NMR analysis of aerosol WSOM. A total of 27 weekly PM2.5 samples were collected during one-year period (average concentrations ranging from 15.0 ± 4.2 to 38.6 ± 15.1 μg m 3). These aerosol samples were grouped together (groups of three and four samples) according to similar ambient conditions, on a total of seven groups, representative of different seasonal periods. After an isolation/procedure of each group of aerosol WSOM samples, the amount of purified material obtained ranged from ca. 50 mg (≈29 mg of carbon) for the summer sample to 200 mg (≈116 mg of carbon) for the winter sample. Sannigrahi et al.[40] batched together a total of eight aqueous extracts of 24 h integrated PM2.5 samples collected at an urban area during summer season. Unfortunately, the authors did not report the amount of aerosol mass and purified WSOM fraction obtained for their solid-state 13C NMR studies. As recently stated, this strategy of combining samples may be considered a weakness of this off-line analytical approach because it hampers any investigation of real-time changes in aerosol chemistry.[12] Another discouraging aspect preventing the widespread use of solid-state CP-MAS 13C NMR in aerosol WSOM studies is the need to make the sample amenable to analysis in order to take full advantage of the potential of this NMR technique. Typically, WSOM material must be isolated from the inorganic ions and prepared as a solid (e.g. freeze-drying) prior to NMR analysis. The isolation procedure is usually timeconsuming and it must allow the recovery of an unbiased and uncontaminated fraction of the WSOM from the original atmospheric aerosol sample.[12] Solid-phase extraction (e.g. hydrophobic bonded-phase silica sorbents and polymer-based packing materials), ion-exchange chromatography, and size-exclusion chromatography have been employed to isolate, and

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simultaneously fractionate, aerosol WSOM samples. A complete survey of the different methods established for the isolation of aerosol WSOM is well beyond the scope of this overview. Readers are encouraged to consult the reviews of Duarte and Duarte,[4,42] Graber and Rudich,[43] and Zheng et al.[44] and references therein to obtain additional information. When compared with 1D solution-state 1H NMR spectroscopy, solid-state 13C NMR spectroscopy has many advantages. Cook[36] provided one of the most comprehensive reviews on the application of NMR to natural organic matter studies, and the reader is encouraged to refer to this work. Here, we highlight that (i) solid-state 13C NMR spectroscopy is a nondestructive technique, leaving the sample available for other complementary chemical analyses and (ii) this technique does not have some of the problems encountered during solution-state NMR analyses of natural organic matter, including solvent effects on the chemical shifts of the sample, potential masking of certain sample chemical shifts due to solvent signals, and limited solubility of the organic material in the selected solvent.[36] Another interesting reason for using the CP-MAS 13C NMR technique is the possibility of obtaining a semiquantitative measure of the relative contribution of the different carbon functional groups to the organic carbon present in a complex organic matrix. As recently reviewed,[12] to establish and improve the quantitative behavior of this technique, a number of different approaches have been recommended, usually through optimization of NMR parameters, such as the rate of MAS, contact time, and recycle delay, so that 13C nuclei in all carbon functional groups will be equally detected. Once again, readers are encouraged to consult the comprehensive reviews of Cook,[36] Simpson and Simpson,[37] and Conte et al.[45] and references therein to obtain a more complete understanding of the recommended procedures to obtain the most possible quantitatively correct solid-state CP-MAS 13C NMR spectra of natural organic matter. In the works published thus far focusing on OA and their water-soluble fractions, MAS frequencies range from 5 to 10 kHz, contact times are generally between 1 and 5 ms, and recycle delay times range from 1 to 5 s. In order to obtain a quantitatively correct CP-MAS 13C NMR spectrum of aerosol WSOM, it is advisable to use a MAS rate of 10 kHz for reducing the number and intensity of spinning sidebands,[42] a contact time of 1.0 ms,[40] and a recycle delay of 5 s.[40] Even if the NMR equipment is properly set and spectra manipulation is not userdependent, the CP technique may still underestimate 13C nuclei remote from 1H, such as those of quaternary aliphatic carbons, carbonyl groups, and condensed polycyclic aromatic structures, because of weak 1H–13C coupling. Notwithstanding the pros and cons of using solid-state CPMAS 13C NMR spectroscopy for gaining insight into the various structural features of aerosol WSOM, it is now appropriate to summarize what has been learned about this complex organic matrix. The works reported thus far demonstrated that almost all CP-MAS 13C NMR spectra are very broad with overlapping peaks, just allowing the identification of typically five to eight types of carbon functional groups. The major carbon functional groups identified in these spectra are alkyl (δC = 0–45 ppm), oxygenated alkyl (δC = 60–95 ppm), alkyl-substituted aromatics (δC = 110–140 ppm), and carboxylic acids (δC = 160–190 ppm). Very minor resonances corresponding to metoxyl groups bonded to aromatic carbons (δC = 55 ppm), anomeric/acetal carbons (δC = 95–110 ppm), and aromatic carbons bonded to oxygen or nitrogen (δC = 140–160 ppm) may be present in the bulk aerosol WSOM and recovered hydrophobic fractions. The

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R. M. B. O. Duarte and A. C. Duarte published CP-MAS 13C NMR data also shows evidence that the WSOM from aerosols exhibit the same main carbon functional groups, but their relative abundances are quite different. Indeed, this NMR technique is effective not only in shedding light on the complexity of the organic fraction of aerosols, but also in revealing the changes in the structural composition of aerosol WSOM as a function of different sources and formation processes, sampling locations (rural, urban, and pristine areas), season events, and meteorological conditions. Figure 3 summarizes the data so far available regarding the relative abundance (as percentage of total NMR peak area) of the main carbon functional groups identified in different OAs and water-soluble fractions: by Duarte et al.[27] at a rural location, by Sannigrahi et al.[40] at an urban area and during a biomass burning event, and by Zhao et al.[41] at heavily industrialized urban area. Besides providing a possible conceptual model of the organic structures that are likely to be representative of a given OA sample,[12] NMR data also enables one to envisage the dominant OA sources at a given location and their seasonal variation (e.g. secondary photochemical reactions in summer because of the more intense solar radiation and, in winter, less photo-chemical oxidation and possibly more concentrated motor vehicle exhaust precursors and/or biomass burning products).[27,40,41,45] Advanced solid-state NMR techniques Recent technical developments and applications of advanced solidstate NMR spectral editing techniques have revealed the promise for targeting and quantifying specific functional groups of sp2 and sp3 hybridized carbons (e.g. CH, CH2, mobile CH3, OCH3, CN,

non-protonated aromatic C–C, and anomeric O–C–O and nonprotonated anomeric O–C(R,R’)–O groups) in humic substances from various origins.[46] Mao and coworkers[46] demonstrated that the use of a systematic solid-state NMR protocol, combining the use of spectral editing techniques, occasionally assisted by 2D 1 13 H- C heteronuclear correlation (HETCOR) NMR, is promising for achieving a new and deeper understanding of the structure, heterogeneity, and domains of such complex organic matrices. These powerful, advanced solids NMR techniques have yet to be applied to aerosol WSOM, even though they can now be implemented using modern commercial spectrometers. The potential of these techniques for studies of OAs will certainly enrich our current understanding on the substructural features of this complex matrix, thus allowing to achieve a better knowledge on yet unaccounted sources, formation pathways, and atmospheric fate of OAs. Applications of nuclei other than 13C Until now, no studies have been found in the literature reporting the use of solid-state NMR of nuclei other than 13C in OAs. For example, nitrogen-containing organic species have been reported as important constituents of WSOM in both aerosol and fog waters samples,[4] and they might have a significant effect on the physicochemical properties of water droplets and aerosol particles by altering their buffering capacity and basicity.[47] These structures are usually difficult to identify by the conventional CP-MAS 15N NMR spectroscopy, mostly because of the very low natural abundance of the 15N isotope (0.36%), its magnetic properties (low and negative gyromagnetic ratio), and the very low content of nitrogen-containing organic species in OAs, all contributing to

Figure 3. Percentage distribution of the main carbon functional groups in OA samples, including their water-soluble organic fractions, at a rural location in summer and winter,[27] at an urban area in summer and during a biomass burning event,[40] and at heavily industrialized urban area.[41]

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Magn. Reson. Chem. (2015)

NMR applications in organic aerosols studies produce CP-MAS 15N NMR spectra with a sensitivity 50 times lower than those of CP-MAS 13C NMR spectra.[48] As suggested in a previous review, the use of spectral editing NMR techniques, such as 13C{14N} saturation pulse induced dipolar exchange with recoupling (SPIDER), can be a promising tool for determining the nature of structures to which nitrogen is bonded.[12] A comprehensive analysis by means of this technique will enable a thorough characterization of nitrogen chemical environments in OAs, allowing improved options for OAs source apportionment.

2D solution-state NMR studies of aerosol WSOM The invaluable contribution of 1D solid-state and solution-state NMR techniques to OA analysis is related to their ability to describe the relative amounts of the most important structural building blocks. The main significance of 2D NMR spectroscopy is to enhance the reliability of NMR assignments, as 2D NMR spectra allow identifying molecular fragments, via homonuclear and heteronuclear connectivity information. To date, only a few studies have applied 2D solution-state NMR techniques to investigate the structural features of atmospheric organic matter, including aerosol WSOM,[15,27,49] atmospheric urban deposits,[23] SOA functionality and formation mechanisms,[50] and dissolved organic matter in rainwater.[51,52] The most typical 2D solution-state NMR experiments applied into atmospheric samples include (i) 1H-1H homonuclear correlation spectroscopy (COSY) and total correlation spectroscopy (TOCSY), which provide connectivity information between protons in neighboring units (COSY) or regarding protons that are interacting within two to three bonds (TOCSY); (ii) 1H-13C heteronuclear single-quantum correlation (HSQC), which detects H–C couplings over one bond and provides chemical shift data for both atoms in a C–H unit; and (iii) 1H-13C heteronuclear multiple bond correlation (HMBC), which provides direct evidence about the bonding of H–C fragments over twobond and three-bond range (i.e. H–C–C or H–C–C–C).[53] It is expected that the combined used of the information provided by 1H-1H homonuclear (COSY and TOCSY) and 1H-13C heteronuclear (HSQC and HMBC) connectivity will allow a higher spectral resolution and, therefore, greater detail on the C–H backbone of the substructures present in the complex atmospheric organic matter.[12] Unfortunately, because of the sophistication of these 2D NMR techniques, atmospheric chemists who wish to apply them to OAs analysis may experience some difficulties in selecting the most appropriate 2D experiments for answering their specific question. In this regard, readers are referred to the excellent reviews of Simpson et al.[28] and Simpson and Simpson[37] and references therein, where they can find suggestions as to the key 2D NMR experiments and pulse programs that may be useful for studying very complex natural organic mixtures. As recently observed, the studies published thus far in the literature for atmospheric organic matter share a common feature: they all exhibit high quality 2D NMR spectra.[12] By combining COSY, HSQC, and HMBC techniques to study the most hydrophobic water-soluble organic fraction of atmospheric aerosols, Duarte et al.[30] were able to identify that the aliphatic material consists of long-chain (carbons greater than three or four) and branched monocarboxylic and dicarboxylic acids, carbonyl, and ester structural types. The presence of such structural fragments was associated to SOA formation. Spectral signatures typical of

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anhydrosugars from cellulose and methoxyphenols from lignin were also clearly identified among the carbohydrate and aromatic moieties, being their presence linked to the occurrence of wood burning emissions. In 1D solution-state NMR techniques, all these specific assignments would be tentative, at best, because of strong peak overlap. Schmitt-Kopplin et al.[49] combined 2D NMR techniques (namely, COSY, TOCSY, HSQC, HMBC, and distortionless enhancement by polarization transfer (DEPT)-HSQC) with highresolution mass spectrometry to investigate the molecular signatures of the water-soluble fraction of SOA. The typical aliphatic chemical environment within the studied samples was heteroatom-substituted functional groups adjacent to highly branched aliphatics, likely in the form of strongly coupled fused alicyclic ring spin systems (e.g. terpenoid-like molecules). These NMR properties of SOA resemble in many aspects the highly oxidized polyfunctional aliphatic molecules found earlier by Duarte and coworkers.[30] Furthermore, aromatics were found to be highly substituted, and electron withdrawing COX and (O)NOx substitution was considerably more common than the presence of electron donating oxygen-containing functional groups (OH, OR, and OSOnR) and neutral substitution (aliphatic carbon).[49] Decesari et al.[15] employed COSY 2D NMR to study the nature of the water-soluble aliphatic compounds isolated from marine OAs. This study was able to trace the occurrence of biogenic fatty acids, diacids, oxo-acids, and hydroxyl-acids in sea spray particles. Simpson and coworkers[23] applied combined solid-state CP-MAS 13C NMR with 2D HR-MAS and solution-state 2D NMR (TOCSY, HSQC) techniques to provide a general overview of the structural components in atmospheric urban deposits. The authors concluded that these deposits represent a complex mixture of anthropogenically and biogenically derived materials, encompassing carbohydrates, various aliphatic groups (namely, acids, alcohols, alkanes, alkenes, and esters), polybutadiene, and functionalized polyaromatic species. Maksymiuk and coworkers[50] applied COSY and HSQC 2D NMR experiments to explore SOA formation from multiphase oxidation of limonene by ozone. The authors concluded that the terminal unsaturation in limonene is oxidized via heterogeneous uptake of ozone to fresh SOA particles. Using COSY 2D NMR, Seaton and coworkers[51] were also able to identify correlations consistent with fatty acids and C5 or higher di-acids or ox-acids in WSOM from rainwater. Within a comprehensive study of rainwater composition, Cottrell and coworkers[52] also employed COSY 2D NMR with spectral database matching, being able to identify over 100 compounds, including primarily carboxylic acids, carbohydrates, and nitrogencontaining compounds. Although COSY, TOCSY, HSQC, and HMBC are the most popular 2D NMR experiments applied into atmospheric samples, being the easiest to apply and interpret, the reader should also be aware that a range of other 2D NMR experiments are available in the solution-state. A more detail discussion on this issue, aimed at those interested in applying solution-state 2D NMR to atmospheric organic matter, is provided in the review works of Simpson et al.[28], and Simpson and Simpson[37] and references therein. At this point, it is clear that 2D NMR-based approaches place the identification of known organic species in atmospheric samples (OAs, rainwater, and fog water) early in the discovery line, simultaneously enabling the targeted isolation and assignment of unknown structures. Indeed, the prospect of mining the molecular structural constituents of the complex atmospheric organic matter, with possible identification of its sources and formation mechanisms, is a truly exciting challenge that justifies the efforts for implementing such sophisticated NMR methods.[12]

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R. M. B. O. Duarte and A. C. Duarte

Conclusions NMR spectroscopy is an invaluable tool in atmospheric organic matter research, and aerosol WSOM in particular. One downside to this off-line approach is the long time resolution required for obtaining sufficient amount of WSOM sample, with the consequent loss of information regarding the very own atmospheric variability of the organic matter composition. Additionally, it may often take several hours or even weeks to collect enough aerosol WSOM – increasing the risk of introducing artifacts in the sample. Nevertheless, the molecular structural assignments obtainable with NMR spectroscopy are key observations that assist in assessing the source, seasonal and regional characterization and formation pathways of these atmospheric samples. While the use of 1D solutionstate NMR has become a commonplace for studies of OAs, the future directions for solution-state NMR are clearly towards the application of 2D NMR. For solid-state NMR, future applications should focus on the use of advanced spectral editing techniques and the study of nuclei other than 13C (e.g. characterization of organic N in OAs). These solid-state techniques have yet to be applied to OAs, but their application will greatly contribute to the identification and quantification of specific functional groups (including those containing nitrogen), which otherwise remain hidden in conventional solid-state CP-MAS 13C NMR spectra. The lack of analytical expertise in the use of these solid-state NMR spectral editing techniques, as well as handling and interpretation of 2D NMR data, are major challenges that need to be overcome in order to achieve a newer and deeper understanding of the structural composition of atmospheric OAs. Furthermore, there are currently over 200 NMR experiments available, and in deciding the specific experiment to be applied, researchers should question the level of information needed to determine the nature, origin and formation mechanisms of atmospheric OAs. If interested in developing a model for linking the molecular structure of OAs to its primary and or secondary origin, then the implementation of a comprehensive analytical approach using solution-state 2D NMR techniques, in combination with chemometric tools for data handling, is highly desirable. However, if interested in the elucidation of the bulk chemical composition of OAs, then simple 1D NMR experiments may be applied. Within this context, the synergistic application of both solid-state and solution-state 2D NMR may play a key role in elucidating OAs composition and structure, as well as identifying molecular fingerprints, indicative of the source and formation mechanism.[4] However, regardless of the NMR method applied to the characterization of OAs and WSOM, the study of the chemical and environmental significance of these complex matrices greatly depends on the limitations imposed by both extraction and isolation procedures. Typically, the organic aerosol material must be isolated if NMR studies of the particulate organic matter are to be conducted. Consequently, the use of an isolation procedure that does not perturb the original sample is necessary.[4] Therefore, the implementation of a robust and standardized analytical method for the extraction and isolation of OAs and its water-soluble fraction is a prerequisite to realize the full potential of these NMR techniques.[4,43,44,54] Researchers interested in atmospheric OAs composition and reactivity also need to carefully consider the use of other complementary analytical tools, such as high-resolution mass spectrometry, and comprehensive 2D liquid or gas chromatography coupled to different detection systems (e.g. diode array detector, fluorescence detector, NMR or mass spectrometry).[4,28,52] The combined

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use of multiple structural characterization techniques will certainly provide a more comprehensive view of atmospheric OAs composition, consequently enhancing our understanding of the impact of OAs’ on the climate, atmospheric processes, and human health. Acknowledgements Centre for Environmental and Marine Studies (PEsT-c/MAR/LA0017/ 2013, University of Aveiro, Portugal) and the Portuguese Science and Technology Foundation (FCT), through the European Social Fund (ESF) and “Programa Operacional Potencial Humano – POPH”, are acknowledged for financial support. This work was also funded by FEDER under the Operational Program for Competitiveness Factors – COMPETE and by National funds via FCT within the framework of research projects ORGANOSOL (PTDC/CTE-ATM/118551/ 2010) and CN-linkAIR (PTDC/AAG-MAA/2584/2012).

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Unraveling the structural features of organic aerosols by NMR spectroscopy: a review.

Our limited understanding of the effect of organic aerosols (OAs) on the climate and human health is largely because of the vast array of formation pr...
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