A Model for the Spectral Dependence of Aerosol Sunlight Absorption

Sep 21, 2017 - ACS Earth Space Chem. , 2017, 1 (9), pp 533–539 ..... Ramanathan , V.; Carmicheal , G. Global and regional climate changes due to bla...
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A Model for the Spectral Dependence of Aerosol Sunlight Absorption August Andersson* Department of Environmental Science and Analytical Chemistry (ACES) and the Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden ABSTRACT: Sunlight-absorbing aerosols, e.g., black and brown carbon (BC and BrC), have a potentially large, but highly uncertain contribution to climate warming. The spectral dependence of the aerosol absorption in the visible and near-UV regime is almost universally well-described with a heuristic power law, where the exponent is termed the absorption Ångström exponent. However, the underlying physicochemical causes for this relation are unknown. Here, a model is presented that predicts the emergence of the power law spectral dependence and unifies the absorption behavior of BC and BrC. Building on the theory of lightabsorption in amorphous materials, the interaction between multiple functional groups upon absorption is predicted to be a key feature for this broad spectral dependence. This aerosol amorphous absorption model is in agreement with recent empirical findings and provides a conceptual basis for the additional research needed to better constrain the optical properties of light-absorbing aerosols and their environmental impact. KEYWORDS: black carbon (BC), brown carbon (BrC), absorption Ångström exponent (AÅE), aerosol absorption optical depth (AAOD), soot, power law, climate change, Humic-like substances (HULIS)

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INTRODUCTION

⎛ f ⎞ AÅE ⎛ λ ⎞−AÅE abs = α ·⎜ = α ·⎜⎜ ⎟⎟ ⎟ ⎝ λref ⎠ ⎝ fref ⎠

Light-absorbing aerosols are associated with multiple environmental effects, including climate warming, glacier melting, shifting of monsoon and storm systems, perturbations of rainfall patterns and the hydrological cycles as well as adverse effects on human respiratory health.1−5 The direct radiative forcing of the main component, black carbon (BC), ranges between 0.2−1.2 W/m2, suggesting a potentially strong but highly uncertain warming effect.1−5 In addition to BC, lightabsorbing organic, brown carbon (BrC), and inorganic aerosols (e.g., dust) also contribute to these effects.6,7 The radiative forcing of BrC is estimated to be ∼20−30% of BC.8,9 Four main sources of uncertainties regarding the Earth system impact of absorbing aerosols have been suggested: (i) sourcespecific emission estimates, for example, bottom-up emission inventories; (ii) atmospheric lifetimes; (iii) optical properties (including mixing/coating); and (iv) the relative contributions from different components, that is, BC, BrC, and dust.10,11 In this paper, points iii and iv are addressed by focusing on the spectral dependence of BC and BrC light-absorption. Atmospheric aerosols are characterized by a vast chemicophysical complexity. It is therefore surprising that the absorption for a large section of the near-UV/visible regime, overlapping with the maximum of the tropospheric solar transmission spectrum, is almost universally found to follow a power law with regard to the wavelength (Figure 1): © XXXX American Chemical Society

(1)

where AÅE is the absorption Ångström exponent, λ is the wavelength, λref is a semiarbitrary reference wavelength, α is the light absorption coefficient (m−1) at the reference wavelength, and f is the frequency. This relation is not applicable to the full UV/vis spectral range, but additional features/mechanisms modulate the spectra at longer and shorter wavelengths.12 This paper focuses on the wavelength-regime where the power law dependence is observed (implying a linear relation in a log−log plot). Equation 1 has been found empirically valid for a wide range of aerosol regimes, e.g., traffic, industry, forest and savannah fires, domestic cooking and remote sites such as the Arctic.7,12−14 In addition, the relation is observed with different instrumental techniques, including columnar aerosol absorption optical depth (AAOD), filter-based absorption, photoacoustic techniques, and in solvent extracts.13−16 However, in contrast to the scattering counterpart, where the scattering Ångström exponent reflects the aerosol size distribution, there is currently, to the author’s knowledge, no model that explains why this relation is observed with such generality for absorption.17 The Received: Revised: Accepted: Published: A

June 6, 2017 August 14, 2017 September 21, 2017 September 21, 2017 DOI: 10.1021/acsearthspacechem.7b00066 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry

Figure 1. Schematic representation of typical power-law wavelength-dependencies (eq 1) of light-absorption by black (BC) and brown (BrC) carbon aerosols. BC is more absorbing than BrC in this wavelength regime with an absorption Ångström exponent (AÅE) ∼ 1. BrC on the other hand is typically less absorptive in the visible spectrum, and characterized by a variable (2−14) AÅE.

the distribution of different energy levels in the material. The DOS indices Gr and Ex refer to ground and excited states, respectively. Equation 2 thus describes the convolution of the energy distributions for the ground- and excited-states material upon excitation by a photon with energy Elight = h·f. The DOS in an amorphous material is inferred from empirical findings, but is typically well-described by a power law:22

aim of this paper is to present a physical model that predicts the emergence of the power law relation described by eq 1. The UV−vis light-absorption spectrum of a single organic molecule, e.g., a polycyclic aromatic hydrocarbon (PAH), shows relatively sharp maxima around the different electron transitions, broadened by, e.g., vibrations. This is in contrast to the broad, featureless spectrum characterized by the power law in eq 1. A fundamental difference is that the absorption of a given ambient aerosol sample represents a bulk property, with a wide variety of chromophores that may or may not interact. The broad spectral features of BC absorption have been discussed using a model for semidirect transitions in amorphous materials (AABC).12,18−20 Given the assumptions underlying this specific model, AABC predicts a power law spectral dependence with AÅE = 1, in good agreement with experimental findings for BC. However, for BrC or BC/BrC mixtures the AABC has limited applicability, since the AÅE for BrC may vary in the range of ∼2−14. In this paper, an extended theory for light-absorption in amorphous materials, the aerosol amorphous absorption model (A3M), is discussed as a possible explanation for the power law relation for both BC and BrC, in which AABC is a special case. The predictions from the A3M are compared with and discussed in light of recent empirical findings.

DOS(E) = k 2·(E − k 3)r

where k2 and k3 ∼ constants. Combining eqs 2 and 3, the following is obtained (k ∼ constant): abs(f ) = k·

THEORY An amorphous material is a solid that lacks long-range order. Models of light-absorption by amorphous materials are based on perturbations of the theory of absorption by crystals and may be expressed as:21 k1 · h·f

∫ DOSGr(E)·DOSEx (E + h·f )dE

(h·f − EOG)r + 1 h·f

(4)

where EOG is the optical gap energy of the material, which defines the energy range where the probability for light absorption is zero. EOG is analogous to the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a molecule. The exponent r depends on multiple factors, including the spatial dimensions of the material, the absorption mechanism, the statistical distribution of various states and the chemical nature of the material. For instance, in crystals the different quantum numbers of r reflect different specific absorption mechanisms; for direct light absorption, r = −0.5, and for semidirect, phonon-assisted absorption, r = 1. Although the absorption of certain amorphous materials reflects these specific r-values, amorphous materials in general are not restricted to these quantum numbers and may vary continuously within a broad range.22−24 For inhomogeneous amorphous systems that include a wide chemical complexity, the mechanistic dependency on r becomes increasingly complex. For the situation when the optical gap is zero, and setting r = AÅE, a means for arriving at eq 1 is obtained.

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abs(f ) =

(3)

(2)

where k1 ∼ constant, h is Planck’s constant, and E is energy. DOS is the density of states, which is a function that describes B

DOI: 10.1021/acsearthspacechem.7b00066 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Amorphous-like Absorption in Aerosols. Amorphous materials of both primary (for example, BC) and secondary origins have been observed in aerosols, but the aerosol phase is generally not an amorphous material.25 The power law absorption spectrum of amorphous materials originates from the mobility of the light-excited electrons in the material and is thus related to conductivity. For instance, the presence of BC increases the rate of electron transfer in environmental samples.26 We hypothesize here that the spectral dependence of aerosol absorption also reflects mobility of light-excited electrons; interactions within a network of functional groups that modulate the absorption. This network is composed of multiple interacting functional groups and chromophores, which may or may not be covalently bonded. In the following discussion, we explore how this model for aerosol amorphous absorption (A3M) fits with empirical findings. Black and Brown Carbon Aerosols. Molecular lightabsorption in the near-UV/visible regime mainly arise due to photon-induced electron-excitations in three types of structures: (i) conjugated (sp2-hybridized) chemical bonds; (ii) charge transfer complexes; and (iii) transition metals. The molecular structure of BC is characterized by conjugated graphene-like aromatic sheets. BrC is more chemically diverse, and multiple, conjugated chromophores have been observed: quinones, phenols, nitro-phenols, imidazoles, epoxy-diol derivatives and PAHs.27−31 In addition to conjugation, charge transfer has also been found to enhance BrC absorption.32,33 Absorption in dust mainly reflects transition metals, for example, iron oxides.6 In this paper, we focus on the absorption of BC and BrC, and thus on conjugation and charge transfer, even though chelation of transition metals by organic compounds may contribute. The phonon-assisted, semidirect amorphous material absorption mechanism, the AABC model (r = AÅE = 1), has been used to explain the spectral dependence for both BC and BrC.34 However, since BrC AÅEs display a wide variability (∼2−14), this special case has limited applicability to ambient mixtures. BC is, furthermore, not a single well-defined state but is operationally defined and is characterized by a continuum ranging from char to graphite, rendering the AÅE-dependence complex.7,35,36 We propose here that both BC and BrC may be considered as parts of a continuum of amorphous-like absorbing states: the A3M model. The AÅE in this model directly reflects the shape of the density of states function in eq 3 and thus how sharply the probability of photon-induced excitation increases with wavelength. This dependency is strongly modulated by the chemical complexity, the disorder, the primary chromophores, and the statistical distribution of different states. For instance, the size of a chromophore is in general coupled to wavelengthdependency. The quantum mechanical particle in a box model suggests absorption red-shifts with increasing molecular size, which is also observed for the wavelength of the absorption maximum of many chromophores, for example, for linear PAHS, such as benzene (1 ring) < naphthalene (2 rings) < anthracene (3 rings) < tetracene (4 rings). Solvent Extracts. The absorption power law relation for aerosols, eq 1, is not only observed for ambient aerosols in situ but is also found for BrC in different solvent extracts. As a tracer for BC, graphene oxide solutions also exhibit this feature.37 Since the wavelength of light in the near UV−vis

regime (∼200−800 nm) is longer than the diameter of dissolved BrC molecules (∼0.2−5 nm), the measured extinction is expected to be dominated by the absorption, even though this has not been found to be true for all solvents.38 This is in contrast to ambient aerosols, where scattering typically dominates extinction. In other words, the spectral dependence of BC and BrC absorption is a molecular property not limited to the aerosol phase. One advantage of solvent extractions is that high spectral resolution (for example, 1 nm) can be obtained, which allows detailed analysis of the spectral characteristics. Solvent extracts therefore provide a good means of examining the molecular origins of the spectral relation. However, large amorphous aggregates (kerogens/BC) are not prone to solubilization. Nevertheless, the power law spectral dependence is still observed in solvent extracts, suggesting that other configurations also contribute to this behavior. BrC Absorption, Formation, and Characteristics. BrC absorption arises from the sum of multiple chromophores. These may either interact with other functional groups or may be considered to be essentially noninteracting. The most likely situation is a mixture of the two. For the scenario where the superposition of noninteracting chromophores dominates the spectrum, multiple local maxima are expected, and it is not clear why such a mixture should follow the smooth and widely observed power law dependence, eq 1. Instead, synergistic modulation of the spectral dependence by interactions may be important. Such interactions occur either within or between molecules. Intermolecular interactions may either be essentially diffusion-limited, or may occur through noncovalent supramolecular assemblies. Intramolecular interactions facilitate interactions through structural confinement. BrC may be emitted as primary particles or may be formed through secondary reactions.7,39,40 A wide array of different absorbing chromophores has been shown to form in laboratory experiments.41,42 The initial absorption spectra in these experiments are typically isolated chromophore-like. However, over time, significant spectral broadening has been observed, which has been associated with increasing molecular weights suggesting oligomerization.30,43 These trends are also observed in the ambient atmosphere, that is, initial jagged spectra that smooth out over time. The smoothing of the spectra is accompanied by a loss of absorption within a time-scale of ∼15 h, indicating the breakdown of individual chromophores, where absorbing components with higher molecular weight are more recalcitrant.30,44−47 However, absorbing molecules also form during air mass transport, for example, through Maillard or condensation reactions.43,48,49 The molecular size-ranges of the absorbing components thus vary with time but also with emission sources, formation mechanisms, and so forth. These trends are further complicated by discrepancies in results obtained by different analytical methods. For example, mass-spectrometry (MS) typically reports molecular weights in the 200−500 Da regime. Other methods, for example, size-exclusion chromatography (SEC), reverse-phase high-performance liquid chromatography (RFHPLC) and fluorescence correlation spectroscopy (FCS), report weights in the +1000 Da range.46,47,50−52 In combination, this suggests possible complications with MS-based BrC weight estimates, including possible fragmentation during MS ionization, selective ionization, or disrupted noncovalent interactions.52−54 C

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ACS Earth and Space Chemistry HULIS. Brown carbon aerosols are sometimes referred to as HUmic LIke Substances (HULIS), since their characteristics are similar to carbonaceous humic materials, commonly found in fresh water systems.40,55 However, BrC and HULIS are technically not the same, as the methodology for isolation can be different.56,57 Nevertheless, comparisons have proven fruitful. For limnic humic materials, fluorescence spectroscopy shows that the spectral dependence of absorption is inconsistent with the noninteracting individual chromophores model, and charge transfer strongly modulates the absorption.58 Similarly, charge transfer interactions account for ∼50% of the absorption for water extracts of ambient BrC aerosols.32,33 There is a debate in the literature as to whether the broad absorption spectra of humic matter reflects large polymers or supramolecular aggregates.59 Given the discrepancy between MS and other methods for determining the molecular weight, supramolecular associations may also be important for BrC.60 MAC and AÅE. The power law relation for aerosol absorption is fully described by two parameters: the absorption coefficient at the reference wavelength (α) and the absorption Ångström exponent (AÅE). By normalizing the absorption to the carbon concentration, the mass-absorption coefficient (MAC) is obtained, which is proportional to the imaginary part of the refractive index. Organic matter in the aerosol phase is characterized by a multitude of atmospheric reactions and transformations. For BrC absorption, both enhancement through formation of chromophores and reduction through oxidation have been observed.41,43,48,49 BC is expected to be more recalcitrant to photochemical oxidation, but the MAC for ambient BC is highly dependent on scattering effects, including internal mixing/coatings effects, such that aging may double the MAC.61−64 Similarly, scattering effects also modulate the BrC MAC.65 A common observation is that the BrC MAC dissolved in methanol is higher than in water.65−68 This suggests that the less polar parts of the BrC continuum are more absorptive. However, solubility is not the only factor at play.38 For instance, at 20 °C the dielectric constant for water is ∼80, while it is ∼30 for methanol, thereby affecting charge transfer interactions, which depend on the inverse of the dielectric constant. When considering other potential interactions, the comparative interpretation of extractions with different solvents becomes complex. For ambient aerosols, the effective solvent properties are poorly constrained, complicating assessments of molecular origins. Both laboratory experiments and field measurements show that the photochemical evolution of BrC absorption, and to a lesser extent BC, is highly dynamic.41,43,46,47,52,69 Photochemical aging reduces the absorption of existing chromophores due to oxidation of conjugated bonds. In addition, oxidation of electron acceptors, for example, carbonyls to hydroxyls, leads to a loss of charge transfer interactions. Comparisons of near-source sites with regional receptor sites suggest lower MACs and higher AÅEs at the receptor sites.68,70−72 The carbon oxidation state, estimated as the O/ C-ratio, has been proposed as a metric to describe the chemical state of organic aerosols.73 It is therefore of interest to investigate the relation of this parameter with the MAC and the AÅE. For instance, a higher O/C ratio for BrC emitted from a propane flame is correlated with a higher AÅE and a lower MAC.74 In contrast, laboratory studies of secondary formation show variable relations of absorption with O/C.41

Clearly, both formation and destruction of BrC absorptive states may be associated with increasing O/C ratios, and therefore the oxidation state. A possible explanation is that different states of oxygen have different effects on absorption, for example, comparing carbonyls with hydroxyls. Similar trends are also expected for other heteroatoms, for example, nitrogen, which is implicated as a key component in BrC.42,43,75,76 In addition to photochemical effects, significant differences for the AÅE are also observed when comparing BrC from different sources.45,68 For ambient aerosols, composed of a mixture of BC and BrC, an AÅE of ∼1−2 is often observed.77 However, the sum of two power laws is not another power law. Although the sum is approximately a power law, it is likely that BC and BrC interact, for example, through electron transfer, forming a joint network of functional groups. In summary, it is clear that the MAC and the AÅE can be highly variable, depending on emission sources, formation mechanism, and atmospheric processing. Why Amorphous-like Absorption in Aerosols? Given the vast chemical/physical complexity of BC/BrC, it is somewhat remarkable that the absorption spectral dependence may almost universally be described by the power law relation, eq 1. This suggests that there is an underlying chemical/ physical mechanism. Recent empirical findings for BrC show common features that govern the emergence of this dependency: (i) the power law relation is a fundamental molecular feature of absorption, independent of scattering; (ii) atmospheric processing smooths initially jagged spectra; (iii) the absorption is strongly dependent on the molecular weight of the absorbing complex, where heavier compounds have longer atmospheric lifetimes; (iv) noncovalent interactions, for example, charge transfer, significantly enhance and modulate the absorption; and (v) the power law relation appears regardless of primary or secondary origins and emission sources. This suggests that the summation of multiple individual chromophores, which represents a broad range of different molecules, is not the cause of the smooth power law relation for BrC; rather it is due to the synergistic interaction between multiple functional groups. These observations support the basic hypothesis of the A3M that light-excited electrons interact with multiple functional groups. Such interactions are facilitated within larger molecules but also within noncovalent supramolecular aggregates. For BC, the amorphous-like features are already present in the fresh emissions, including electron mobility, which is a key feature of graphene-like layers. Taken together, these observations suggest that the power-law absorption spectral dependence of both BC and BrC have common molecular origins and may be described by the A3M. Implications. The A3M predicts the onset of a power law absorption spectral dependence, but a direct interpretation of the AÅE is not clear. Mathematically, the AÅE directly reflects the functional form of the density of states, which in turn depends on several specifics. Empirical findings show that the AÅE is highly influenced by atmospheric transformations and source variability. This complicates AÅE-based deconvolution techniques, including source differentiation and the delineation of BC and BrC contributions to mixtures.78,79 In particular, the A3M, in agreement with empirical findings, suggests that absorption by BC and BrC is fundamentally nonlinear with synergy between interacting chromophores. Overall, this model provides a testable framework for further empirical research D

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with the goal of constraining the environmental impact of BC and BrC.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

August Andersson: 0000-0002-4659-7055 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS A.A. acknowledges financial support from the Swedish research council FORMAS (contract 942-2015-1070) and the Swedish Research Council (contract 348-2013-114). I am grateful for the many fruitful discussions with Elena N. Kirillova, Patrik Winiger, Carme Bosch, Srinivas Bikkina, Wenzheng Fang, Sanjeev Dasari, and Ö rjan Gustafsson.

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DOI: 10.1021/acsearthspacechem.7b00066 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsearthspacechem.7b00066 ACS Earth Space Chem. XXXX, XXX, XXX−XXX