Color of brown carbon: A model for ultraviolet and visible light absorption by organic carbon aerosol

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L17813, doi: /2007gl029797, 2007 Color of brown carbon: A model for ultraviolet and visible light absorption by organic carbon aerosol Haolin Sun, 1 Laura Biedermann, 1,2 and Tami C. Bond 1 Received 1 March 2007; revised 16 June 2007; accepted 3 August 2007; published 12 September [1] We recommend ultraviolet and visible absorption spectra to represent particular types of atmospheric organic particles. Spectra of liquids and particles can be compared using the absorption coefficient of bulk material divided by material density. Reported absorption by absorbing organic aerosol from combustion is greater than that of organic material isolated by humic acid extraction. We examine ultraviolet and visible spectra of 200 organic compounds, concluding that visible absorption may be attributable to n! p* electronic transitions in a small fraction of oxygenated compounds. Absorption spectra can be communicated using the band-gap and Urbach relationships instead of the absorption Angstrom exponent. Water-soluble atmospheric aerosol has a band-gap of about 2.5 ev; insoluble aerosol may have a lower band-gap and higher absorption. Although different types of organic carbon may exhibit a continuum in absorption, there is a sharp distinction between the mostabsorbing organic carbon and black carbon. Citation: Sun, H., L. Biedermann, and T. C. Bond (2007), Color of brown carbon: A model for ultraviolet and visible light absorption by organic carbon aerosol, Geophys. Res. Lett., 34, L17813, doi: / 2007GL Introduction [2] Carbonaceous aerosols comprise a large fraction of atmospheric aerosols, which affect the radiative balance of the Earth by absorbing and scattering light. Although organic carbon (OC) has been thought to have a cooling effect, some organic compounds absorb light. Their absorption is strongly wavelength-dependent, being greater at near-ultraviolet and blue wavelengths. They appear brownish, evoking the name brown carbon [Andreae and Gelencsér, 2006]. Visible absorption is far more important for the energy balance than ultraviolet (UV) absorption; nearly 40% of solar energy is found at wavelengths between nm. UV absorption affects photolysis, but wavelengths below 400 nm comprise provides only about 4% of solar energy. Mie calculations confirm that a small amount of absorption can affect the radiative balance. At a wavelength of 550 nm, the simple forcing efficiency of Bond and Bergstrom [2006] for a 150-nm particle is negative if the refractive index is i and positive if it is i. 1 Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA. 2 Now at Department of Physics, Purdue University, West Lafayette, Indiana, USA. Copyright 2007 by the American Geophysical Union /07/2007GL [3] UV and visible absorption results from electronic transitions. These are often p! p* transitions at UV and near-uv wavelengths, and n! p* transitions at visible wavelengths. Jacobson [1999] identified nitrated and aromatic compounds as likely absorbers. Bond [2001] suggested differing levels of aromatization to explain wavelength-dependent absorption. This paper provides quantitative recommendations for the properties of this material for use in radiative-transfer modeling. 2. Absorption Coefficients [4] The absorption coefficient a(l) (cm 1 ) of bulk organic liquid is a function of wavelength, l. It can be derived from measurements of dissolved compounds if the density r (g/cm 3 ) of the substance is known. a can be determined from spectroscopic data, using either the molar absorption coefficient, e, and the molecular weight M w,or the absorbance A, the optical path length L (cm), and the concentration c (g/l): al ð Þ ¼ 1000 ln ð 10 Þel ð Þ ¼ 1000 lnð10þ AðlÞ M w cl [5] The complex refractive index, m = n + ik, is needed to model absorption and scattering by liquid particles. The imaginary part k is proportional to the absorption coefficient: a = 4pk/l. To compare the bulk absorption coefficient a with the particulate absorption coefficient, consider the mass-normalized absorption cross-section MAC (m 2 /g). In the small-particle limit, MAC is [e.g., Bohren and Huffman, 1983]: MAC ¼ 6p l Im m2 1 m 2 þ 2 Expanding the term in brackets and substituting for a yields: MAC ¼ a x where x is a weak function of k: ð1þ ð2þ ð3þ 9n x ¼ ðn 2 k 2 þ 2Þ 2 ð4þ þ4n 2 k 2 Thus, MAC can be compared with a only after considering the particulate effect, x. For n = 1.5 and particles small relative to the wavelength, x ranges from about 0.69 to The density used to derive k from e or A must be consistent L of5

2 Figure 1. (a) Absorption by humic acid and by water-soluble atmospheric organic aerosol. (b) Summary of data in (a) along with two projections from band-gap and Urbach models. For band-gap #1, Eg = 2.5 ev; for band-gap #2, Eg = 1.65 ev. Both use B = 250 (cm 2 g 1 ev 1 ) 1/2. Upper branch of each band-gap model shows inclusion of the Urbach tail with E 0 = 0.4 ev. Cumulative fraction of solar irradiance at 1.5 atm is shown on the right axis for reference. with that used in the radiative-transfer model. We suggest the value a/r, which avoids such discrepancies. 3. Observations 3.1. Soluble Organic Carbon [6] Figure 1a shows spectra from extracts of atmospheric aerosols from Varga et al. [2001], who found that absorption at longer wavelengths was associated with morehydrophobic material; Havers et al. [1998], who measured the alkaline-extractable fraction; and Hoffer et al. [2006], who extracted the humic-like fraction from biomass burning aerosols. This absorption is similar to that of humic fraction isolated from pulp mill effluent [Duarte et al., 2003] Combustion Aerosol [7] Figure 1a also shows absorption by organic aerosol from combustion. Kirchstetter et al. [2004] inferred absorption by the acetone-extractable fraction of biomass-burning aerosol. We used an intermediate value of x to obtain a/r, and accepted their correction for filter enhancement of absorption (factor of 2). The error bars show likely values of x. We also show absorption by aerosol generated in propane combustion [Schnaiter et al., 2006] (C/O = 0.5, stage 1). [8] Absorption by organic aerosol from these combustion sources is higher than that of soluble aerosol by about a factor of 5. Analytical artifacts might cause the apparent high absorption. Kirchstetter et al. [2004] and Roden et al. [2006] captured particles on filters. These measurements suffer from (1) enhancement of absorption, (2) a small fraction of scattering measured as absorption [Bond et al., 1999], and (3) deformation of absorbing liquid material [Subramanian et al., 2007]. Absorption could be attributed to organic carbon by Kirchstetter et al. [2004] if the acetone wash removed an organic coating, decreasing black carbon absorption. However, enhancement by a factor of ten above the value for suspended, uncoated particles is unlikely. We suggest that unidentified compounds, not extractable by water or alkali, must contribute this absorption. [9] Roden et al. [2006] provide additional evidence for high absorption from smoldering biofuel. Although a/r could not be inferred from their study, their low values of single-scatter albedo ( ) could not come from particles absorbing like humic-like substances (HULIS), unless they were unreasonably small and scattered less (around 30 nm diameter). [10] Graber and Rudich [2006] pointed out that atmospheric HULIS are less aromatic and light-absorbing than reference fulvic acids. Perhaps the composition of combustion aerosol with stronger absorption is closer to that of humic or fulvic acids. However, we could not predict sufficient absorption to match the combustion aerosol in Figure 1a for even the largest fulvic acids studied by Ewald et al. [1988] or the most aromatic humic acids measured by Peuravuori and Pihlaja [1997] Individual Spectra [11] Larger chromophores portions of molecules that absorb yield both higher absorption and a spectral shift to longer wavelengths. Diagnostic relationships between molecular structure and absorption have been pursued for decades, but most are insufficient for our needs. For example, the Woodward-Fieser rules [e.g. Woodward, 1942] describe shifts in peaks, but do not give the full absorption spectrum. [12] We examined the absorption spectra of organic compounds tabulated in the Perkampus [1992] atlas of UV-Visible absorption (hereinafter Perkampus). Each spectrum can be approximated by a sum of Gaussian peaks. We recorded magnitude, peak wavelength and band width for up to six peaks in over 550 spectra. Here, we discuss mainly the 200 spectra containing C, H and O. The additional spectra containing nitrogen are summarized in Supplemental Information. We also recorded the number of rings, unsaturation number, and composition of each compound. We then calculated a/r normalized to carbon mass. [13] We use Perkampus with caution, because it is not representative of atmospheric aerosol. It focuses on absorbing compounds, including dyes, and may exclude those with weak absorption. The compounds tend to be aromatic and contain many functional groups; the average unsaturation number divided by the number of carbon atoms is Biomass products are probably represented in Perkampus, since wood-derived absorbing compounds have been used as dyes for centuries. 2of5

3 Figure 2. Cumulative distribution of absorption at visible wavelengths for compounds in the Perkampus [1992] atlas. Humic curve is upper bound of humic-acid and watersoluble curves in Figure 1a; combustion curve lies midway between acetone-soluble and propane combustion curves. Many compounds have absorption comparable to atmospheric aerosol at ultraviolet wavelengths, but few can match visible observations. [14] Figure 2 summarizes absorption at visible wavelengths by compounds in Perkampus, also marking absorption from Figure 1a. At 400 nm, absorption by about twothirds of the C-H-O compounds is below that of extracts, and 90% are lower than the more-absorbing organic carbon from combustion. For absorption at 550 nm, only 10% of the compounds have as much or more absorption as humic extracts. [15] We next examined the 10% of compounds with sufficient absorption at 550 nm in order to identify the molecular structures that result in light absorption like that of brown carbon. Figures S1 through S3 in the auxiliary material 1 depict the composition of these highly absorbing compounds. Most are highly oxygenated (3 or more O atoms), are large (18+ carbon atoms), or contain one or more nitrogen atoms. High absorption also requires conjugation of unsaturated bonds or non-bonding electron pairs. In compounds containing O or N, non-bonding orbitals allow the n! p* transition that occurs at visible wavelengths. [16] According to the Perkampus tabulation, compounds with visible absorption like that of brown carbon are oxygenated, large or multifunctional. We suggest that this conclusion will also hold for atmospheric aerosol, even if it contains different compounds than Perkampus. Compared with compounds identified in biomass combustion aerosol [e.g. Oros and Simoneit, 2001], most of the compounds in Perkampus are similar in size (600 amu and lower) and more unsaturated, yet they absorb far less light than humiclike aerosol. [17] The color of brown carbon could result from many chromophores with similar absorbance, but the fraction identified by speciation has lower absorbance. Therefore, we conclude that much of the visible absorption can be attributed to a subset of carbonaceous compounds, and that these compounds absorb much more than the average of humic-like substances. These compounds are very specialized, and they are either oxygenated or nitrated. They absorb light even before undergoing atmospheric reactions, so we suggest that oxygenated rather than nitrated species are the more likely contributors to absorption. Because these compounds must be large and oxygenated, they are probably difficult to identify by speciation. Orthoquinones are primarily responsible for wood pulp color [Lebo et al., 1988], and a similar attribution may be possible for lignin pyrolysis products. del Vecchio and Blough [2004] attributed absorption at long wavelengths to quinoid acceptors. 4. Spectra of Complex Aerosol [18] While examining individual spectra can be instructive, it is unlikely that molecular identification can predict average absorption. Organic speciation identifies 20% or less of aerosol components. Large, complex molecules, likely responsible for visible absorption, are particularly difficult to identify. Even if the full organic composition were known, spectra would not be available for each species. Next, we examine relationships that have been used to describe absorption by complex mixtures, and we rely on these instead. The first two (band-gap and Urbach tail) are more often used to describe amorphous solids. Organic aerosol is similar in that a continuum of electronic states leads to featureless absorption, and Leboeuf and Weber [2000] have shown that naturally-occurring organic matter exhibits amorphous or glass-like behavior Band-Gap [19] The band-gap relationship is frequently used to describe the absorption spectrum of semiconducting amorphous carbon [Robertson, 2003]. We write these equations as a function of photon energy E (E = hc/l), as is conventional. The band-gap E g is the difference between the highest occupied and lowest unoccupied energy state, and it affects the absorption coefficient a(e): pffiffiffiffiffiffiffiffiffiffiffiffiffi aðeþe ¼ BE E g where B is a constant. This theory is appropriate to describe p! p* transitions in clustered, conjugated, unsaturated molecules. Like the width of individual spectra [Knapp, 1984], E g is proportional to the square root of the number of clustered atoms participating in absorption [Robertson, 2003]. Minutolo et al. [1996], Bond [2001], and Schnaiter et al. [2006] suggested this relationship to describe flamegenerated carbon Urbach Tail [20] Because a few energy states exist within the band gap, spectra exhibit a tail beyond E g rather than a sharp ð5þ 1 Auxiliary materials are available in the HTML. doi: / 2007GL of5

4 cutoff [Urbach, 1953]. This empirical relationship describes absorption below cm 1 : aðeþ ¼ a cr expð ðe cr EÞ=E 0 Þ ð6þ where a cr and E cr are values at the band-gap, and E 0 is a characteristic decay width. Larger E 0 indicates a slower decrease in absorption with wavelength (smaller slope). Schnaiter et al. [2006] suggested this tail to explain absorption by combustion aerosol, while Kinoshita et al. [1987] used it to describe absorption by organic dyes. Mullins et al. [1992] postulate that the Urbach tail is also applicable to asphaltenes because the geologic processes that create chromophores are thermally activated and result in a broad molecular distribution. Production of atmospheric chromophores may also be thermally activated, whether it occurs at atmospheric temperatures [Gelencser et al., 2003] or in pyrolysis Angstrom Exponent [21] The spectral dependence of atmospheric absorption a is sometimes represented as a power law using the absorption Angstrom exponent, Åap: al ð 1 Þ=aðl 2 Þ ¼ ðl 1 =l 2 Þ A ap For particles small relative to the wavelength, and for constant refractive index, Åap = 1; this is the case for strongly-absorbing black carbon. Åap > 1 indicates that the refractive index decreases as wavelength increases [Bond, 2001]. Åap for mixed aerosol from combustion of lignite, hard coal, and biomass ranges from 1.0 to 2.5 [Kirchstetter et al., 2004]. If the black carbon were removed, Åap for the remaining material would be greater than 2.5. Equation 7 parallels similar relationships for optical depth or scattering, but Åap has no physical basis. Nevertheless, Equations (5), (6), and (7) can be manipulated to yield relationships between Åap, E 0 and E g (see auxiliary material) Humic-Like Substances and Water-Soluble OC [22] We determined E g from absorption data for 62 humic-acid extracts and natural water samples given by Peuravuori and Pihlaja [1997]. g Eranged from 2.0 to 2.6 ev and was correlated with the square root of aromaticity (R = 0.75) with an average B 2 = cm 2 g 1 ev 1. The model is not perfect; B is not constant but depends slightly on aromaticity. Measurements that are highly resolved with respect to wavelength may better elucidate the form of the spectrum. [23] Next, we assume that chromophores in humic acids and in atmospheric water-soluble OC are similar due to their common origin. Limbeck et al. [2005] showed that atmospheric WSOC was about twice as absorbing per carbon as humic acids; humic acid may contain more non-chromophoric carbon. Figure 1b shows some of the data from Figure 1a, along with a prediction from Equation 5 ( Bandgap #1 ) using E g = 2.5 ev and B 2 double that of humic acids (57800 cm 2 g 1 ev 1 ). E g is similar to that of the least aromatic humic acid. Figure 1b also shows the cumulative fraction of solar irradiance lower than each wavelength to ð7þ demonstrate the relative importance of predicting visible absorption. [24] The band-gap curves in Figure 1b show two tails. The lower tail is the band-gap relationship alone. The upper tail is the Urbach relationship for a/r below cm 1.It is tempting to use the correlation of E 0 with E g for amorphous carbon, given by Robertson [2003], which would yield E 0 = 0.25 ev. Such a correlation may not work for atmospheric aerosol, if the electronic states allowing p! p* (aromatic) transitions are independent of those allowing n! p* (donor-receptor) transitions. Visible characteristics of humic acids do not correlate with those at UV wavelengths [ Peuravuori and Pihlaja, 1997]. The slope at visible wavelengths for particles forming in an ethyleneoxygen flame [Apicella et al., 2004] is consistent with E 0 = 0.45 ev. For asphaltenes, E 0 is about 0.3 ev [Mullins et al., 1992]. We choose an intermediate value of 0.4 ev, which fits the observations reasonably well Absorbing Combustion Aerosol [25] We have suggested that changes in absorption result from increasing aromatization [Bond, 2001]. Chhowalla et al. [2000] reported that the band-gap decreases when unsaturated bonds cluster together. We do not limit this clustering to aromatic bonds. Our investigation of the Perkampus [1992] data showed that spectral peak correlated with unsaturation number, and we use this as a metric. [26] We now assume that some biomass combustion aerosol and water-soluble OC have the same building blocks: chromophores with E g = 2.5 ev. Evans and Milne [1987] summarized mass spectroscopic data from pyrolysis of the same wood at 400, , and C. The 90th percentile (an arbitrary value) of unsaturation number is about 6 for medium-temperature products, which are slightly pyrolyzed but still oxygenated and may be similar to HULIS. For high-temperature products, unsaturation number is about 14 (see auxiliary material, Figure S4.) Assuming that E g scales with the square root of unsaturation number yields E g = We use the same value of B as for Band-gap #1, since this new material is a transformation of the original material. The resulting curve is shown as Band-gap #2. We use an Urbach decay width, E 0,of the same magnitude as Band-gap #1. However, molecules with energy states within the apparent band-gap, such as oxygenated aromatic molecules, may be transformed in combustion and change E 0. This spectrum is not applicable to all combustion aerosol, only that from the specific conditions shown here. 5. Discussion and Recommendations [27] We recommend the Band-gap #1 spectrum with the Urbach tail to represent water-soluble, humic-like organic carbon, and Band-gap #2 without the tail to represent morepolymerized organic carbon. Values of absorption coefficient are given in the auxiliary material. The apparent value of absorption Angstrom exponent is about 6 for Band-gap #1 (with tail), and about 4 for the Spectrum #2 (without tail). However, this value varies across the spectrum. [28] Because humic-acid extraction does not isolate the more-polymerized material, its optical and physical properties may be closer to those of humin, which requires 4of5

5 extraction with heat in addition to alkali. If it exists, it probably results from specific high-temperature conditions, and may be extractable with non-polar solvents. The evidence for high-absorption, non-polar material is limited but intriguing, and should be pursued. [29] The Band-gap #2 curve does not match combustion measurements exactly, but again, the measured data in Figure 1 are imperfect. Kirchstetter et al. [2004] used filter-based absorption techniques which have known artifacts. The particles measured by Schnaiter et al. [2006] are not expected to have the same B as aerosol of biomass origin. Our recommendations for modeling atmospheric aerosol rely on physical reasoning, not these measured data. The absorption cross-section for 150-nm particles, obtained from Mie calculations and weighted by the solar spectrum, is 0.4 m 2 /gc for Band-gap #1. It is 10% lower and 40% higher, respectively, than that for the highest and lowest observed humic-like spectra. The weighted cross-section for Band-gap #2 is 1.2 m 2 /gc, 15% lower and 10% higher than the two combustion aerosol spectra. [30] We found no evidence of absorbing organic carbon with E g lower than 1.6 ev; black carbon is thought to have E g = 0. Van der Wal [1996] reports a rapid transition between soot precursors and flame soot in diffusion flames, so there may be a sharp division between organic and black carbon. However, a continuum of organic carbon between humic-like, water-soluble material and strongly-absorbing, less-soluble material may exist. [31] Acknowledgments. We are grateful for support from the National Aeronautics and Space Administration (ACMAP, project ) and the National Science Foundation (Atmospheric Chemistry, ATM ). References Andreae, M. O., and A. Gelencsér (2006), Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols, Atmos. Chem. Phys., 6, Apicella, B., M. Alfe, R. Barbella, A. Tregrossi, and A. 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Mathews Avenue, Urbana, IL 61801, USA. (yark@uiuc.edu) 5of5