Article pubs.acs.org/est
Brown Carbon Formation by Aqueous-Phase Carbonyl Compound Reactions with Amines and Ammonium Sulfate Michelle H. Powelson, Brenna M. Espelien, Lelia N. Hawkins,† Melissa M. Galloway, and David O. De Haan* Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San Diego California 92110 S Supporting Information *
ABSTRACT: Reactions between small water-soluble carbonyl compounds, ammonium sulfate (AS), and/or amines were evaluated for their ability to form light-absorbing species in aqueous aerosol. Aerosol chemistry was simulated with bulk phase reactions at pH 4, 275 K, initial concentrations of 0.05 to 0.25 M, and UV−vis and fluorescence spectroscopy monitoring. Glycolaldehyde−glycine mixtures produced the most intense absorbance. In carbonyl compound reactions with AS, methylamine, or AS/glycine mixtures, product absorbance followed the order methylglyoxal > glyoxal > glycolaldehyde > hydroxyacetone. Absorbance extended into the visible, with a wavelength dependence fit by absorption Ångstrom coefficients (Åabs) of 2 to 11, overlapping the Åabs range of atmospheric, watersoluble brown carbon. Many reaction products absorbing between 300 and 400 nm were strongly fluorescent. On a per mole basis, amines are much more effective than AS at producing brown carbon. In addition, methylglyoxal and glyoxal produced more light-absorbing products in reactions with a 5:1 AS-glycine mixture than with AS or glycine alone, illustrating the importance of both organic and inorganic nitrogen in brown carbon formation. Through comparison to biomass burning aerosol, we place an upper limit on the contribution of these aqueous carbonyl−AS−amine reactions of ≤10% of global light absorption by brown carbon.
■
aqueous-phase reactions, including oligomerization.9 BrC compounds in clouds and aerosol have chemical, lightabsorbing, and fluorescent properties similar to humic and fulvic acids found in terrestrial soils and sediments, but tend to have smaller molecular masses.9 For this reason, and because BrC is typically found in submicrometer aerosol, the transfer of humic and fulvic acids to the atmosphere by wind or other processes has been ruled out as a significant source of BrC.9 The major precursors and products of secondary BrC formation in the atmosphere are unknown, although several reactions have been proposed based on laboratory studies. Many organic compounds that are routinely found in clouds and aerosol have the potential to form fluorescent BrC products. In polluted areas, radical coupling of phenolic compounds may form oligomerized BrC.10 Small, water-soluble carbonyl compounds such as hydroxyacetone (HA), glycolaldehyde (GAld), methylglyoxal (MG), glyoxal (GX), acetaldehyde (AAld), or perhaps even formaldehyde (FAld) 11 could undergo Maillard-type browning reactions or aldol condensation reactions in the presence of ammonium salts,12−17 amino acids such as glycine (Gly),18−21 or primary amines such as methylamine (MAm).20−22 Cloudwater and
INTRODUCTION Atmospheric aerosols play a key role in the formation of clouds, degrade visibility, and affect climate by reflecting and absorbing light.1 They have also been linked to adverse health effects.2−4 Black and brown carbon are two categories of condensed-phase compounds that cause light absorption, a phenomenon that is most pronounced over wide areas of south Asia where atmospheric brown clouds are observed.5 The term “black carbon” (BC) refers to the radiation-absorbing components of soot, which consists of elemental carbon and associated condensed organics.5 “Brown carbon” (BrC) refers to carbonaceous compounds that are present in atmospheric aerosol from both anthropogenic and natural sources that absorb light with a strong wavelength dependency.6 Although BrC is less absorbing than BC on a per mole basis, BrC is more abundant in the atmosphere 7 and thus is likely of comparable importance with BC in terms of visible light absorption in the atmosphere.6,8 BC and BrC compounds in the atmosphere are the second-largest anthropogenic contributors to climate change, after carbon dioxide.5 Since aerosol particles have ∼1 week lifetimes in the atmosphere,5 reductions in the amounts of BC and BrC in the atmosphere could mitigate climate change in the short term, in addition to benefitting human health. However, emission reductions are possible only when the sources are identified. Atmospheric BrC is thought to have two major sources: primary emissions from biomass burning, and secondary formation from a variety of precursors via particle- or © 2013 American Chemical Society
Received: Revised: Accepted: Published: 985
August 28, 2013 December 11, 2013 December 18, 2013 December 18, 2013 dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
where Abse is base e absorbance at wavelength λ, b is path length in cm, and Cmass is the total concentration of reactants in g/cm3. The wavelength dependence of absorption over the range 300−700 nm (or from the top of the longest-wavelength peak to the end of measurable absorbance) was expressed using absorption Ångstrom coefficients, which are the negative slope of base e absorbance vs ln(wavelength) plots.6,24
aerosol concentrations of these substances are listed in Table 1. These reactions are complex: measurements with ammonium Table 1. Chemical Abbreviations and Reported Atmospheric Concentrations reactant
abbrev.
acetaldehyde formaldehyde glycolaldehyde glyoxal hydroxyacetone methylglyoxal ammonium sulfate glycine methylamine product
AAld FAld GAld GX HA MG AS Gly MAm
2,2′-biimidazole imidazole-2-carboxaldehyde a
cloud conc. (μM)
aerosol conc. (pmol/m3)
■
a
0.2−1.5 1.2−230a,b 0.3−3.6a 0.2−270b,d 0.2−0.6a 0.1−130b,e 1.2−18f 0.06−1.1f 0.03−0.06f abbrev.
RESULTS Carbonyl + AS Reactions. Figure 1 shows the change in UV−visible absorption spectra and any fluorescence produced
0−1300c 0−1600c 0−1200c 0−2600c 3200−7700g 0.2−170g,h 56 ± 52h
BI IC
Ref 11. bUrban, ref 33. cRef 36. dRef 35. eRef 34. fRef 40. gRef 41. Ref 42.
h
sulfate (AS)/Gly mixtures indicate that ammonium ions catalyze GX + Gly reactions,23 while studies of MG + amine reactions found that AAld was generated as an intermediate and incorporated into reaction products.20 In this work, we use absorbance spectroscopy and emission-excitation matrix (EEM) fluorescence spectroscopy of aqueous reaction mixtures at pH 4 to identify the carbonyl + amine + AS combinations most likely to produce BrC by Maillard or aldol condensation dark reactions in atmospheric aerosol.
■
MATERIALS AND METHODS All chemicals were used as received from Sigma-Aldrich unless otherwise designated. Aqueous stock solutions were made in 18-MΩ d.i. water by hydrolyzing glyoxal trimer dihydrate (Fluka >95%), glycolaldehyde dimer, or para-formaldehyde (95%), or from pure compounds (acetaldehyde (Fluka ≥99.5%), ammonium sulfate (≥99%), glycine (>99%), hydroxyacetone (90%), imidazole (>99%)), or concentrated solutions (methylamine (40% w/w), methylglyoxal (Alfa Aesar 40% w/w)).21 To simulate atmospheric aerosol conditions, reactions ran at 275 K with 0.05−0.25 M initial concentrations and pH adjusted to 4.0 with acetic or oxalic acid, which are known to be inert in these systems.21 A spectrofluorometer (JASCO FP-6500) was used in EEM mode (0.01 s response, high sensitivity, 200 nm/min emission scan speed, 5 nm data pitch) to map the development of fluorescence over a full range of excitation (λexc) and emission wavelengths (λem) (typically 220−700 nm) with the limit λem ≥ (λexc + 5 nm) to avoid scattering signals. Due to differences in signal intensity between samples, data were taken with either 1 or 3 nm spectral bandwidths; narrow bandwidth data was then corrected for differences in instrument sensitivity. A UV−vis diode array spectrometer (HP8452A) recorded changes in absorbance between 200−800 nm in capped quartz 1-cm cuvettes. Absorbance data was converted to mass absorption coefficients (MAC, in cm2/g) using the following:16 MAC(λ) =
Figure 1. Top Panel: Change in UV−visible absorption of 0.25 M carbonyl compound + 0.25 M ammonium sulfate mixtures after 4 d at pH 4. Bottom panels: Fluorescence maps, same conditions (except GAld + AS, 9 d reaction), colors indicate signal intensity. Black dotted lines indicate movement of peaks as a function of reaction time; numeric labels (in days) have colors indicating peak intensities at various time points during the reaction. No fluorescent products were observed in the reactions of AAld, FAld, or HA with AS.
by six different carbonyl compounds after reacting separately with 0.5 M AS for 4 days at pH 4. Time-dependent absorbance data for GAld and MG reactions is shown in Supporting Information, SI, Figure S1; changes in sample absorbance slowed with time, and after 4 d only minor changes in spectra were observed. In the top panel of Figure 1, increasing absorbance (positive ΔAbs) indicate the production of absorbing material, while negative ΔAbsnotable for AAld + AS at 280 nmindicate the reactive loss of an absorbing reactant. For the other five carbonyl compounds, absorbance was not observed to decline significantly at any wavelength during reaction with AS. This suggests that if any absorbing reactants were lost, the loss was offset by equal or greater absorption by product molecules. For FAld + AS reactions, absorbance increases were observed only below 240 nm. This reaction produces hexamethylenetetramine,25 with a peak absorbance at 214 nm and little absorbance beyond 240 nm. Reactions of the five other carbonyl compounds with AS generated products absorbing
Abse(λ) bCmass 986
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
Carbonyl + Methylamine Reactions. Figure 2 top shows the change in UV−visible absorption spectra of the six carbonyl
across the range 300−520 nm or more, with peak locations listed in SI Table S1. After 4 d of reaction with AS at pH 4, AAld, HA, and GAld generated products that absorbed weakly in the visible range. GX and especially MG + AS reactions, however, generated absorbing products with greater visible absorbance, measurable out to at least 700 nm. Absorbing products have been characterized previously for the GX + AS system. The major product, imidazole, absorbs most strongly at 207 nm,26 while secondary products such as imidazole-2-carboxaldehyde (IC)26 and 2,2′-biimidazole (BI)27 absorb at 273 and 280 nm, respectively. Since these peak locations are near a broad ΔAbs peak at 256 nm and a narrow peak at 222 nm (likely shifted by strong reactant absorbance at 200 nm), our data is consistent with these dominant products. In the MG + AS system, we tentatively assign the multiple peaks between 200 and 280 nm to analogous methylimidazole products20 (e.g., 4-methylimidazole, 4-methyl-2-acetylimidazole), and the strong 330 nm peak (which has no counterpart in the GX + AS reaction spectrum) to the formation of ammonium-catalyzed aldol condensation products.28 The long visible absorbance tails observed in GX and MG + AS reactions may be due to either additional, unidentified products or extensive peak broadening of these UV peaks.27 Figure 1 also shows fluorescence signals produced by GAld, GX, and MG + AS reaction products after 4 d at pH 4. No fluorescent products were observed for reactions of AS with the other three carbonyl compounds. The fluorescence intensities of MG and GAld + AS reaction products matched the order of absorption intensity at λ > 300 nm, but GX + AS reaction products were much more fluorescent. No new fluorescence signals below 240 nm were observed upon reactant mixing in any AS reaction. Thus, we can conclude that the products absorbing below 240 nm in all AS reactions (Figure 1 top panel) are not fluorescent, while at least some of those absorbing above 300 nm are fluorescent. Interestingly, GX and MG are themselves fluorescent in the absence of AS. GX by itself exhibited a very weak fluorescence peak at λexc = 335/λem = 405 nm (SI Table S1). Within an hour of mixing with AS, a new fluorescence peak appeared at 295/ 350 nm (Figure 1), which grew in intensity at the same location for several days. The new fluorescence signal was near a pH 4 imidazole standard (285/340 nm), consistent with imidazole and imidazole derivatives being the major products of glyoxal + AS reactions at pH 4.21,26,29 An aqueous solution of MG fluoresces at 345/420 nm. Upon mixing with AS, fluorescence at this position grows in intensity, and then shifts to longer wavelengths over several days (Figure 1), concurrent with a shift in the location of the UV/vis absorbance peak. This behavior has been attributed to the lengthening of conjugated regions of product molecules as aldol condensation reactions progress.18 The GAld + AS reaction generates products that fluoresce weakly at 340/425 nm after a few days reaction time at pH 4. GAld can conceivably participate in acetal, imine, and aldol condensation reaction pathways, but only aldol condensation reactions generate the conjugated double bonds necessary for fluorescence in organic molecules. Thus, the observation of fluorescence (and the similarity to MG + AS aldol condensation product fluorescence) is a strong indicator that aldol condensation reactions are occurring in the GAld + AS system, but provides no evidence for or against the other pathways.
Figure 2. Top: Changes in absorbance spectra of 0.25 M carbonyl compound + 0.25 M methylamine mixtures after 6 d at pH 4. Bottom panels: Fluorescence maps, same conditions, t = 7 d, colors indicate signal intensity. Black dotted lines indicate movement of peaks as a function of reaction time; labels (in days) have colors indicating peak intensities at various time points during the reaction. No fluorescent products were observed in the reactions of AAld or FAld with methylamine.
compounds reacting separately with MAm for 6 d at pH 4. Changes in absorbance signals are suppressed below 260 nm by strong absorbance from the higher concentrations of oxalic acid, relative to AS experiments, required to reach pH 4. Above 260 nm, the ΔAbs spectrum for FAld + MAm reaction products stays near zero, while AAld + MAm goes negative due to the loss of an absorbing product, in both cases similar to reactions with AS. No fluorescence was observed for either of these systems, consistent with the lack of absorbing products. For GX, HA, GAld, and MG, significantly more light absorption was produced in reactions with MAm than with AS. For GX + MAm, production of light-absorbing species is observed through the appearance of a new peak at 314 nm tailing into the visible. MAm reactions with HA, GAld, and MG form strong new peaks at 346, 374, and 382 nm, respectively, but only GAld and MG + MAm reaction products tail strongly (and nearly identically) into the visible. (The ΔAbs spectrum for MG + MAm drops off and exhibits noise below 325 nm due to already strong absorbance by MG in this region.) Figure 2 lower panels show fluorescence produced by GAld, GX, HA, and MG + MAm reactions after 5−7 d of reaction at pH 4. (No fluorescence was observed in reactions of either FAld or AAld with MAm.) For these four reactions, the peak λexc are within 20 nm of the longest-wavelength absorbance peak (Figure 2 top panel), suggesting that absorbing products 987
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
responsible for these peaks are also fluorescent. Peak λem and especially λexc shift upward over time during reactions of HA, GAld or MG with MAm, suggesting that MAm-catalyzed aldol condensations are taking place. However, in the glyoxal + methylamine reaction, the position of the fluorescent peak remained fixed at 340/405 nm. Previous work indicates that the major products of this reaction at pH 4 are 1,3-dimethylimidazole and formic acid.21 The addition of methyl groups to imidazole’s two nitrogen atoms likely explains the observed increase in fluorescence wavelengths relative to imidazole (285/ 340 nm). A smaller increase was reported for the addition of methyl groups to carbons on a quinoline molecule,30 another N-containing aromatic compound. An absorbance peak at 310 nm was assigned to a similar glycine-derivatized imidazole by Trainic et al.23 For these three reasons, we assign the 340/405 nm fluorescence peak to 1,3-dimethylimidazole. Carbonyl + Glycine Reactions. Figure 3 (top panel) shows the change in UV−visible absorption spectra of the six
Gly reactions was assigned to 1,3-diglycine-imidazole,23 while aldol condensation reactions catalyzed by amino acids have been observed to efficiently produce light-absorbing products from acetaldehyde.18 The MG + Gly reaction produces strong absorbers with spectra that are qualitatively similar but less intense than the spectrum of absorbers produced with MAm (Figure 2). The similarity suggests that the same products are formed at slightly slower rates, consistent with aldol condensation reactions where different aminium ions act as catalysts and are not themselves incorporated into absorbing product species. The GAld + Gly reaction produces absorbing species with a peak absorbance at 405 nm, along with a huge tail of visible absorption extending out to at least 650 nm. This substantial extension of absorption wavelengths into the visible suggests that Gly is especially efficient at catalyzing GAld aldol condensation reactions. The behavior of fluorescence peaks (Figure 3 lower panels) matches the absorbance peak assignments, since peak fluorescence wavelengths shift continuously for peaks assigned to aldol condensation products. A GX + Gly reaction product fluoresces at 350/420 nm between 3 and 30 d, and λexc is near the absorbance peak assigned to 1,3-diglycine-imidazole,23 as was also observed for 1,3-dimethylimidazole formed by GX + MAm reactions. In contrast, the fluorescence peaks produced by reactions of MG, GAld, and AAld with Gly all shift to longer wavelengths at different rates, with notable fading intensity at longer wavelengths for GAld and AAld + Gly reactions. These wavelength shifts are expected for aldol condensation reactions, which can produce lengthening, conjugated oligomers as the reactions proceed.18 More Complex Mixtures. Since large changes in absorbance spectra suggest that carbonyl−AS reactions can form different products than carbonyl−amine reactions, it is desirable to determine which products are most likely to form from AS/amine mixtures. In addition, aldehyde mixtures can also influence product distributions. Figure 4 shows UV−visible absorption spectra of aldehyde + AS reactions as Gly and/or formaldehyde are added at pH 4. Aldehyde absorption spectra are also shown for comparison. For GX (Figure 4 top), all experiments that include AS show strong absorbance near 222 nm due to imidazole production. The GX + AS reaction product spectrum also has a small peak at 285 nm, likely due to IC and BI production.26,27 The addition of FAld is observed to suppress this peak, as expected because neither IC nor BI can form when FAld is incorporated into the product. Instead, FAld is expected to steer the reaction toward imidazole production, observed as a slight broadening of the 222 nm peak. The strongest fluorescence upon FAld addition (Figure 5), however, is not from imidazole (285/340 nm) but from a new peak (330/410 nm). This new fluorescence peak may be due to an imidazole + FAld reaction forming 1N-formaldehyde-substituted imidazole (FI), (eq 1), analogous to the equilibrium between imidazole and GX with K = 0.22.26
Figure 3. Top: Changes in absorbance spectra of 0.25 M carbonyl compound + 0.25 M glycine mixtures after 7 d at pH 4. Bottom panels: Fluorescence maps, same conditions, colors indicate signal intensity. Black dotted lines indicate movement of peaks as a function of reaction time; labels (in days) have colors indicating peak intensities at various time points during the reaction. No fluorescent products were observed in the reactions of HA or FAld with glycine.
carbonyl compounds reacting separately with Gly for 7 days at pH 4. The ΔAbs spectrum produced by the HA + Gly reaction is almost identical to HA + AS; as with that reaction, no fluorescent products are observed with HA + Gly. The FAld + Gly reaction produces no change in absorbance above 250 nm and no fluorescent products, as was the case with FAld reactions with MAm or AS. The production of UV bands with strong tailing into the visible is seen with GAld, GX, and MG, while AAld generates UV-absorbing products. The 320 nm peak observed in GX +
The addition of Gly (at a 1:5 Gly/AS mole ratio) to the GX + AS system (Figure 4 top) causes a 5-fold increase in absorbance at 285 nm, further broadening of the absorbance peak at 207 nm, the appearance of a shoulder at 360 nm, and 988
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
Figure 5. Top: fluorescence of glyoxal (0.25 M) + ammonium sulfate (0.25 M) + glycine (0.05 M) after 1 d. Middle: fluorescence of glyoxal + AS + formaldehyde (0.25 M each, 30 d). Bottom: fluorescence of 2.5 mM imidazole. No peak locations changed significantly with time. All panels include fluorescence location markers for products of glyoxal + AS (purple +) and glyoxal + glycine (black × ) reactions.
Figure 4. UV−visible absorption of aldehyde (0.25 M) + ammonium sulfate (0.25 M) reactions in the presence of glycine (0.05 M) and/or formaldehyde (0.25 M) at pH 4. Glycolaldehyde and methylglyoxal data sets were collected at 4 d. Glyoxal measurements were taken at 7 d, but AS/formaldehyde/glyoxal (red dots) was measured at 8 d.
the presence of GX alone (Figure 2). However, the shift in the location of the absorbance maxima from 320 nm (GX + Gly) to 285 nm (GX + AS + Gly) suggests that ammonium ions are not only acting as catalysts. Since the peak absorbance at 285 nm and fluorescence wavelengths (310/355 nm, Figure 5) for GX + AS + Gly are between those of imidazole and 1,3-diglycineimidazole, we tentatively assign these peaks to 1-glycineimidazole. Finally, the addition of FAld to the GX + AS + Gly reaction significantly reduces the features at 285 and 360 nm, along with the visible absorbance tail, likely by reacting with 1glycine-imidazole and suppressing IC and BI formation, as seen for GX + AS in the absence of Gly.
enhanced visible absorbance. The appearance of the shoulder at 360 nm is consistent with observations by Trainic et al. 23 that adding small amounts of Gly to AS particles exposed to gasphase GX greatly increased the aerosol optical extinction measured at 355 nm. The addition of Gly also caused an increase in mass spectrometric signals for products containing C−N bonds,23 causing Trainic et al. 23 to suggest a mechanism where ammonium ions catalyze GX + Gly reactions. We observe a similar effect in our data, where 0.05 M Gly produces a larger absorbance in the range 280−320 nm in the presence of 0.25 M GX and AS (Figure 4 top) than 0.25 M Gly does in 989
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
formation. FAld addition has little effect on absorbance above 270 nm, and even increases it near 300 and 340 nm.
UV−visible absorption spectra of MG + AS reactions with Gly and/or FAld were recorded after 4 days of reaction at pH 4 (Figure 4 middle). In these spectra, MG + AS reaction products cause a significant increase in absorbance between 300 and 450 nm relative to the spectrum of MG alone. We have assigned the absorbance peak at 330 nm to an aldol condensation product. The addition of FAld causes some suppression of MG + AS product absorbance between 330 and 370 nm, likely because FAld promotes competing 4-methylimidazole formation. (Methylimidazoles absorb between 200−300 nm, which in this system is masked by MG absorbance). The addition of 0.05 M Gly to the 0.25 M MG + AS system results in a huge increase in BrC formation, reaching even higher absorbance levels than in 0.25 M MG + Gly mixtures at longer reaction times (Figure 3). The strong (and fluorescent) absorbance band that appears at 380 nm was also observed in MG reactions with MAm or Gly alone (Figures 2 and 3), and is even observed by fluorescence in MG + AS reactions after t = 18 d. We assign this peak to a larger, more conjugated aldol condensation product that forms quickly with additional, efficient amine catalysis. Remarkably, the addition of FAld (at a 1:1:1 molar ratio with MG and AS) seems to turn off the extra catalytic effect of Gly, perhaps due to inactivation of Gly via oxazolidinone formation (eq 2).
■
DISCUSSION On a per mole basis, our results suggest that amines in general are more effective than AS at forming BrC via reactions with small carbonyl compounds. This greater effectiveness has been observed previously for the specific system GX + Gly.23 Glycine appears to be most effective at catalyzing aldol condensation reactions with GAld and AAld, while MAm is more effective with HA and MG. Bones et al. 24 found that amino acid effectiveness at BrC formation followed the order Gly ∼ alanine > cysteine > arginine. In atmospheric cloud droplets and aerosol particles, aldehydes are more likely to react with amino acids than with other primary amine compounds.21 We therefore use the carbonyl + Gly results at pH 4 to rank carbonyl compounds in terms of BrC production by this mechanism on a per mole basis: GAld > MG > GX > HA ∼ AAld. A similar order (GAld > MG ≫ GX) was noted at pH 6 for reactions with lysine.32 In our experiments with AS, BrC production follows the order MG > GX > GAld ∼ HA ∼ AAld. Aqueous phase concentrations of the three fastest browning aldehydes (GAld, MG, GX) are often correlated in the atmosphere.11,33−35 The geometric mean GX/MG molar ratio calculated from these field measurements is 1.5, indicating that glyoxal is 50% more abundant than MG in atmospheric aqueous droplets, on average. Field measurements of rural and urban aerosol particles indicate that the GX/MG molar ratio is 1.2 ± 0.4.36,37 However, the visible absorbance of aldehyde-ASGly reaction products is much larger for MG than for GX at pH 4 (Figure 4). Our recent study of MG reaction kinetics with AS, MAm, Gly, serine, and arginine 21 found that these reactions were significantly faster than corresponding GX reactions at pH ≤ 5.5, and this difference grew larger at low pH. If BrC production is correlated to aldehyde reaction rates, then we predict that the difference between MG and GX BrC production rates will only grow larger in acidic atmospheric aerosol particles. Thus, we do not expect that the typical excess of GX in atmospheric aqueous droplets and aerosol particles will make up for its slower production of BrC. We conclude that MG will usually be a more important source of secondary BrC than GX under most atmospheric conditions. Condensed-phase concentrations of GAld and MG in the atmosphere are nearly equal. The geometric mean ratio of GAld/MG calculated from field measurements of cloud, fog, dew, and rain is 0.8,11,34 and for atmospheric particulates it is 1.3 ± 0.7.36,37 Since MG + AS (Figure 1) and MG + AS + Gly (Figure 4) reaction products absorb more visible light than corresponding GAld reactions, MG is expected to be generally more important than GAld in terms of secondary BrC production in atmospheric aerosol. Similarly, we can predict that GX > GAld in its potential for BrC production due to similar concentrations in atmospheric aerosol36,37 but more absorbing products produced by GX than GAld in reactions with AS (Figure 1). Hydroxyacetone is 2 to 3 times less abundant in the atmosphere than GAld. The HA/GAld molar ratio in cloudwater is 0.3 ± 0.2,11 and in particulates is 0.4 ± 0.2.36,37 The HA + AS reaction generates approximately double the absorbance as GAld + AS between 200 and 300 nm, but HA + Gly generates approximately 10 times less absorbance than GAld + Gly. Since AS is present in greater concentrations than
As expected, FAld addition had almost no effect on the aldol condensation product absorbing at 330 nm. Similar data for GAld + AS reactions were recorded after 7 d at pH 4 (Figure 4 bottom). Only slightly enhanced absorption below 260 nm is observed from GAld + AS reaction products. The addition of FAld reduces absorbance below 260 nm, while marginally increasing it above 280 nm. The addition of 0.05 M Gly provides significant increases in absorbance between 210 and 420 nm. Formation of derivatized pyrazine and pyridine products 31 are likely responsible for the increased absorbance below 270 nm. Significantly, the massive browning seen in 0.25 M GAld + Gly reaction mixtures in the absence of AS (Figure 3) does not reoccur. While it is tempting to attribute this effect to interference by AS in GAld + Gly reactions, this explanation would be inconsistent with fluorescence data. Observations of peak fluorescence wavelengths and intensities were very similar for GAld + Gly and GAld + Gly + AS reactions when [Gly] concentrations were equal (495/575 nm at 17 d), but at much shorter wavelengths for GAld + AS reactions under the same conditions (345/420 nm). This indicates that the presence of AS does not hinder GAld + Gly reactions. Rather, the lower absorbance observed in Figure 4 is likely due to [Gly] being 5× lower, and the relatively poor catalytic activity of AS toward GAld reactions. The loss of absorbance at 400 nm is closer to a factor of 25, suggesting that BrC formation is second order in Gly. For AAld, this concentration dependence has been observed when [amino acids] ≥ 0.3 M, and was attributed to a Mannich reaction pathway.18 This alternative aldol condensation mechanism produces identical reaction products via an imine−imine reaction. Our results suggest GAld + Gly → absorbing product formation kinetics are second order in Gly at amino acid concentrations down to at least 0.05 M. The addition of formaldehyde to make a 5:5:5:1 FAld/GAld/AS/ Gly mixture greatly decreases absorbance between 220 and 270 nm, consistent with disruption of pyrazine and pyridine product 990
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
was recently shown to occur in GAld ∼ HA ∼ AAld. This is the same order that was observed for BrC production by carbonyl compound + AS reactions at 0.25 M. Absorption Ångstrom coefficients, Åabs, are used to differentiate between black and brown carbon, and to model the effects of light-absorbing aerosol on atmospheric radiative transfer. While Åabs = 1 for BC, the range for atmospheric BrC is 2 ≤ Åabs ≤ 7,6 and as high as 11.4 for pine smoke extracts.39 Our aqueous-phase reaction mixtures cover this entire BrC range. For MG and GX, Åabs averaged 2.0 for reactions with AS, 9.1 for reactions with Gly (SI Table S1), and 7.9 for reactions with both AS and Gly present. For reactions of other carbonyl compounds (AAld, GAld, HA) with Gly and reactions of GX and MG with MAm, Åabs averaged 11.3. In our bulk phase experiments, the presence of FAld reduced BrC production from GX and MG. However, because FAld in atmospheric aqueous droplets is very likely to transfer to the gas phase when water evaporates from the droplet (converting formaldehyde hydrate to volatile formaldehyde), it is unlikely that FAld will effectively suppress BrC formation in atmospheric aerosol unless it is present in significant quantities. Since AS and amino acid concentrations in marine cloudwater 40 are typically the same order of magnitude as the sum of non-FAld carbonyl compounds,11,35,36 the study of 1:1 carbonyl/AS and carbonyl/Gly mixtures is atmospherically relevant. Furthermore, the concentration of all amino acids together is typically less than that of AS,40−42 justifying the 1:5 Gly/AS mixtures used. In this work, we used starting concentrations of 0.25 M, which are comparable to reported concentrations of AS and amino acids in aerosol particles in California’s Central Valley.43 These concentrations are a factor of 104 larger than those reported in rain.40 Given that 20 and 35% of MG and GX, respectively, present in a simulated cloud droplet is trapped in residual aerosol particles produced by droplet evaporation,44 it is likely that aldehyde enrichment in aerosol relative to rain is at least 103 for MG and GX. Given MG and GX concentrations in cloud and fogwater of 1 to 300 μM,11,33 MG and GX concentrations in atmospheric aerosol particles can therefore be estimated to fall within the range 0.001 to 0.3 M.45 This range overlaps concentrations used in this and several other studies of aldehyde aerosol chemistry.14,15,27 Thus, even though these carbonyl compounds are predominantly in the gas phase, where photochemical sinks dominate, the development of visible absorbance over a few days in these experiments by particle phase carbonyl compounds is therefore atmospherically relevant, because atmospheric aerosol particles have lifetimes of several days. However, atmospheric aerosol particles also cycle through different relative humidity (RH) environments, including cloud processing events. The production of UV absorbers in drying glyoxal + AS droplets
■
ASSOCIATED CONTENT
S Supporting Information *
Time-dependence UV−vis spectra, spectra in ΔMAC units, and a table of measured Ångstrom coefficients, absorbance and fluorescence peak wavelengths, intensities, and peak assignments for each reaction. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Department of Chemistry, Harvey Mudd College, 301 Platt Blvd., Claremont CA 91711. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
REFERENCES
This work was supported by NSF Grants ATM-0749145 and AGS-1129002.
(1) Dinar, E.; Taraniuk, I.; Graber, E. R.; Katsman, S.; Moise, T.; Anttila, T.; Mentel, T. F.; Rudich, Y. Cloud condensation nuclei properties of model and atmospheric HULIS. Atmos. Chem. Phys. 2006, 6, 2465−2481. (2) Dockery, D. W.; Pope, C. A.; Xu, X.; Spengler, J. D.; Ware, J. H.; Fay, M. E.; Ferris, B. G.; Speizer, F. E. An association between air pollution and mortality in six U.S. cities. N. Engl. J. Med. 1993, 329 (24), 1753−1759. (3) Perri, M. J.; Seitzinger, S.; Turpin, B. J. Secondary organic aerosol production from aqueous photooxidation of glycolaldehyde: Laboratory experiments. Atmos. Environ. 2009, 43 (8), 1487−1497.
991
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
(24) Bones, D. L.; Henricksen, D. K.; Mang, S. A.; Gonsior, M.; Bateman, A. P.; Nguyen, T. B.; Cooper, W. J.; Nizkorodov, S. A. Appearance of strong absorbers and fluorophores in limonene-O3 secondary organic aerosol due to NH4+-mediated chemical aging over long time scales. J. Geophys. Res.−Atmos. 2010, 115 (D5), D05203/1− 14. (25) Nielsen, A. T.; Moore, D. W.; Ogan, M. D.; Atkins, R. L. Structure and chemistry of the aldehyde ammonias. 3. Formaldehydeammonia reaction. 1,3,5-hexahydrotriazine. J. Org. Chem. 1979, 44 (10), 1678−1684. (26) Yu, G.; Bayer, A. R.; Galloway, M. M.; Korshavn, K. J.; Fry, C. G.; Keutsch, F. N. Glyoxal in aqueous ammonium sulfate solutions: Products, kinetics, and hydration effects. Environ. Sci. Technol. 2011, 45, 6336−6342. (27) Kampf, C. J.; Jakob, R.; Hoffmann, T. Identification and characterization of aging products in the glyoxal/ammonium sulfate systemimplications for light-absorbing material in atmospheric aerosols. Atmos. Chem. Phys. 2012, 12, 6323−6333. (28) Krizner, H. E.; De Haan, D. O.; Kua, J. Thermodynamics and kinetics of methylglyoxal dimer formation: A computational study. J. Phys. Chem. 2009, 113 (25), 6994−7001. (29) Galloway, M. M.; Chhabra, P. S.; Chan, A. W. H.; Surratt, J. D.; Flagan, R. C.; Seinfeld, J. H.; Keutsch, F. N. Glyoxal uptake on ammonium sulphate seed aerosol: reaction products and reversibility of uptake under dark and irradiated conditions. Atmos. Chem. Phys. 2009, 9, 3331−3345. (30) Rautela, R.; Joshi, H. C.; Joshi, N. K.; Tewari, N.; Pant, S. Effect of positional substitution of methyl group on the fluorescence properties of quinolinium ion. J. Lumin. 2012, 132 (8), 2151−2154. (31) Nagai, R.; Hayashi, C. M.; Xia, L.; Takeya, M.; Horiuchi, S. Identification in human atherosclerotic lesions of GA-pyridine, a novel structure derived from glycolaldehyde-modified proteins. J. Biol. Chem. 2002, 277 (50), 48905−48912. (32) Takagi, M.; Morita, N. Lysine-catalyzed Maillard browning of sugar-related compounds smaller than tetrose, in neutral and alkaline solutions. Dev. Food Sci. 1986, 13, 49−57. (33) Igawa, M.; Munger, J. W.; Hoffmann, M. R. Analysis of aldehydes in cloud- and fogwater samples by HPLC with a postcolumn reaction detector. Environ. Sci. Technol. 1989, 23 (5), 556−561. (34) Matsumoto, K.; Kawai, S.; Igawa, M. Dominant factors controlling concentrations of aldehydes in rain, fog, dew water, and in the gas phase. Atmos. Environ. 2005, 39, 7321−7329. (35) Munger, J. W.; Jacob, D. J.; Daube, B. C.; Horowitz, L. W.; Keene, W. C.; Heikes, B. G. Formaldehyde, glyoxal, and methylglyoxal in air and cloudwater at a rural mountain site in central Virginia. J. Geophys. Res. 1995, 100 (D5), 9325−9333. (36) Matsunaga, S.; Mochida, M.; Kawamura, K. Variation on the atmospheric concentrations of biogenic carbonyl compounds and their removal processes in the northern forest at Moshiri, Hokkaido Island in Japan. J. Geophys. Res. 2004, 109, D04302. (37) Ortiz, R.; Enya, K.; Sekiguchi, K.; Sakamoto, K. Experimental testing of an annular denuder and filter system to measure gas−particle partitioning of semivolatile carbonyls in the atmosphere. Atmos. Environ. 2009, 43, 382−388. (38) Sareen, N.; Schwier, A. N.; Lathem, T. L.; Nenes, A.; McNeill, V. F. Surfactants from the gas phase may promote cloud droplet formation. Proc. Natl. Acad. Sci. U.S.A. 2013, 110 (8), 2723−2728. (39) Chen, Y.; Bond, T. C. Light absorption by organic carbon from wood combustion. Atmos. Chem. Phys. 2010, 10 (4), 1773−1787. (40) Mopper, K.; Zika, R. G. Free amino acids in marine rains: Evidence for oxidation and potential role in nitrogen cycling. Nature 1987, 325, 246−249. (41) Matsumoto, K.; Uematsu, M. Free amino acids in marine aerosols over the western North Pacific Ocean. Atmos. Environ. 2005, 39, 2163−2170. (42) Zhang, Q.; Anastasio, C. Free and combined amino compounds in atmospheric fine particles (PM2.5) and fog waters from Northern California. Atmos. Environ. 2003, 37, 2247−2258.
(4) Poschl, U. Atmospheric aerosols: Composition, transformation, climate and health effects. Angew. Chem. Int. Ed. 2005, 44 (46), 7520− 7540. (5) Ramanathan, V.; Carmichael, G. Global and regional climate changes due to black carbon. Nat. Geosci. 2008, 1, 221−227. (6) Andreae, M. O.; Gelencser, A. Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols. Atmos. Chem. Phys. 2006, 6, 3131−3148. (7) Hoffer, A.; Gelencser, A.; Guyon, P.; Kiss, G.; Schmid, O.; Frank, G. P.; Artaxo, P.; Andreae, M. O. Optical properties of humic-like substances (HULIS) in biomass-burning aerosols. Atmos. Chem. Phys. 2006, 6, 3563−3570. (8) Alexander, D. T. L.; Crozier, P. A.; Anderson, J. R. Brown carbon spheres in East Asian outflow and their optical properties. Science 2008, 321, 833−836. (9) Graber, E. R.; Rudich, Y. Atmospheric HULIS: How humic-like are they? A comprehensive and critical review. Atmos. Chem. Phys. 2006, 6, 729−753. (10) Chang, J. L.; Thompson, J. E. Characterization of colored products formed during irradiation of solutions containing H2O2 and phenolic compounds. Atmos. Environ. 2010, 44, 541−551. (11) van Pinxteren, D.; Plewka, A.; Hofmann, D.; Muller, K.; Kramberger, H.; Svrcina, B.; Bachmann, K.; Jaeschke, W.; Mertes, S.; Collett, J. L., Jr.; Herrmann, H. Schmucke hill cap cloud and valley stations aerosol characterisation during FEBUKO (II): Organic compounds. Atmos. Environ. 2005, 39, 4305−4320. (12) Noziere, B.; Dziedzic, P.; Cordova, A. Formation of secondary light-absorbing “fulvic-like” oligomers: A common process in aqueous and ionic atmospheric particles? Geophys. Res. Lett. 2007, 34, L21812. (13) Noziere, B.; Dziedzic, P.; Cordova, A. Products and kinetics of the liquid-phase reaction of glyoxal catalyzed by ammonium ions (NH4+). J. Phys. Chem. 2009, 113 (1), 231−237. (14) Shapiro, E. L.; Szprengiel, J.; Sareen, N.; Jen, C. N.; Giordano, M. R.; McNeill, V. F. Light-absorbing secondary organic material formed by glyoxal in aqueous aerosol mimics. Atmos. Chem. Phys. 2009, 9, 2289−2300. (15) Sareen, N.; Schwier, A. N.; Shapiro, E. L.; Mitroo, D.; McNeill, V. F. Secondary organic material formed by methylglyoxal in aqueous aerosol mimics. Atmos. Chem. Phys. 2010, 10, 997−1016. (16) Nguyen, T. B.; Lee, P. B.; Updyke, K. M.; Bones, D. L.; Laskin, J.; Laskin, A.; Nizkorodov, S. A. Formation of nitrogen- and sulfurcontaining light-absorbing compounds accelerated by evaporation of water from secondary organic aerosols. J. Geophys. Res. 2012, 117, D01207. (17) Woo, J. L.; Kim, D. D.; Schwier, A. N.; Li, R.; McNeill, V. F. Aqueous aerosol SOA formation: Impact on aerosol physical properties. Faraday Discuss. 2013, 165, 357−367. (18) Noziere, B.; Cordova, A. A kinetic and mechanistic study of the amino acid catalyzed aldol condensation of acetaldehyde in aqueous and salt solutions. J. Phys. Chem. 2008, 112 (13), 2827−2837. (19) De Haan, D. O.; Corrigan, A. L.; Smith, K. W.; Stroik, D. R.; Turley, J. T.; Lee, F. E.; Tolbert, M. A.; Jimenez, J. L.; Cordova, K. E.; Ferrell, G. R. Secondary organic aerosol-forming reactions of glyoxal with amino acids. Environ. Sci. Technol. 2009, 43 (8), 2818−2824. (20) De Haan, D. O.; Hawkins, L. N.; Kononenko, J. A.; Turley, J. J.; Corrigan, A. L.; Tolbert, M. A.; Jimenez, J. L. Formation of nitrogencontaining oligomers by methylglyoxal and amines in simulated evaporating cloud droplets. Environ. Sci. Technol. 2011, 45 (3), 984− 991. (21) Sedehi, N.; Takano, H.; Blasic, V. A.; Sullivan, K. A.; De Haan, D. O. Temperature- and pH-dependent aqueous-phase kinetics of the reactions of glyoxal and methylglyoxal with atmospheric amines and ammonium sulfate. Atmos. Environ. 2013, 77, 656−663. (22) De Haan, D. O.; Tolbert, M. A.; Jimenez, J. L. Atmospheric condensed-phase reactions of glyoxal with methylamine. Geophys. Res. Lett. 2009, 36, L11819. (23) Trainic, M.; Riziq, A. A.; Lavi, A.; Rudich, Y. Role of interfacial water in the heterogeneous uptake of glyoxal by mixed glycine and ammonium sulfate aerosols. J. Phys. Chem. 2012, 116, 5948−5957. 992
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
Environmental Science & Technology
Article
(43) Zhang, Q.; Anastasio, C.; Jimenez-Cruz, M. Water-soluble organic nitrogen in atmospheric fine particles (PM2.5) from northern California. J. Geophys. Res. 2002, 107 (D11), 4112. (44) De Haan, D. O.; Corrigan, A. L.; Tolbert, M. A.; Jimenez, J. L.; Wood, S. E.; Turley, J. J. Secondary organic aerosol formation by selfreactions of methylglyoxal and glyoxal in evaporating droplets. Environ. Sci. Technol. 2009, 43 (21), 8184−8190. (45) Kroll, J. H.; Ng, N. L.; Murphy, S. M.; Varutbangkul, V.; Flagan, R. C.; Seinfeld, J. H. Chamber studies of secondary organic aerosol growth by reactive uptake of simple carbonyl compounds. J. Geophys. Res. 2005, 110, D23207. (46) Lee, A. K. Y.; Zhao, R.; Li, R.; Liggio, J.; Li, S.-M.; Abbatt, J. P. D. Formation of light absorbing organo-nitrogen species from evaporation of droplets containing glyoxal and ammonium sulfate. Environ. Sci. Technol. 2013, 47 (22), 12819−12826. (47) Kreidenweis, S.; Tyndall, G.; Barth, M.; Dentener, F.; Lelieveld, J.; Mozurkewich, M. Aerosols and Clouds. In Atmospheric Chemistry and Global Change; Brasseur, G. P., Orlando, J. J., Tyndall, G., Eds.; Oxford University Press: New York, 1999; pp 117−155. (48) Fu, T.-M.; Jacob, D. J.; Wittrock, F.; Burrows, J. P.; Vrekoussis, M.; Henze, D. K. Global budgets of atmospheric glyoxal and methylglyoxal, and implications for formation of secondary organic aerosols. J. Geophys. Res. 2008, 113, D15303. (49) Yokelson, R. J.; Christian, T. J.; Karl, T. G.; Guenther, A. The tropical forest and fire emissions experiment: Laboratory fire measurements and synthesis of campaign data. Atmos. Chem. Phys. 2008, 8 (13), 3509−3527.
993
dx.doi.org/10.1021/es4038325 | Environ. Sci. Technol. 2014, 48, 985−993
