Article pubs.acs.org/est
Formation of Light Absorbing Organo-Nitrogen Species from Evaporation of Droplets Containing Glyoxal and Ammonium Sulfate Alex K. Y. Lee,*,† Ran Zhao,† Richard Li,† John Liggio,‡ Shao-Meng Li,‡ and Jonathan. P. D. Abbatt† †
Department of Chemistry, University of Toronto, Toronto, Canada Air Quality Research Division, Atmospheric Science and Technology Directorate, Science and Technology Branch, Environment Canada, Toronto, Canada
‡
S Supporting Information *
ABSTRACT: In the atmosphere, volatile organic compounds such as glyoxal can partition into aqueous droplets containing significant levels of inorganic salts. Upon droplet evaporation, both the organics and inorganic ions become highly concentrated, accelerating reactions between them. To demonstrate this process, we investigated the formation of organo-nitrogen and light absorbing materials in evaporating droplets containing glyoxal and different ammonium salts including (NH4)2SO4, NH4NO3, and NH4Cl. Our results demonstrate that evaporating glyoxal-(NH4)2SO4 droplets produce light absorbing species on a time scale of seconds, which is orders of magnitude faster than observed in bulk solutions. Using aerosol mass spectrometry, we show that particlephase organics with high N:C ratios were formed when ammonium salts were used, and that the presence of sulfate ions promoted this chemistry. Since sulfate can also significantly enhance the Henry’s law partitioning of glyoxal, our results highlight the atmospheric importance of such inorganic−organic interactions in aqueous phase aerosol chemistry.
1. INTRODUCTION Atmospheric droplets including aqueous aerosol, fog, and cloud droplets can be a significant sink of water-soluble, volatile organic compounds (VOCs). Subsequent aqueous-phase reactions of the dissolved VOCs can lead to the formation of low-volatility organics and secondary organic aerosol (SOA).1−4 In particular, glyoxal, which is an α-dicarbonyl compound (C2H2O2) produced via gas-phase oxidation of various precursors such as isoprene and aromatics, has been used as a model precursor to investigate SOA formation through aqueous-phase chemistry2,3 due to its atmospheric abundance and high effective Henry’s law constant for dissolving in inorganic aerosol droplets.5,6 Previous laboratory studies have shown that aqueous-phase OH oxidation of glyoxal can generate less volatile products with high oxygen-to-carbon (O:C) ratios such as glyoxylic and oxalic acids, and therefore, it may be a substantial source of highly oxidized ambient organic aerosol that is not readily produced through traditional gasphase oxidation pathways.7−11 In addition to the photo-oxidation mechanisms for producing highly oxygenated organics, glyoxal and its mixtures with methylglyoxal can undergo self- and/or cross-oligomerization to produce dimers and trimers upon droplet evaporation.12−14 It is well-known that droplet evaporation in response to lowered relative humidity (RH) of the surrounding air can result in a highly concentrated and supersaturated solute environment inside the shrinking droplets.15,16 Due to the hygroscopic nature of inorganic salts, the dissolved glyoxal likely coexists with many inorganic components in atmospheric © 2013 American Chemical Society
aqueous aerosol, fog, and cloud droplets. The chemical processing that occurs upon droplet evaporation may thus play a critical role in accelerating the chemistry between glyoxal and inorganic ions. Since ammonium sulfate ((NH4)2SO4) is a major atmospheric aerosol component that can significantly enhance the partitioning of glyoxal into water,5,6 it is important to understand the interactions between glyoxal and ammonium (NH4+) and sulfate ions (SO42‑) within evaporating droplets. Formation of particle-phase organo-nitrogen and light absorbing organics has been proposed via aqueous glyoxal chemistry in the presence of ammonium salts, and it is generally believed that aqueous-phase ammonia (NH3, in equilibrium with NH4+) can react with glyoxal monomer to produce imidazole and its derivatives.17−22 Specifically, mixing of high concentrations of glyoxal and ammonium salts (e.g., >1 M) in bulk solutions produces strongly absorbing compounds in the ultraviolet and near-visible if given sufficiently long reaction time on the order of days.19,20 Recently, a modeling study from Woo et al.23 showed that glyoxal uptake with subsequent aqueous processing can be a significant source of light absorbing materials in the atmosphere. As pointed out above, droplet evaporation could provide a highly concentrated solute environment as a function of RH on a relatively short time scale on the order of seconds. Acceleration of the formation rates of Received: Revised: Accepted: Published: 12819
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temperature for about 1 day prior to water extraction. The filters were weighed before and after sampling to estimate the total concentration of glyoxal and its related products in the aerosol extracts using the molecular weight of glyoxal (MW = 58) and (NH4)2SO4 (MW = 132) and assuming the particles were completely dried and had the same organic mass fraction as in the bulk solution for atomization. Given that some glyoxal monomer can escape to gas phase upon droplet evaporation and glyoxal retained in the dried particles can be in its hydrated form, the estimated concentrations likely represent upper limits. A liquid waveguide capillary cell (LWCC, World Precision Instruments) with an optical path of 50 cm coupled to a temperature controlled UV−vis spectrophotometer (USB2000+, Ocean Optics) was used to measure the light absorption properties of the aerosol extracts from 230 to 850 nm. A deuterium tungsten halogen light source (DT-Mini-2, Ocean Optics) that produces stable output between 200 and 2200 nm was utilized. The three optical components were connected by optical fibers for light transmission. A syringe filter (2 μm pores) was used to remove solid particles when the extracts were injected to the LWCC. The bulk solutions before and after atomization, and the filter blank were also studied. 2.2. Bulk Experiments. Aqueous solutions of glyoxal (50− 200 mM) and individual inorganic salts ((NH4)2SO4, NH4Cl, and Na2SO4, 0.5−1.5 M) were prepared in much higher concentrations than those prepared for the droplet evaporation experiments. All solution mixtures were kept in capped glass bottles in the dark at room temperature until their light absorption properties were measured using a conventional UV−vis spectrophotometer (Lamba 12 UV−visible Spectrometer, Perkin-Elmer). The scan range of the spectrometer was set to 190−400 nm, and a quartz curvette with an optical path length of 1 cm was used. For the case of (NH4)2SO4, the initial pH of the solutions was adjusted to either 3 or 4 using HCl in order to determine the effects of acidity on the formation kinetics of light absorbing materials.
light absorbing materials in evaporating glyoxal-ammonium salt mixed droplets is a potentially relevant mechanism for atmospheric chemical processing. Indeed, Nguyen et al.24 recently demonstrated that water evaporation of SOA extracts generated via ozonolysis of D-limonene can enhance the formation rate of chromophores by at least 3 orders of magnitude compared to the reaction in bulk aqueous solution. De Haan et al.25 reported that reactions of methylglyoxal with amino acids, methylamine, and (NH4)2SO4 can take place in simulated evaporating cloud droplets to produce organonitrogen species and suggested the proposed reactions can be the source of light absorbing materials in the atmosphere. In this study, we performed a series of droplet evaporation experiments to investigate the formation of organo-nitrogen compounds and light absorbing materials in different types of evaporating glyoxal-ammonium salt (including (NH4)2SO4, NH4NO3, and NH4Cl) droplets. The experimental approach qualitatively simulates the mixing of glyoxal and inorganics in relatively dilute atmospheric droplets followed by evaporation to form SOA. Complementary bulk solution experiments were also performed to support the observations from the droplet evaporation experiments. Specifically, by comparing our results observed with different inorganic counterions of ammonium salts, we address the effects of sulfate ions on the formation of organo-nitrogen species and light absorption properties in the evaporating droplets.
2. EXPERIMENTAL SECTION 2.1. Droplet Evaporation Experiments. A series of solutions of glyoxal and individual ammonium salts including (NH4)2SO4, NH4NO3, and NH4Cl were prepared using purified water (18 MΩ-cm) with total organic carbon (TOC) less than 1 ppb. The initial concentrations of glyoxal and NH4+ were 10 and 20 mM in all solutions, respectively. A glyoxal and Na2SO4 (both 10 mM) solution was also prepared as a control. Each solution was atomized by compressed air using a TSI constant output atomizer (Model 3076). The volume mean diameter of the droplets was estimated to be 1−2 μm based on the theoretical calculation of atomizer output. A portion of the aqueous droplets was diluted by compressed air and then passed through a diffusion dryer at a flow rate of 800 sccm. The RH of the dried aerosol stream was below the crystallization RH of pure (NH4)2SO4. The dried particles were subsequently analyzed by an Aerodyne high resolution-aerosol mass spectrometer (HR-ToF-AMS) to determine their nonrefractory composition including sulfate, nitrate, ammonium, and organics. With the aerosol drying process, such an approach qualitatively simulates the mixing of glyoxal and inorganic salts in dilute solution droplets (e.g., aqueous aerosol particles at high RH, or fog droplets) followed by evaporation of water and other volatile species to form SOA. The working principle of the AMS has been reviewed by Canagaratna et al.26 The HR-ToF-AMS was operated in Wmode for determining the elemental composition of organics remaining in the evaporated droplets. Elemental analysis was performed using AMS data analysis software (Squirrel, version 1.51H, and Pika, version 1.10H) with the corrected air fragment column of the standard fragmentation table.27 For the experiments with mixed glyoxal and (NH4)2SO4, the dried particles were collected on two 47 mm Teflon filters (2 μm pores) in parallel for 2−4 h. While one filter sample was extracted using 10 mL of purified water immediately after collection, another one was kept in the dark and at room
3. RESULTS AND DISCUSSION 3.1. Rapid Formation of Light Absorbing Materials in Evaporated Droplets. The aerosol particles formed from the evaporated solution droplets were collected on Teflon filters to investigate whether droplet evaporation can lead to significant formation of light absorbing materials in the case of glyoxal(NH4)2SO4 solutions. Figure 1a shows the UV−vis absorption spectra (230−450 nm) of the aerosol filter extract, which has a strong absorption signal at ∼280 nm and a small absorption band peaked at ∼360 nm. These spectral characteristics match the light absorption properties of this chemical system observed in previous bulk studies using solute concentrations in molar level (e.g., >1 M) with reaction times on the order of hours to days.18−20 In addition, Figure 1a illustrates that the absorption spectra of bulk solutions before and after atomization are very similar to each other with a relatively weak absorption at 280 nm. No significant absorption at 280 nm was observed in the filter blank. Note that the concentrations of the bulk solutions are somewhat higher than that estimated for the aerosol filter extract and some reaction products may have hydrolyzed after the extraction. We conclude that the droplet evaporation mechanism (i.e., the drying process) significantly accelerates the formation of light absorbing materials in this chemical system. A potential explanation for these observations is that an evaporating droplet provides a highly concentrated solute 12820
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by AMS because of the hard ionization technique being used. However, glyoxal oligomers are unlikely the major contributors to the observed light absorption properties of the aerosol extracts because the total concentration of glyoxal in the aerosol extracts was on the level of mM, at which level most glyoxal is in the form of monomer and its hydrated structures. This is also supported by the UV−vis measurements that there is no significant absorption at 280 nm in the bulk glyoxal solution with initial concentrations of 10 mM (Figure 1a). Figure 1b shows the UV−vis absorption spectra (subtracted by the scaled filter blank spectrum) of filter samples extracted by water immediately and 1 day after collection. Similar absorption intensity at 280 nm between the two cases suggests that the light absorbing materials predominantly formed in droplets during evaporation instead of on the filter, after collection. Note that the residence time of particles in the diffusion dryer is approximately 7 s, representing an upper limit for the evaporation time scale kinetics; it is possible that the light absorbing materials are formed on a time scale of seconds or less. This observation also implies that aerosol liquid water is necessary for generating light absorbing materials in this chemical system. In particular, there was no additional formation of such species over a period of a day on the filter when they were either completely dried or had extremely low liquid water content. 3.2. Elemental Composition and Mass Spectral Features of Evaporated Droplets. The nitrogen-to-carbon (N:C) ratios of organics (i.e., the N in the form of NH+, NH2+, and NH3+ was excluded in the calculation because ammonium ion is the major contributor to those nitrogen-containing fragments) remaining in the evaporated droplets in the presence of different inorganic salts are shown in Figure 2. Note that the total organic mass spectrum of glyoxal(NH4)2SO4 particles is very similar in all cases. The average N:C ratio of pure glyoxal particles in all experiments was approximately 0.005, likely due to a small organo-nitrogen impurity in the water with perhaps some additional contribution arising from uncertainties in the high resolution fitting procedure of the AMS data. When (NH4)2SO4 was added to the bulk glyoxal solution, the N:C ratio of particle mixtures increased to ∼0.022 immediately, indicating rapid formation of organo-nitrogen compounds. Under a similar NH4+ concentration, the N:C ratio only increased to ∼0.011 upon addition of NH4Cl or NH4NO3 to the glyoxal solution. Further addition of Na2SO4 to those solutions (i.e., glyoxalNH4Cl or -NH4NO3 mixtures) enhanced their N:C ratios immediately to the level comparable to that observed with (NH4)2SO4, as shown in Figure 2a. Addition of Na2SO4 alone to a glyoxal solution did not result in observable changes in the N:C ratio, suggesting that the NH4+ ion is essential to produce organo-nitrogen species in our experiments. Overall, these observations indicate that inorganic sulfate plays an important role to enhance the production of organo-nitrogen compounds via aqueous glyoxal chemistry (see later discussion). Mass spectral features of evaporated droplets provide useful information to understand the formation mechanisms of organo-nitrogen species. Figure 3 reports the mass spectra of different evaporated droplets showing the nitrogen-containing peaks only (i.e., CxHyNn+ and CxHyOzNn+). All mass spectra were normalized by the total particle-phase organic measured by the AMS. Note that AMS uses electron impact ionization and so most species are detected as fragment ions. Figure 3a shows the mass spectrum of the evaporated glyoxal-(NH4)2SO4
Figure 1. (a) The absorption spectra measured in glyoxal-(NH4)2SO4 solution experiments including aerosol filter extract (blue), filter blank (gray), and bulk solutions before (green) and after atomization (orange). (b) The absorption spectra of aerosol filter samples (subtracted by the scaled filter blank spectrum) extracted immediately (blue) and 1 day after the collection (red).
environment which will greatly enhance the molecular interactions between aqueous-phase NH3 and glyoxal, thus accelerating the formation of light absorbing materials (see later discussion for possible reaction mechanism and concentration effects). In particular, it is well-known that water evaporation can lead to supersaturation of solutes in aerosol particles, whereas without atomization the glyoxal-(NH4)2SO4 solutions were probably too dilute (both at mM level) to rapidly facilitate the reactions. Furthermore, the formation of light absorbing materials likely involves condensation reactions or dehydration, which can be accelerated by actively removing water from the reaction system. De Haan and co-workers have shown that droplet evaporation can lead to transformation of dihydrated glyoxal (i.e., the most dominant form in dilute solution with relatively low reactivity) to monohydrated form as an intermediate product for producing oligomers.12 Note that the nonhydrated and monohydrated forms of glyoxal retain reactive carbonyl moieties that can be attacked by NH3 nucleophile to produce light absorption products (see later discussion). Glyoxal oligomers were expected to be formed inside the evaporating droplets,12−14 but they cannot be easily identified 12821
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Figure 2. (a) The time series profile of nitrogen-to-carbon (N:C) ratios of evaporated droplets with different inorganic composition. All bulk solutions (except for the case of Na2SO4 as a control experiment) for atomization had initial NH4+ ion concentration of 20 mM. Note that there was only a single addition of inorganic salt in the cases of Na2SO4 (red) and (NH4)2SO4 (gray). For the cases of NH4Cl (blue) and NH4NO3 (green), NH4Cl or NH4NO3 was first added to the glyoxal solution (1st addition) followed by Na2SO4 addition (2nd addition). (b) The average N:C ratios of different evaporated droplets. The error bars represent one standard deviation from different experiments. (Note: AS = (NH4)2SO4, SS = Na2SO4, AN = NH4NO3, ACl = NH4Cl).
mixed droplets. The chemical formula and proposed structures of some major peaks are listed in Table 1. In particular, the relatively strong signals at m/z 41 (C2H3N+) and 68 (C3H4N2+) are probably due to the presence of imidazole (C3H4N2) and imidazole derivatives. The AMS mass spectrum of atomized imidazole solution in the presence of (NH4)2SO4 is shown in Figure 3d for comparison. The N:C ratio of imidazole measured by HR-ToF-AMS is ∼0.44, which is lower than its theoretical value (0.67), suggesting that the N:C ratios of the evaporated mixed droplets may be underestimated. Galloway et al.17 proposed that fragmentation of 1H-imidazole-2-carboxaldehyde, which has a molecular weight of 96, can be a significant source of m/z 68. The peak at m/z 96 (C4H4N2O+) was also detected in the evaporated glyoxal-(NH4)2SO4 mixed droplets, indicating the potential formation of 1H-imidazole-2carboxaldehyde (Figure 3a). More recently, Kampf et al.21 observed dimers of imidazole and its derivates such as 2,2′bimidazole in their bulk experiments, which may also contribute to the imidazole-related fragments observed in this study. Formation of imidazole and 1H-imidazole-2-carboxaldehyde from this chemical system has been reported.17,20,21 Although there is no definitive mechanism for this reaction (see Figure 4), it is thought that aqueous-phase NH3, in equilibrium with NH4+, undergoes nucleophilic attack at the carbonyl moiety of
Figure 3. The mass spectra (CxHyNn+ and CxHyOzNn+ peaks only) of evaporated droplets (a) glyoxal-(NH4)2SO4, (b) glyoxal-NH4NO3, (c) glyoxal-NH4NO3-Na2SO4, and (d) imidazole. The major nitrogencontaining peaks labeled in green color are not due to the presence of imidazole. The proposed structures of some major peaks are shown in Table 1
the glyoxal monomer to form diimine, which then condenses with another glyoxal monomer to give imidazole and its derivatives (e.g., 1H-imidazole-2-carboxaldehyde). 17,20 Although not previously reported, it is also possible that an oxazole, which has a 5-member ring structure consisting of an oxygen and a nitrogen atom, can form as a byproduct from this multicomponent condensation.28 Oxazole species possibly contributed to the signal at m/z 69 (C3H3NO+, Table 1). Furthermore, m/z 57 (C2H3NO+) and 74 (C2H6N2O+) may correspond to the formation of imines (i.e., the intermediate products between glyoxal and NH3) or they can be the fragments of some higher molecular weight organo-nitrogen compounds. The relatively strong intensities of nitrogencontaining fragments at the lower mass range (i.e., m/z 26, 27, 29, 30, and 46) imply that the evaporated glyoxal12822
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glyoxal-NH4NO3−Na2SO4 (or glyoxal-NH4Cl−Na2SO4) and glyoxal-(NH4)2SO4 are almost identical, confirming that the three systems may undergo similar aqueous chemistry. The Na2SO4 addition experiments suggest that most of the peaks shown in Figure 3b (i.e., glyoxal-NH4NO3 and -NH4Cl droplets) are likely the intermediate products such as imines whereas the imidazole and its derivatives are the later generation products. Given that the reaction between NH3 and glyoxal monomer is one of the key steps to generate imines for subsequent production of imidazole and its derivatives, it is important to understand the potential influence of Na2SO4 (or sulfate ions) addition on the aqueous-phase concentrations of NH3 and glyoxal monomer in the evaporating droplets, and hence the overall reaction kinetics. In particular, a recent study has reported that the production rate of imidazole has a secondand first-order dependence on the concentrations of glyoxal and NH3, respectively.20 In the current study, we predicted the aqueous concentration of NH3 in different types of inorganic droplets in the absence of glyoxal as a function of relative humidity using the Extended-Aerosol Inorganic Model (EAIM).29 Assuming equilibrium established between gas- and aqueous-phase NH3 and no solid salt formation upon water evaporation, the NH3 concentrations in the presence of Na2SO4 or (NH4)2SO4 are ∼2−3 times and up to an order of magnitude higher than in droplets consisting of NH4NO3 or NH4Cl only (Figure S1), and thus might explain the higher N:C ratios observed in the evaporated Na2SO4 and (NH4)2SO4 droplets. In addition, it has been experimentally shown that inorganic sulfate favors the hydration of glyoxal (i.e., to form its gem-diol) in bulk solution due to the strong hydrogen bonding between hydrated glyoxal molecules and sulfate ions, shifting the equilibrium from oligomers to hydrated form in aqueous solution.20 However, it is not clear whether this equilibrium shifting can also increase the concentration of nonhydrated glyoxal monomer in the evaporating glyoxal-(NH4)2SO4 droplets for accelerating the imine formation chemistry described in the previous section. 3.4. Formation of Light Absorbing Materials in Bulk Solutions. To investigate the formation of light absorbing materials without evaporation, a series of bulk solutions of glyoxal and ammonium salts was prepared for UV−vis spectrophotometry analysis. To observe significant change in absorption properties on a reasonable time scale, the initial concentrations were much higher than in the evaporation experiments. Figure 5a shows the absorption spectra of a glyoxal-(NH4)2SO4 (200 mM: 3 M) solution measured in different time intervals (from time 0 to ∼266 h). Similar to the absorption spectra observed in the droplet evaporation experiments (Figure 1a), this solution had a strong absorption band at 280 nm and a relatively weak signal at ∼360 nm, whose intensities increased with reaction time. The rate of increase of these absorption signals increased with the initial concentrations of NH4+ and glyoxal, varying from 0 to 2 M and 0 to 200 mM, respectively (see Figures S2 and S3 for peaks at 280 nm). In addition, Figure 5b shows that the rate of increase of light absorption at pH 4 is much faster than that observed at pH 3. This is consistent with the aqueous-phase NH 3 concentration dropping in a more acidic solution (i.e., the equilibrium shifts toward NH4+ in low pH). Note that the bulk solutions became more acidic across the reaction time due to the formation of organic acids as previously reported,20 and hence this may slow down the formation kinetics of light
Table 1. Chemical Formula and Proposed Chemical Structures of the Major Nitrogen-Containing Peaks Observed in the Evaporated Glyoxal-(NH4)2SO4 Droplets
#
Those AMS peaks were previously reported in the study of glyoxal uptake on (NH4)2SO4 particles.17
Figure 4. A simplified reaction schematic of glyoxal in the presence of aqueous NH3. The proposed structures of organo-nitrogen observed by HR-ToF-AMS are shown inside the dashed line region.
(NH4)2SO4 droplets may be composed of a significant amount of unidentified organo-nitrogen species. 3.3. Potential Effects of Sulfate Ions on the Formation of Organo-Nitrogen. For the glyoxal-NH4NO3 mixed droplets (Figure 3b), all the nitrogen-containing peaks have lower intensities compared to those observed in the case of (NH4)2SO4, thus resulting in lower overall N:C ratios (Figure 2a). Specifically, the relative contribution of imidazole and its subsequent products (e.g., m/z 41, 68, and 96) to the overall N:C ratio is reduced by a factor of 4−5, suggesting that the formation of those compounds was promoted in the evaporated glyoxal-(NH4)2SO4 mixed droplets (e.g., the reactions between a diimine and a glyoxal monomer). Nevertheless, when Na2SO4 was added to the atomized glyoxal-NH4NO3 solution, the mass spectral features of imidazole and 1H-imidazole-2-carboxaldehyde arose as shown in Figure 3c. The evaporated glyoxalNH4Cl mixed droplets also gave the identical mass spectral characteristics and response to the addition of Na2SO4 (data not shown). By comparing Figure 3a and c, the mass spectra of 12823
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at ∼280 nm observed in the glyoxal-(NH4)2SO4 solution mixtures. Figure 6 shows the time series profiles of the absorption band at 280 nm measured in the glyoxal-NH4Cl solution (200 mM:3
Figure 6. Time series profiles of absorption signals at 280 nm: glyoxal(NH4)2SO4 solutions with initial pH at 4 (gray cross), glyoxal-NH4Cl solutions with initial pH at 3.3 (red open diamond), and addition of Na2SO4 to glyoxal-NH4Cl solutions (pH = 2.9 after addition, orange solid diamond). Note that the pH of original glyoxal-NH4Cl solutions is 2 at the point of Na2SO4 addition. The dashed line is a fit to the measurements.
M) with an initial pH of 3.3. The observed profile is similar to that observed in the glyoxal-(NH4)2SO4 solution (200 mM:1.5M) with the initial pH of 3 under the same initial NH4+ concentration (Figure 5b). Furthermore, the low initial pH of the glyoxal-NH4Cl solution resulted in a much slower formation of light absorbing species than that observed in the glyoxal-(NH4)2SO4 solution with the initial pH of 4, further confirming a significant role of acidity in affecting the chemistry. After ∼11 days, the glyoxal-NH4Cl solution was divided into two portions. Na2SO4 was added to one sample to make the sulfate concentration 2 M and the other portion was not disturbed. The pH values of the solutions with and without Na2SO4 were about 2.9 and 2, respectively, at the time of Na2SO4 addition. Figure 6 clearly illustrates that the formation rate of light absorbing materials was enhanced significantly in the presence of Na2SO4. One possible explanation for the enhancement is the pH difference between the two solutions. However, it is important to point out that the Na2SO4containing solution with an initial pH of 2.9 is more acidic than the glyoxal-NH4Cl solution at time zero (pH = 3.3) and yet had a greater formation rate of light absorbing materials compared to the glyoxal-NH4Cl solution at the time zero. This implies that solution pH was not the only governing factor in the reaction kinetics. Rather, based on the AMS droplet measurements that show enhancement in the N:C ratio upon addition of Na2SO4 to the glyoxal-NH4Cl and -NH4NO3 solutions, we believe that the sulfate ions play a critical role in controlling the formation of imidazole and its subsequent products.
Figure 5. (a) Absorption spectra of bulk solutions of pure glyoxal (200 mM) (blue line) and its solution with 1.5 M of (NH4)2SO4 (red line at ∼266 h). The gray dashed lines represent the absorption spectra of glyoxal-(NH4)2SO4 mixtures at different time intervals (i.e., less than 266 h). (b) Time series profile of absorption signals at 280 nm: pure glyoxal (green open triangle), and glyoxal-(NH4)2SO4 solutions with initial pH at 3 (blue open circle) and 4 (gray cross). The dashed line is a fit to the measurements.
absorbing materials. Furthermore, the absorption band at ∼210 nm became saturated within a few hours in the glyoxal(NH4)2SO4 bulk solution (Figure 5a), which agrees with previous studies.18,19 Nozière et al.18 has assigned this absorption feature to the CN bond, indicating reaction between glyoxal and NH3. There are three important implications from the above observations. First, similar absorption characteristics between the evaporated glyoxal-(NH4)2SO4 droplets and the bulk solutions at high concentrations indicates that the chemical composition of the light absorbing materials produced in the two experiments were similar to each other. Second, the effects of the initial concentrations of NH4+ and glyoxal support our hypothesis that the drying process can accelerate the formation rate of light absorbing materials in the evaporating glyoxal(NH4)2SO4 droplets due to the highly concentrated or supersaturated solute environment within the aerosol particles. Lastly, both the concentration and acidity effects imply that the light absorption properties observed in the bulk solution (Figure 5a) and the evaporated glyoxal-(NH4)2SO4 mixed droplets (Figure 1a) are likely due to organo-nitrogen species generated via the reactions between aqueous NH3 and glyoxal. Specifically, Yu et al.20 have proposed that imidazole-2carboxaldehyde and some unknown species with similar chemical structures likely contribute to the absorption band
4. ATMOSPHERIC IMPLICATIONS Organic aerosol components that absorb light in the nearultraviolet to lower visible wavelengths have been referred to as brown carbon,30 which can lead to positive radiative forcing in the atmosphere, enhancing the forcing from greenhouse gases and particulate black carbon. Incomplete and smoldering 12824
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NH3 and glyoxal monomer, which is consistent with previous bulk studies.19,20 The N:C ratios of condensed-phase organics increased when ammonium salts (e.g., (NH4)2SO4, NH4Cl, and NH4NO3) coexisted with glyoxal in aqueous droplets. Sulfate ions likely play an important role in enhancing the formation of organo-nitrogen species in the evaporating droplets (i.e., higher N:C ratios in the sulfate-containing droplets, Figure 2). In the absence of sulfate ions, the formation of imidazole and its subsequent organo-nitrogen species was somewhat diminished (Figure 3). Note that the light absorption properties observed are likely due to the presence of later generation organonitrogen products (e.g., imidazole-2-carboxaldehyde) as proposed by Yu et al.20 Furthermore, the Na2SO4 addition experiment in the bulk study further suggests an unidentified sulfate effect which can enhance the formation kinetics of light absorbing materials. It is well-known that (NH4)2SO4 is one of the major hygroscopic components in atmospheric aerosol and it can significantly enhance the Henry’s law partitioning of glyoxal into aqueous aerosol droplets.5,6 Overall, our observations highlight the importance of improving our fundamental understanding of the effect of sulfate on the formation of light absorbing materials and organo-nitrogen via aqueous glyoxal chemistry.
combustion, especially that associated with biomass burning, is likely a direct emission source.31−33 There is also growing laboratory evidence for the formation of brown SOA through a variety of gas-phase, aqueous-phase, and heterogeneous reactions. For instance, gas-phase oxidation of toluene with NOx,34 biogenic SOA that was subsequently aged in the presence of NH4+ under humidified condition,35 acid-catalyzed reactions of isoprene,36 and reactions of atmospherically relevant dicarbonyls with NH3, amine and amino acids in aqueous solution19,37,38 can all produce strong light absorbers that are brown in color. Recent field observations further suggest that water-soluble brown carbon may be a general feature of fresh anthropogenic SOA in urban-influenced area.39 However, our fundamental understanding of the brown carbon formation remains far from clear. By comparing the absorption spectra of the atomized solutions and aerosol filter extracts in this study, it can be concluded that the droplet evaporation mechanism significantly accelerates the formation of light absorbing materials and organo-nitrogen species inside the evaporating droplets possibly due to the highly concentrated and supersaturated solute environment. The time scale of droplet evaporation is on the order of a second, whereas the reaction time of glyoxal and (NH4)2SO4 in our bulk experiments is a few hours to days prior to UV−vis spectrophotometry analysis. The E-AIM model predicts that the concentrations of inorganic salts can be on the order of 100−101 M in the evaporating droplets,29 which is an order of magnitude higher than those used in the bulk experiments. The concentration enhancement factor of glyoxal is expected to be similar to that of inorganic salts even though some glyoxal monomer may escape to the gas-phase upon droplet evaporation. Nevertheless, it is not clear whether the condensation or dehydration reactions, which may involve the production of light absorbing materials/organo-nitrogen, can be accelerated by actively removing water from the system. Atmospheric aerosol particles frequently go through many water uptake and evaporation cycles throughout their lifetime, especially via cloud cycling. These experiments demonstrate that the reactive uptake of glyoxal into aqueous (NH4)2SO4 droplets followed by water evaporation may be a source of light absorbing materials in ambient aerosol particles. Even though the initial concentrations of glyoxal and inorganics in the bulk solution for atomization are more aerosol relevant, our results can also be applied to describe the evaporation of fog and cloud droplets because the formation of light absorbing products likely took place under a very concentrated solute environment during the drying process. Furthermore, the aerosol particles collected on the filter were either completely dried or had extremely low liquid water content, but our droplet evaporation experiments (fresh vs 1 day old samples, Figure 1b) show that liquid water is an essential component to produce light absorbing materials. Since atmospheric droplets (i.e., aerosol, fog and cloud droplets) are unlikely completely dried under most atmospheric conditions, our observations highlight that it is important to understand the critical water content (or RH) required to facilitate this chemistry. In particular, some of our observations may be captured by the model used in Woo et al.,23 but it is hard to be definitive without a good sense of the RH at which the light absorbing materials is produced. The complementary bulk phase study provides evidence that the light absorption properties of evaporated glyoxal(NH4)2SO4 droplets are likely driven by organo-nitrogen species produced through the reactions between aqueous
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ASSOCIATED CONTENT
S Supporting Information *
Additional E-AIM data and time series profiles. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Environment Canada under the Clean Air Regulatory Agenda (CARA) Science Program and NSERC. Infrastructure was made available through a grant from the Canada Foundation for Innovation.
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REFERENCES
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