ARTICLE pubs.acs.org/est
Glyoxal in Aqueous Ammonium Sulfate Solutions: Products, Kinetics and Hydration Effects Ge Yu,† Amanda R. Bayer,† Melissa M. Galloway,† Kyle J. Korshavn,‡ Charles G. Fry,† and Frank N. Keutsch†,* † ‡
Department of Chemistry, University of Wisconsin—Madison, Madison, Wisconsin 53706, United States Department of Chemistry, Valparaiso University, Valparaiso, Indiana 46383, United States
bS Supporting Information ABSTRACT: Reactions and interactions between glyoxal and salts in aqueous solution were studied. Glyoxal was found to react with ammonium to form imidazole, imidazole-2carboxaldehyde, formic acid, N-glyoxal substituted imidazole, and minor products at very low concentrations. Overall reaction orders and rates for each major product were measured. Sulfate ions have a strong and specific interaction with glyoxal in aqueous solution, which shifts the hydration equilibria of glyoxal from the unhydrated carbonyl form to the hydrated form. This ion-specific effect contributes to the observed enhancement of the effective Henry’s law coefficient for glyoxal in sulfate-containing solutions. The results of UVvis absorption and NMR spectroscopy studies of solutions of glyoxal with ammonium, methylamine, and dimethylamine salts reveal that light absorbing compounds require the formation of nitrogen containing molecules. These findings have implications on the role of glyoxal in the atmosphere, both in models of the contribution of glyoxal to form secondary organic aerosol (SOA), the role of nitrogen containing species for aerosol optical properties and in predictions of the behavior of other carbonyls or dicarbonyls in the atmosphere.
1. INTRODUCTION Organic aerosol (OA) has been detected in substantial concentrations, which has impact on human health and climate. Secondary organic aerosol (SOA) formed from oxidation of volatile organic compounds (VOCs) contributes significantly to OA. However, formation and aging of SOA are still poorly understood. Glyoxal, the simplest and one of the most abundant dicarbonyls found in the atmosphere, is produced largely via photochemical oxidation of VOCs. It has recently been at the focus of numerous studies due to its potential to form SOA.112 In addition, chamber and laboratory studies have investigated SOA formation from glyoxal uptake on aerosol and cloud-processing.1,3,5,6,10 Important aspects for quantifying the role of glyoxal in forming SOA are the chemical processing of glyoxal in aerosol and the effect of electrolyte composition on the partitioning (Henry’s law) coefficient. Studies have shown that the effective Henry’s law coefficient (KH*) of glyoxal is >50 times larger in 10 mM sulfate solutions than in water and >12 times larger than in 50 mM chloride solutions.13,14 In chamber studies of glyoxal uptake on liquid ammonium sulfate ((NH4)2SO4, AS) seed aerosol, a similar enhancement of KH* has been determined.2,4 This indicates a sulfate-specific interaction with glyoxal, which has important implications for partitioning of glyoxal to either sulfatecontaining aerosol or cloud droplets. However, little work has focused on this effect or whether it is also active for other species. Another important role of glyoxal in aerosol is the effect of condensed-phase chemistry on aerosol optical properties. r 2011 American Chemical Society
Aqueous glyoxal/AS mixtures result in the formation of yellowbrown products, indicating a potential contribution to atmospheric “brown carbon”.8,1517 Shapiro et al.17 used UVvis and mass spectrometry to propose that aldol condensation and oligomerization of glyoxal are responsible for the formation of light absorbing compounds in glyoxal/AS mixtures and Noziere et al.15 proposed an ammonium or iminium catalytic pathway. The only reaction product of glyoxal and AS that has been identified via a chemical standard is imidazole-2-carboxaldehyde (IC) by Galloway et al.4 The authors proposed a mechanism via reaction of glyoxal and ammonium ion based on the work of Debus,18 who also mentioned the formation of light-absorbing compounds. De Haan et al.19 also reported formation of brown reaction products including imidazole derivatives formed from glyoxal and methylamine. Identification of these proposed absorptive compounds is important to our understanding of SOA chemistry. In this work, we utilize UVvis spectrometry, Electrospray Ionization Time-of-Flight Mass Spectrometry (ESI-TOF-MS) and NMR spectroscopy to identify and analyze the products and kinetics of the reaction of glyoxal with AS, including formation of light-absorbing species. NMR spectroscopy is particularly well suited to this work as it is easy to use in a quantitative mode, can Received: March 24, 2011 Accepted: June 10, 2011 Revised: May 31, 2011 Published: July 01, 2011 6336
dx.doi.org/10.1021/es200989n | Environ. Sci. Technol. 2011, 45, 6336–6342
Environmental Science & Technology
ARTICLE
provide better structural (molecular) information than the other used techniques, and it is an in situ technique that obtains measurement in the sample-solutions rather than indirectly as is the case for ESI. Additionally, we explore the combination of glyoxal in various electrolyte solutions via NMR spectroscopy in order to investigate the Henry’s law enhancement of glyoxal by sulfate.
2. MATERIALS AND METHODS All reagents were obtained from Sigma-Aldrich. Glyoxal trimer dihydrate was dissolved in water and stored in dark conditions at room temperature. Closed reaction vessels were used and glyoxal blank solutions did not turn brown or show product signals in NMR spectra, indicating good protection from atmospheric contamination. We did not observe evaporation of water or a change in ammonium concentration over at least six months. The lab is temperature controlled, and any small daily fluctuations were averaged out. All reaction solutions were well-mixed at the beginning of the reaction and no further stirring was conducted. H2O was used as solvent for UVvis studies, and spectra were taken with 1 mm path length cuvettes (ES quartz, Precision Cells). D2O was used for NMR studies with 1,4-dioxane as an internal standard. All 1D and 2D NMR spectra were collected on Varian 500 MHz spectrometers. All 1H NMR measurements were taken quantitatively and details of this method are included in the Supporting Information, SI. A selected experiment list is shown in Table S1. 2.1. Glyoxal/AS Product Identification and Kinetics Studies. The major products and kinetics of the reaction between
glyoxal (11.5 M) and AS (11.6 M) were studied via 1D NMR techniques (1H and 13C) and 2D NMR techniques (COSY and HSQC). Experiments conducted with 0.17 M glyoxal/3.3 M AS were measured simultaneously by NMR and UVvis. Overall reaction orders and rate constants were determined by correlating initial product formation rates with the concentration of starting reagents during the first hour of reaction (see Supporting Information for details). Activity coefficients of ions were calculated with the E-AIM model developed by Clegg et al.20,21 The pH dependence of rate constants was determined by experiments with 1.5 M glyoxal and 1 M AS at pH = 1.5, 1.9, 2.8, 3.5 (initial pH of glyoxal/AS), 4.0 and 4.5. pH was controlled via addition of dichloroacetic acid (DCA) or concentrated KOH solution (typically
