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Letter
Formaldehyde and Acetaldehyde Increase Aqueous-Phase Production of Imidazoles in Methylglyoxal – Amine Mixtures: Quantifying a Secondary Organic Aerosol Formation Mechanism Alyssa A Rodriguez, Alexia de Loera, Michelle H. Powelson, Melissa M Galloway, and David O De Haan Environ. Sci. Technol. Lett., Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 29, 2017
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Environmental Science & Technology Letters
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Formaldehyde and Acetaldehyde Increase Aqueous-
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Phase Production of Imidazoles in Methylglyoxal –
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Amine Mixtures: Quantifying a Secondary Organic
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Aerosol Formation Mechanism.
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Alyssa A. Rodriguez, Alexia de Loera, Michelle H. Powelson, Melissa M. Galloway,+ David O.
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De Haan*
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Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San
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Diego CA 92110
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+Currently at Department of Chemistry, Lafayette College, 730 High St, Easton, PA 18042
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*Corresponding_Author,
[email protected], (619) 260-6882
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KEYWORDS: secondary organic aerosol, SOA, imine, Maillard chemistry, aqueous aerosol,
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cloud processing, dicarbonyl
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ABSTRACT. Formaldehyde and acetaldehyde are commonly found in cloud droplets due to
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reversible partitioning and hydration reactions. An SOA formation pathway was recently
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identified where these common aldehydes are irreversibly incorporated into imidazole
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derivatives formed by reaction with dicarbonyl species and ammonium salts or amine species.
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Here we use UV-Vis and NMR kinetics measurements to determine the influence of
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formaldehyde and acetaldehyde on aqueous methylglyoxal chemistry. The presence of
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formaldehyde increases imidazole product formation rates by factors of 2 and ≥5 in reactions
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with ammonium sulfate and amines, respectively, and increases imidazole product yields in
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methylglyoxal + amine reactions by more than an order of magnitude. Acetaldehyde is less
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likely to be incorporated into imidazole products, and increases formation rates and yields only
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in reactions involving amines. We estimate that aqueous imidazole formation could generate as
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much as 1.05 TgC/yr SOA from formaldehyde, and 3.8 TgC/yr or 7 Tg/yr SOA overall, limited
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by the availability of aqueous phase glyoxal and methylglyoxal. While this upper limit
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represents a negligible formaldehyde sink, it is ~5% of current estimates of global SOA
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formation. Formaldehyde’s channeling of aqueous dicarbonyl chemistry towards production of
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imidazoles limits the formation of other oligomer products, including brown carbon species.
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Introduction
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Formaldehyde is the most common aldehyde species in the atmosphere,1-3 and in cloudwater it
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is typically present in concentrations equal to that of all other aldehyde species combined.2-7
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Acetaldehyde is also commonly identified in both gas-phase and cloudwater measurements.3-8
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Both species can be taken up by highly acidic aerosol particles,9-11 and partition into cloud
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Environmental Science & Technology Letters
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droplets because of their favorable hydration reactions.12 Cloud uptake is usually assumed to be
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reversible13 unless these aldehydes are oxidized by reactions with aqueous OH radicals.14 Other
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reactions have been suggested: both species may be incorporated into oligomers15 via acetal16-18
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or aldol11, 18-20 pathways; and both species react with dissolved SO2.21, 22 However, studies of
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dilute, evaporating droplets containing ammonium sulfate (AS)23 found that these aldehydes
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returned entirely back to the gas phase, although acetaldehyde took longer than formaldehyde to
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do so. In contrast, dicarbonyl species such as methylglyoxal formed oligomers so rapidly in AS-
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containing evaporating droplets that 18% was trapped in the dried residual aerosol particle.23
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Earlier studies have suggested that methylglyoxal may react with acetaldehyde and
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formaldehyde, forming acetal17 or imidazole products,24,
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pathways for SOA formation by these two smallest aldehyde species. At slightly acidic pH,
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imidazole products are formed at higher yields in dicarbonyl + amine reactions than at neutral
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pH, but the overall reactant loss rates are slower.25 In this study, we explore the effects of adding
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acetaldehyde or formaldehyde to aqueous solutions containing methylglyoxal and AS, glycine,
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or methylamine at pH 5, using NMR and UV-Vis to characterize rates of reactant loss, imidazole
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product formation, and browning. We find that formaldehyde accelerates imidazole formation
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rates and yields while reducing brown carbon formation.
providing additional aqueous
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Materials and Methods
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All chemicals were used as received from Sigma-Aldrich unless otherwise designated.
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NMR.
Samples containing 0.25 M methylglyoxal (MeGly, diluted from 40% aqueous
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solution, Alfa-Aesar), 0.50 M of a reduced nitrogen containing species (methylamine, diluted
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from 40% aqueous solution and neutralized to pH 5.4 with acetic acid); glycine, >99%; or AS,
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>99%), and 0.25 M of formaldehyde (FAld, hydrolyzed from para-formaldehyde, 95%) or
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acetaldehyde (AAld, >99.5%, Fluka), and dimethylsulfoxide as an internal standard, all in D2O
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(Cambridge Isotopes, >99.9%), were mixed in glass vials (t = 0), pH-checked, and transferred to
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NMR tubes.
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intervals that increased from 1 to 60 min over 16 h.
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methylamine, formaldehyde, acetaldehyde, and imidazole products signals were quantified at
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chemical shifts listed in Table 1 and converted to concentrations via comparison to internal
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standard signals. Initial reaction rates in M s-1 were extracted from the first 1.5 h of data.
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Calculations of imidazole yields are described in the Supplemental Information.
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H NMR spectra (500 MHz Varian Inova) were recorded at 298 K over time Changes in methylglyoxal, glycine,
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UV-Vis. Aqueous (18 MΩ water) samples containing 0.25 M methylglyoxal and AS, 0.05 M
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glycine, and either zero or 0.25 M formaldehyde were mixed, adjusted to pH 4 with acetic acid,
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placed in capped quartz cuvettes, and analyzed every 3 min by UV-Vis spectrometry (HP8452A,
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200 – 800 nm) for 12 hours at 298 K, then once every 1-3 days. The diode array instrument
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irradiated samples for
