Formaldehyde and Acetaldehyde Increase Aqueous-Phase

Apr 27, 2017 - ... AlmodovarTiffany N. StewartAlexia de LoeraAudrey C. De HaanMathieu CazaunauAline GratienEdouard PanguiJean-François Doussin...
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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 is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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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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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