Maillard Chemistry in Clouds and Aqueous Aerosol As a Source of

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Maillard chemistry in clouds and aqueous aerosol as a source of atmospheric humic-like substances. Lelia Nahid Hawkins, Amanda N. Lemire, Melissa Marie Galloway, Ashley L. Corrigan, Jacob J. Turley, Brenna M. Espelien, and David O De Haan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00909 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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Environmental Science & Technology

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Maillard chemistry in clouds and aqueous aerosol as

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a source of atmospheric humic-like substances.

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Lelia N. Hawkins,a,b,* Amanda N. Lemire,b Melissa M. Galloway,a,c Ashley L. Corrigan,a Jacob J.

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Turley,a Brenna M. Espelien,a David O. De Haana

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a. Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San

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Diego CA 92110

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b. Currently at Department of Chemistry, Harvey Mudd College, 301 Platt Blvd., Claremont CA

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91711

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c. Currently at Department of Chemistry, Lafayette College, 730 High St, Easton, PA 18042

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*Corresponding Author:

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Lelia Hawkins: Harvey Mudd College, 301 Platt Blvd., Claremont, CA 91711-5990, Phone:

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909.621.8522, Fax: 909.607.7577, [email protected].

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KEYWORDS: secondary organic aerosol formation, oligomers, light-absorbing aerosol, cloud

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processing, STXM.

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ACS Paragon Plus Environment

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ABSTRACT. The reported optical, physical, and chemical properties of aqueous Maillard

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reaction mixtures of small aldehydes (glyoxal, methylglyoxal, and glycolaldehyde) with

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ammonium sulfate and amines are compared with those of aqueous extracts of ambient aerosol

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(water-soluble organic carbon, WSOC) and the humic-like substances (HULIS) fraction of

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WSOC. Using a combination of new and previously published measurements, we examine

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fluorescence, X-ray absorbance, UV/vis, and IR spectra, complex refractive indices, 1H and 13C

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NMR spectra, thermograms, aerosol and electrospray ionization mass spectra, surface activity,

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and hygroscopicity. Atmospheric WSOC and HULIS encompass a range of properties, but in

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almost every case, aqueous aldehyde-amine reaction mixtures are squarely within this range.

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Notable exceptions are the higher UV-visible absorbance wavelength dependence (Angstrom

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coefficients) observed for methylglyoxal reaction mixtures, the lack of surface activity of glyoxal

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reaction mixtures, and the higher N/C ratios of aldehyde-amine reaction products relative to

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atmospheric WSOC and HULIS extracts. The overall optical, physical, and chemical similarities

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are consistent with, but not demonstrative of, Maillard chemistry being a significant secondary

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source of atmospheric HULIS. However, the higher N/C ratios of aldehyde-amine reaction

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products limits the source strength to ≤ 50% of atmospheric HULIS, assuming that other sources

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of HULIS incorporate only negligible quantities of nitrogen.

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Introduction

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In spite of significant efforts, the molecular components of atmospheric organic aerosol remain

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largely unidentified. A primary difficulty is the presence of thousands of different compounds,

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some of which react in aqueous or particulate-phase reactions to form complex oligomers. For

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example, single particle measurements in late summer and fall of 2005 indicate that half of

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organic aerosol particles in Los Angeles were significantly oligomerized (1), and atmospheric

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oligomers have been found to contain organic nitrogen (2). Aerosol oligomers are largely water-

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soluble, associated with light-absorbing organic compounds (brown carbon, BrC), and similar to

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terrestrial humic and fulvic acids, resulting in the acronym HULIS (humic-like substances) (3) as

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a collective term for oligomerized aerosol material. It is now clear, however, that lofted

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terrestrial humic and fulvic acids are not a major source of oligomerized aerosol or BrC, since

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terrestrial acids have larger molecular weight, higher aromaticity, lower surface activity, poorer

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droplet activation ability, and longer fluorescence wavelengths than typical aerosol extracts (4).

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Additionally, growing evidence suggests a secondary, HULIS BrC production pathway exists in

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clouds and aerosol (4-6). The production of light-absorbing and oligomeric compounds has

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important implications for climate (7-9), aerosol-cloud interactions (10), and the rate of diffusion

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of water and reactive gases into aerosol particles due to increasing viscosity and associated phase

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changes (11). A comprehensive review of current measurements and models of brown carbon

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(including HULIS BrC) composition, reactivity, and optical properties can be found in Laskin

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2015 (12). A better understanding of the sources and properties of HULIS BrC is needed to

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improve predictions of climate change and particulate air pollution.

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Several aqueous and particle-phase reactions have been suggested as sources of brown and/or

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oligomerized material in the atmosphere. For example, browning occurs when limonene SOA is


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exposed to NH3(g) or NH4+(aq) (13) and when small aldehydes undergo aldol condensation

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catalyzed by amino acids and ammonium ions (14-16).

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efficient for producing BrC in solutions containing glycolaldehyde and methylglyoxal (17). OH

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radical oxidation of phenols quickly produces brown products (18), and aldehyde-radical

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reactions produce oligomers under aerosol-like conditions (19-21). Glyoxal, methylglyoxal,

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glycolaldehyde, or hydroxyacetone react with ammonium sulfate (AS) (17, 22-27) or primary

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amines (17, 28, 29) to produce imine oligomers, N-containing aromatic derivatives, and a small

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amount of highly absorbing brown products. The lifetime of both brown and high molecular

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weight products from these reactions may be short during daytime in both clean and polluted air

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masses due to photolysis (30), though more studies are needed to determine if all AS-aldehyde

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systems and any amine-aldehyde systems behave as methylglyoxal-AS. These latter processes

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are key elements of Maillard reactions (sugars + proteins), since sugars and proteins break down

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upon heating into small aldehydes and individual amino acids. While aldehyde-amine reactions

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are slow in bulk aqueous-phase studies at room temperature, they are accelerated in drying

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aerosol particles by a factor of ~103 beyond any concentration effect (28), increasing the mass of

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the residual dried aerosol (31). Although future studies are needed to determine the acceleration

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of these reactions under intermediate relative humidity conditions, this acceleration makes

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Maillard reactions potentially competitive with OH radical reactions as sinks for small aldehydes

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in non-acidified aerosol (32).

Aldol condensation is especially

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Here we analyze the products of bulk aqueous-phase reactions of the common atmospheric

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aldehydes glycolaldehyde (GAld), glyoxal (GX), and methyglyoxal (MG) with AS and amines.

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Rather than study the kinetics of formation, our goal is to compare the reported characteristics of

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these products with published measurements of reference humic substances and atmospheric


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particulates (including extracts) to determine whether these reactions are a plausible atmospheric

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source of secondary HULIS-type BrC. Using a combination of new and previously published

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measurements, we find that Maillard reaction mixtures of atmospheric aldehydes and amines

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match a remarkable array of properties observed for atmospheric HULIS BrC, with very few

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exceptions.

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Materials and Methods

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Sample preparation procedures for all new measurements are described below. All chemicals

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were used as received from Sigma-Aldrich unless otherwise designated.

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Fluorescence. 1M stock solutions were made by hydrolyzing para-formaldehyde polymer

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(95%) or glyoxal trimer dihydrate (>95%, Fluka) in 18 MΩ deionized water. Each reaction

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mixture contained 0.25 M of an aldehyde and either glycine (>99%) or ammonium sulfate

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(>99%), was buffered to pH 4 with acetic acid, and reacted at room temperature. Excitation –

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emission fluorescence maps from 220 nm were collected twice per week in 1×1 cm quartz

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cuvettes (JASCO FP-6500, 0.01 s response, high sensitivity, 200 nm/min emission scan speed, 5

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nm data pitch, 3 nm spectral bandwidths).

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FTIR. Solutions contained 0.05 M of an aldehyde freshly mixed with 0.1 M of an amino acid

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in 18 MΩ deionized water, and had pH = 5.3 to 6.0. 1 µL aliquots were dried for 10 min. under

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ambient conditions on attenuated total reflectance (ATR) 9-reflection diamond crystals

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(DuraSamplIR, Smiths Detection) and analyzed by Fourier Transform Infrared (FTIR)

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spectroscopy (JASCO 4800, 0.5 cm-1 resolution).

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Nuclear Magnetic Resonance and Electrospray Ionization-MS. Aqueous samples containing

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0.5 M methylglyoxal (diluted from 40% aqueous solution, Alfa-Aesar), glyoxal, or


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glycolaldehyde (hydrolyzed from dimer) and 0.5 M amine (methylamine, diluted from 40%

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aqueous solution; glycine, serine, >99%; arginine, >98%; or ornithine-HCl, 99%) were mixed in

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glass vials and evaporated under ambient conditions. Dried residues were redissolved in D2O

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(Varian Inova 500 MHz, 5000 scan 13C or 8 scan 1H spectra recorded) or 18 MΩ deionized water

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at 1 mg/mL (Thermo Finnigan LCQ ESI-MS, +4 kV, 250 °C capillary, syringe-pumped at 1

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µL/min, sheath flow at 20).

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STXM-NEXAFS. Aqueous samples were prepared and dried following the NMR protocol.

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The dried residues were redissolved in 18 MΩ deionized water, atomized using a Brechtel BMI

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Aerosol Generation System, collected by impaction on silicon nitride slides (Si3N4, Silson, Ltd.,

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Northampton, England), and frozen until spectroscopic analysis. Scanning Transmission X-ray

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Microscopy – Near Edge X-ray Absorption Fine Structure (STXM-NEXAFS) has previously

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been employed to characterize individual organic aerosol particles at the carbon K-edge (33-36).

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Particles were analyzed on Beamline 5.3.2 at the Lawrence Berkeley National Laboratory

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Advanced Light Source facility. Details of the energy calibration, sample handling, and data

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analysis are presented in (34). Each particle is scanned from 280 to 320 eV; the scan results in a

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pixelated image of each particle based on carbon absorption intensity at any point along the

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carbon K-edge. A post-analysis Matlab (Mathworks) processing algorithm was used to define the

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particle in each stack of images and obtain particle-average spectra (33).

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Results and Discussion Fluorescence.

Many atmospheric aerosol particles, especially in biogenic-dominated air

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masses, are strongly fluorescent.

The fluorescence of atmospheric HULIS BrC was

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characterized by Graber and Rudich (4) with excitation / emission peaks at 340/450 nm (“fulvic-


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like”) and 250/430 nm (“humic-like”). SOA produced in the α-pinene + O3 system becomes

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strongly fluorescent in the presence of NH3 (13). Aqueous-phase reactions of small aldehydes

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with AS and amines also generate highly fluorescent products (17). The fluorescence of a few of

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these reaction systems is compared with atmospheric WSOC collected in Portugal (37) in Figure

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1. Glyoxal-AS and glyoxal-glycine reaction products fluoresce at wavelengths that match the

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fulvic-like fluorescence peak commonly seen in atmospheric HULIS BrC, while formaldehyde-

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AS reaction product fluorescence matches the humic-like peak.

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fluorescence wavelengths of the products of these three reactions are stable even at long reaction

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times in the bulk phase (17). Other aldehydes, such as MG and GAld, also generate intensely

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fluorescent product mixtures when reacting with AS and amines, but peaks shift to longer

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wavelengths during the course of reaction due to lengthening of conjugated products by aldol

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condensation (17).

Additionally, the peak

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UV-Vis. Many BrC candidate aqueous-phase reactions studied thus far in the lab generate

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product mixtures exhibiting discrete absorption bands in the UV or visible region, such as

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limonene SOA + AS (13, 38), syringol + OH (18), or acetaldehyde + amino acids (15). These

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“banded” spectra contrast with the featureless continuum usually observed in BrC extracted from

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atmospheric aerosol (39).

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absorption bands could produce a mixture exhibiting a smooth absorbance continuum, even if

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many such reactions are happening concurrently (40, 41). Instead, such featureless absorption

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and fluorescence spectra have been attributed to the formation of a complex mixture of products

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with carbonyl and hydroxyl functional groups, energetically linked to function as charge transfer

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complexes (42).

It is unlikely that reactions producing compounds with discrete


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Continuum absorbance spectra are well-fitted by a power law with exponent -λ, where λ is the

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Angstrom coefficient (4). While λ ~ 1 for black carbon (12), absorption spectra of BrC are

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characterized by steeper wavelengths dependence with 4 < λ < 8 (5, 43, 44). This type of data

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produces a straight line when graphed on a log scale (Figure 2) with slopes that correlate with –λ.

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Thus, one way such “featureless” chemical systems might be distinguished is by these slopes. In

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Figure 2, published aldehyde-AS and aldehyde-amine absorption data (17) is compared with

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urban WSOC (5). Linear fits are shown for each aldehyde. The wavelength dependence (or

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slope) of the absorbance of GX-amine and GX-amine-AS reactions (shown in red) between 300

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and 600 nm matches perfectly that observed in aqueous extracts of urban and biomass-burning-

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influenced aerosol (shown in green) (5), while MG-amine reactions (shown in black) have a

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significantly steeper wavelength dependence. GAld-amine reactions (shown in blue) produce

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~10× more light absorption per mole than glyoxal reactions, yet the average slope is still close to

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that observed in atmospheric HULIS BrC. This analysis suggests that it is at least plausible that

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glyoxal- and glycolaldehyde-amine reactions are major contributors to the atmospheric BrC

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sampled by Hecobian et al. (5), but that methylglyoxal-amine reactions are not dominant sources

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of BrC in these aerosol samples.

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Refractive index. Complex refractive indices were determined previously by cavity ringdown

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spectroscopy at 532 nm on products of GX and MG + amine reactions conducted without pH

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control (45). Here we compare those values to reported refractive indices for atmospheric

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samples (at 532-550 nm). For unbuffered reactions with glycine and methylamine, pH ranges

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from 3 - 5 and 8 – 10, respectively. With aldehyde-glycine reactions, acidity comes from the

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pyruvic acid impurity in MG and from products such as formic acid (22).

The complex


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refractive indices for GX-amine and MG-glycine aerosol (Figure 3) match that of atmospheric

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HULIS BrC extracted from smoke and from polluted environments, while aerosol particles

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generated from MG-methylamine reaction products (at high pH) were slightly more absorbing

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and less scattering.

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collected in the Amazon, and much less absorbing than BrC spheres (“tar balls”) or black carbon

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/ soot. From these measurements we conclude that aldehyde-amine aqueous BrC materials

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appear to be optically indistinguishable from BrC associated with biomass burning and urban

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BrC.

The aldehyde-amine aerosol were much more absorbing than HULIS

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FTIR. The FTIR spectra of four aqueous droplets containing an aldehyde and an amino acid,

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dried on the ATR crystal surface, are shown in Figure 4, along with those of urban aerosol

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HULIS extracts (39, 46).

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simultaneously in evaporating cloud droplets, the average of these spectra is also shown for

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comparison. While similarities in FTIR spectra of complex mixtures do not necessarily imply

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similar structures (4), spectral differences do imply significant differences in the predominant

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functional groups present. It is notable that the Guangzhou HULIS extract has a stronger

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shoulder at 3550 cm-1 due OH or NH stretching, while the Copenhagen HULIS extract matches

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the reaction samples more closely in this range. The glyoxal – amine samples are similar to the

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Copenhagen HULIS extract in the 2450 – 2900 cm-1 range where carboxylate OH and protonated

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amine NH stretches are dominant. Both HULIS extracts and 3 out of 4 aldehyde – amine reaction

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mixtures exhibit a peak (or shoulder) at 1720 cm-1, likely due to protonated carboxylic acid

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functional groups. Additional discussion of peak assignments can be found in the SI. Overall, the

Since many aldehyde – amino acid reactions could take place


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aldehyde – amino acid products have significantly higher C-N and C-O signals and slightly

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lower O-H signals than the HULIS extracts.

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NMR. A 1H-NMR spectrum of a dried MG – methylamine reaction mixture redissolved in

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D2O is shown in Figure 5, along with atmospheric organic aerosol spectra taken from the

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literature ((3, 47). In the MG – methylamine reaction mixture, signals assigned to unreacted MG

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(1.39 and 5.25 ppm) (32), unreacted methylamine (2.59 ppm), and bulk-phase product 1,3,4-

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trimethylimidazole (7.16 and 8.53 ppm) (32), are not observed. Instead, prominent narrow peaks

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are seen at 0.96, 1.75, 2.03, 2.42, and 8.27 ppm along with broad groups of unresolved signals

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centered at 1.25, 2.6, and 3.6 ppm that indicate a very complex mixture of products.

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Interestingly, the narrow 2.42 ppm peak is the strongest (non-solvent) signal in both the MG-

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methylamine reaction products spectrum and the HULIS extract from NIST 1648 urban dust (3).

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A broader peak at 1.25 ppm is also present in both spectra. While the large number of aerosol

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NMR spectra used to generate the PMF factor spectrum eliminate most narrow NMR peaks due

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to individual compounds, the resulting three broad peaks labeled “HC- O” “HC-C=O” and “HC”

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in the HULIS factor (Figure 5c) match the location of broad peaks in the MG-methylamine

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reaction mixture more closely than the other PMF factor spectra do. The relative sizes of the

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broad peaks are different, however, with the atmospheric HULIS spectra containing greater

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signals from aliphatic hydrogens (“HC”).

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In Figure 6 we compare an overlay of 13C NMR spectra for aldehyde-amine reaction mixtures 13

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with rural HULIS extracts (48). The strongest

C peak observed in the HULIS extract is the

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unsubstituted aliphatic carbon peak centered at 30 ppm, which matches the location of aliphatic

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C in arginine and ornithine side chains in the reaction mixtures. Signals due to carbon atoms


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singly bound to an N or O atom appear between 40 and 65 ppm. Most of these peaks might

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contribute to the broad 30 and 55 ppm peaks observed in the HULIS extract, but not the broad

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peak at 70 ppm, which we assign to C singly bound to both N and O. Only aldehyde-arginine

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products have similar shifts, due to carbons labeled A and B in Figure 7 inset (49). The strong

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peak at 90.5 ppm is observed only in GX reaction mixtures, and is due to the dissolved and fully

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hydrated glyoxal monomer. In solid aerosol extracts, this peak would be replaced by glyoxal

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acetal oligomer peaks in the range 90 – 105 ppm, caused by carbons singly bonded to two

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oxygen atoms. Aromatic C peaks from 110 – 160 ppm in the HULIS extract were more

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prominent in autumn samples influenced by biomass burning, likely due to the presence of lignin

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breakdown products which also enhanced carbonyl signals near 200 ppm. Aromatic signals in

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aldehyde – amine reaction mixtures are due to the formation of imidazole derivatives, with C=C

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and C=N carbons appearing near 123 and 137 ppm, respectively. No carbonyl signals are

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observed near 200 ppm in aldehyde – amine reaction mixtures because of favorable hydration of

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aldehyde functional groups in these aqueous samples. Both aldehyde – amine reaction mixtures

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and the rural HULIS extracts exhibit a carboxylic acid peak centered at 175 ppm. To summarize,

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the most notable difference in 13C NMR signals between the HULIS extract and the aldehyde +

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amino acid reaction mixtures is the large peak observed in the HULIS extract at 70 ppm,

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assigned to aliphatic carbons bonded to O and N. Very few signals are observed in this region in

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aldehyde + amino acid reaction mixtures.

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AMS. Factor analysis of aerosol mass spectrometry (AMS) field spectra has been used to

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identify a typical HULIS mass spectrum associated with continental secondary organic aerosol

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production (47).

The predominant fragments in the HULIS AMS spectrum are m/z = 44


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(carboxylate group) and other oxygenated or heteroatom fragments (m/z = 17, 18, 29, and 40),

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but it was determined that polycarboxylic acids, which compose a significant fraction of HULIS

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extracts, made up only 27% of this type of aerosol material. De Haan et al. (29) reported N/C

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and O/C ratios of several aldehyde + amine aerosols using HR-ToF-AMS. These values are

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compared in Figure S3 to elemental analysis results of rural and urban HULIS extracts (50-52).

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HULIS extracts have O/C ratios (0.45 – 0.58) that are consistent with those observed in freshly

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generated aerosol containing only MG, MG-AS, or any MG-amine mixture (except ornithine).

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N/C ratios in atmospheric HULIS extracts (0.04 – 0.05) are at least a factor of 2 lower than those

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of any aerosol generated from 1:1 mixtures of MG and AS or amine. The lower N/C ratios of

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HULIS could be due to aldehyde-amine reactions occurring under conditions where

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aldehyde:amine ratios are above 1:1, or to mixing of aldehyde-amine reaction products with non-

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nitrogen-containing material.

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methylglyoxal-amine reaction products and HULIS extracts can be used to place an upper limit

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on the contribution of the reactions of C1 – C3 carbonyl compounds with amines and AS to the

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atmospheric formation of HULIS. Using the lowest N/C ratio observed in lab experiments (MG

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+ serine bulk reaction, N/C = 0.091) and the higher of the HULIS average N/C ratios (0.045), an

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upper limit of ≤ 50% is calculated, which assumes that other HULIS-producing reactions do not

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significantly incorporate nitrogen. We note that many other proposed sources of HULIS, such as

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acid-catalyzed terpenoid or IEPOX reactions (53) or OH oxidation of phenols (18) or aldehydes

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(19-21), do not generate nitrogen-containing products. Additionally, the consistency of O/C

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ratios suggests that the “aging” influence of aldehyde-amine and aldehyde-AS reactions on the

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elemental composition of atmospheric HULIS BrC will be to increase the N/C ratio while having

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little effect on the O/C ratio.

In the latter case, the difference in N/C ratios between


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ESI-MS. The clearest evidence for oligomerization in HULIS extracts of atmospheric aerosol

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has come from electrospray ionization (ESI-) MS studies. Oligomers measured in negative ion

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mode show a broad distribution of masses centered around m/z 300 – 350, with series of peaks 2,

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14, and 16 amu apart (39, 54, 55). Most aldehyde + amine mixtures produce complex mixtures

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of oligomers with similar mass patterns (28, 29), although higher N/C ratios result in better

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detection in the positive ion mode. Figure 7 compares three HULIS extracts (negative mode)

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(39) with a dried and redissolved glycolaldehyde + methylamine mixture (positive mode). The

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lab generated sample shows a similar complexity and has a mass distribution centered at 330 ±6,

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in between those of the HULIS extracts. Laskin et al. (56) used high-resolution positive ion

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mode ESI-MS to identify N-heterocyclic alkaloid compounds as a substantial fraction of

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nitrogen-containing organic compounds in biomass-burning aerosol samples.

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abundant homologous series of oligomers were methylated imidazoles and pyrazines, which are

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also major aqueous-phase products of dicarbonyls and hydroxyaldehydes reacting with AS and

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amines (22, 25, 49). While the study notes that alkaloid compounds like these are abundant in

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pine foliage, these compounds could also be produced by the Maillard reaction of carbohydrates

285

in the presence of proteins at elevated temperatures, conditions where BrC would also be

286

produced. Since N-heterocyclic alkaloid compounds are produced by both biomass burning and

287

aqueous aldehyde-amine Maillard chemistry, these compounds are not usable as tracers for only

288

one of these BrC sources.

The most

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STXM-NEXAFS. Carbon K-edge X-ray absorbance spectra from GX-methylamine reaction

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mixtures share more features with atmospheric HULIS samples than with humic reference


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material (Figure 8). Based on relative peak area, it is clear that fulvic acid fractions have fewer

293

and narrower peaks and a greater aromatic content than atmospheric samples, consistent with

294

previous studies (4). The atmospheric samples shown were all influenced by biomass burning

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(36, 57). In the region between 288.5 and 290 eV containing both O-substituted alkyl carbon (36)

296

and C-N carbon absorbances (58), fulvic acid standards have little absorbance while atmospheric

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samples have significant absorbance.

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absorbance at 285.1 eV than either reference material or atmospheric samples, indicating less

299

aromatic C=C content. However, the atmospheric HULIS sample containing slightly aged smoke

300

shares the broad, featureless absorbance between 284 and 287 eV. The absorbance between 286

301

and 287 eV can be attributed to C=N bonds and O-substituted aromatic carbon, among other

302

groups, making this region typically too ambiguous for specific group assignment. The GX-

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methylamine reaction mixture product particles appear to have less carboxyl carbonyl and more

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amide carbonyl signature, with the large peak near 288 eV shifted to lower energy than in the

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atmospheric samples. This is consistent with elemental analyses showing the higher N/C ratio of

306

aldehyde-amine reaction products relative to atmospheric samples and with the proposed

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products (28). The overall similarity between the GX-methylamine product spectrum and the

308

atmospheric samples supports the notion that products formed from small aldehydes and amines

309

are chemically similar to many types of atmospheric HULIS.

Glyoxal-methylamine reaction mixtures have less

310 311

HTDMA. Water uptake measurements of HULIS show continuous uptake without distinct

312

deliquescence, efflorescence, or hysteresis (59, 60). This type of behavior is observed for

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complex mixtures. The reactions of GX, MG, and GAld with amines produce complex mixtures

314

with very similar continuous water uptake behavior (61). A useful parameter, κ, was introduced


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by Petters and Kreidenweis (62) in order to describe hygroscopicity of dried aerosol using a

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single term. To determine if these Maillard reactions produce material with similar

317

hygroscopicity to atmospheric HULIS, hygroscopicity kappa parameters (κ) are calculated from

318

measured growth factors for all of these systems reported in (61). The results of this comparison

319

are shown in Figure S1. Atmospheric HULIS extracts show a wide range of measured kappa

320

parameters, from 0.02 (winter rural forest) to 0.26 (polluted urban) (59). This range includes

321

kappa parameters measured for GX, MG, and GAld-amine reaction products (κ = 0.05 to 0.2),

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but also the hygroscopicity of many other organic species, including ozonolysis SOA generated

323

from a variety of precursors (63).

324 325

Thermal profiling. Thermogravimetric analysis (TGA) of aldehyde – amine aqueous reaction

326

products under an inert atmosphere (31) has shown that peak thermal breakdown depends on the

327

reaction system, but occurs between 180 and 370 °C. Thermograms are shown in Figure S2,

328

normalized at the starting and ending experimental temperatures (105 and 450 °C, respectively).

329

Thermograms of fine continental organic aerosol have two thermal breakdown peaks, the first

330

attributed to volatiles and the second (at ~260 °C) to oligomerized material (64). A study of

331

rural / coastal WSOC also observed a third region of thermal breakdown (near 500 °C) attributed

332

to stable aromatics (65). Volatilization of aldehyde-amine reaction mixtures occurs over a range

333

roughly consistent with organic aerosol oligomers, but not with stable aromatic WSOC species.

334 335

Surface Activity. HULIS have been found to be surface active (4), especially the polyacidic

336

fraction (66). The uptake by AS aerosol of MG or acetaldehyde from the gas phase at 65% RH

337

enhances aerosol CCN activity (67), an effect attributed to surface activity of the aldehydes


15


Page 16 of 31

338

themselves. However, while GX and its reaction products are not surface active (23), the

339

combination of reactions involving GX and MG would still be expected to produce some

340

surface-active products.

341 342

The notable similarity between atmospheric HULIS BrC extracts and the products of aqueous-

343

phase aldehyde–amine indicates that these reactions are plausibly significant sources of

344

atmospheric HULIS BrC. Aerosol-based kinetic studies under atmospheric conditions will be

345

necessary to quantitate the size of this source, however. The similarity across a wide range of

346

optical, physical, and chemical parameters summarized above suggests that the practice of using

347

aldehyde + amine reactions to produce “lab-generated” nitrogen-containing HULIS BrC material

348

(when extracts of atmospheric aerosol are not available in adequate quantities) is reasonable.

349 350

Acknowledgments and Funding Sources

351

This work was funded by NSF grants AGS-1129002 and AGS-1523178. LNH thanks Lynn M.

352

Russell for assistance with Beamline 5.3.2 and David Kilcoyne, beamline scientist, for access to

353

the Advanced Light Source. LNH was funded in part by Research Corporation grant 22473 and

354

the Barbara Stokes Dewey foundation.

355 356 357 358


16

Page 17 of 31


359

Supporting Information

360

Additional details regarding STXM-NEXAFS analysis are provided in the SI. We also provide

361

additional discussion of the shape of UV/visible absorption spectra, specific functional group

362

identification of FTIR spectra, and additional discussion of hygroscopicity measurements. Figure

363

S1 contains the hygroscopicity parameter κ for samples in this study and in other atmospheric

364

HULIS measurements. Figure S2 shows the results of thermogravimetric analysis (TGA) for

365

amine-aldehyde products compared with rural PM1 aerosol. Figure S3 shows the N/C and O/C

366

elemental ratios for atmospheric HULIS and methylglyoxal-amine reaction product material.

367

This information is available free of charge via the Internet at http://pubs.acs.org.

368 369 370 371


17


Page 18 of 31

500 450 300

350

250

300 200 250

HCHO + AS

Glyox + AS 150

500 450

100

400

Fluorescence Intensity

Emission Wavelength (nm)

400

50

350 300

0

250

Glyox + Glycine 250

300 350 400 250 Excitation Wavelength (nm)

Urban WSOC (Duarte et al., 2004) 300

350

400

372 373

Figure 1: Fluorescence maps of 0.25 M room-temperature pH 4 reaction mixtures.

374

Formaldehyde-AS after 6 d, (top left); glyoxal-AS after 4 h (top right); glyoxal-glycine after 3 d

375

(bottom left, 1 nm exc. bandwidth); and WSOC extract of urban/coastal particles collected in

376

Portugal (bottom right, ref (37) reprinted with kind permission from Springer Science+Business

377

Media).

378

wavelengths) between t = 2 and 6 d and grows stronger to reach maximum intensity at t = 14 d.

The fluorescence signal of formaldehyde-AS mixtures appears (at the same peak

379


18

Page 19 of 31


380 381

Figure 2: UV-visible absorbance spectra of WSOC collected in Decatur, Georgia (ref (5)) on

382

biomass-burning-influenced (thin green) and non-BB-influenced days (thick green line),

383

compared with 0.25 M aqueous aldehyde-amine mixtures after 3 – 7 days of reaction at 22 C and

384

pH 4 (reference (17)).

385

glycolaldehyde reactions. Bold dotted lines are linear fits to geometric averages of absorbance in

386

all reactions involving a given aldehyde. For glyoxal, only reaction mixtures including amines

387

were averaged.

Red:

glyoxal reactions;

black: methylglyoxal reactions;

blue:


19


Page 20 of 31

388 389

Figure 3. Complex indices of refraction measured between 532 and 550 nm for aldehyde-amine

390

reaction products (colored diamonds, ref (45)), atmospheric HULIS samples, and fulvic acid

391

standards (labeled). Vertical dashed lines: absorption components, ref (68). Organic acids, ref

392

(69); smoke influence (70); HULIS extracted from Amazonian biomass burning aerosol, ref (71);

393

Suwanee River fulvic acid, HULIS pollution and HULIS smoke, ref (72); BrC spheres, ref (73);

394

and soot, ref (74).

395 396


20

Page 21 of 31


397 398

Figure 4: FTIR-ATR spectra of dried aldehyde – amino acid droplets and atmospheric HULIS.

399

Before drying, 1 µL droplets contained 0.05 M glyoxal with 0.1 M glycine, serine, or arginine-

400

HCl, or 0.05 M methylglyoxal with 0.1 M arginine-HCl. Dried HULIS extracted from urban

401

aerosol (Guangzhou, China) using HRB sorbent and 2% v/v NH3 / MeOH eluent, pressed into a

402

KBr pellet (black line, ref (46) reprinted with permission from Elsevier); and HULIS extracted

403

from urban aerosol (Copenhagen, Denmark) dried on a CaF2 window (blue line, ref (39)

404

reprinted with kind permission from Springer Science+Business Media) are also shown.


21


Page 22 of 31

405 1

406

Figure 5:

H NMR spectra of (a) methylglyoxal – methylamine reaction mixture, (b) HULIS

407

extracted from NIST 1648 urban dust sample (ref (3) reprinted with kind permission from

408

Springer Science+Business Media), and PMF factor spectra from PM1 aerosol collected in

409

Cabauw, Netherlands (ref (47)). Top two spectra include HDO solvent peak near 4.7 ppm.

410


22

Page 23 of 31


411 412

Figure 6: Overlay of 13C NMR spectra produced in individual reactions between GX or MG and

413

amino acids (glycine, serine, arginine, or ornithine) (narrow black and gray peaks), along with.

414

CPMAS 13C solid-state NMR spectra of WSOC extracted from autumn (red) and summer (black)

415

rural aerosol (Portugal, ref (48) reprinted with permission from Elsevier). Inset: GX – arginine

416

reaction product.


23


Page 24 of 31

417 418

Figure 7: Positive ion mode electrospray mass spectra (ESI-MS) for dried and redissolved

419

glycolaldehyde – methylamine mixture (red), compared with negative ion mode electrospray

420

mass spectra for HULIS extracts (from ref (39)) from Copenhagen (“urban,” black), Melpitz,

421

Germany (“rural,” green), and Storm Peak Laboratory, Colorado (“remote,” blue), normalized by

422

air sampling volumes. All ion counts are given as 106.


24


π

π

σ

π

Page 25 of 31

423 424

Figure 8: STXM-NEXAFS carbon K-edge spectra from four GX-methylamine particles are

425

averaged (black trace) and compared with previously published spectra including atmospheric

426

HULIS (teal), fractions F2 (solid grey) and F5 (dashed grey) of Suwanee River fulvic acid (57),

427

and aerosol labeled as “tar balls” collected during biomass burning episodes in Yosemite, CA

428

(pink) and Flagstaff, AZ (green) (36). Vertical lines indicate the approximate location of various

429

functional groups. Spectra from (57) and (36) were reproduced with kind permission from

430

Elsevier and Csiro publishing, respectively.

431 432


25


433

Page 26 of 31

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654 655


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