Reactive Uptake of Glyoxal by Ammonium-Containing Salt Particles as

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Environmental Processes

Reactive uptake of glyoxal by ammonium containing salt particles as a function of relative humidity Masao Gen, Dandan Huang, and Chak Keung Chan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00606 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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Environmental Science & Technology

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Reactive uptake of glyoxal by ammonium containing salt particles as a

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function of relative humidity

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Masao Gen†, Dan Dan Huang†,‡, and Chak K. Chan†,*

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†School

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Kowloon, Hong Kong, China

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‡ Current:

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*Author

of Energy and Environment, City University of Hong Kong, Tat Chee Avenue,

Shanghai Academy of Environmental Sciences, Shanghai 200233, China

to whom correspondence should be addressed.

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Email: [email protected]

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Telephone: +(852)-3442-5593.

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Fax: +(852)-3442-0688.

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ACS Paragon Plus Environment


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ABSTRACT

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Reactions between dissolved ammonia and carbonyls, which form light-absorbing species in

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atmospheric particles, can be accelerated by actively removing water from the reaction system.

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Here, we examine the effects of relative humidity (RH) on the reactive uptake of glyoxal (Gly)

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by aqueous particles of ammonium sulfate (AS), ammonium bisulfate, sodium sulfate,

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magnesium sulfate, ammonium nitrate (AN), and sodium nitrate. In-situ Raman analysis was

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used to quantify particle-phase Gly and a colored product, 2,2’-biimidazole (BI) as a function

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of uptake time. Overall, the Gly uptake rate increases with decreasing RH, reflecting the

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“salting-in” effect. The BI formation rate increases significantly with decreasing RH or aerosol

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liquid water (ALW). Compared to that at 75% RH, the BI formation rate is enhanced by factors

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of 29 at 60% RH and 330 at 45% RH for AS particles and 65 at 60% RH, 210 at 45% RH, and

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460 at 30% RH for AN particles. These enhancement factors are much larger than those

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estimated from increased reactant concentrations due to decreases in RH and ALW alone. We

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postulate that the reduction in ALW at low RH increases the Gly uptake rate via the “salting-

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in” effect and the BI formation rate by facilitating dehydration reactions.

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INTRODUCTION

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Atmospheric particles have significant, direct impacts on the global radiative budget through

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the scattering and absorption of solar radiation. Light scattering by particles causes a negative

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forcing on climate (cooling), while light absorption transforms a fraction of solar energy in

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heat in the atmosphere (warming).1 One of the largest uncertainties in the overall aerosol

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radiative forcing is the highly variable fraction of light-absorbing species, such as mineral dust,

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black carbon, and brown carbon, in atmospheric aerosols.2–6

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Brown carbon (BrC) consists of colored organic carbon compounds that absorb light in

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the near-UV and visible ranges.7 BrC can arise from primary sources (e.g., emission from

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combustion8 and biomass burning2) and secondary formation through gas-particle conversion

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and heterogeneous (multiphase) chemical reactions.9 Particular attention has been given to the

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secondary formation of BrC by primary amine-mediated mechanisms.7,9 Reactions between -

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dicarbonyls, such as glyoxal10–13 and methylglyoxal,14,15 and amino acids, methylamine,

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dimethylamine, and ammonium salts have been shown to produce BrC. Glyoxal reaction

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products such as imidazole, imidazole-2-carboxaldehyde (IC), and 2,2’-biimidazole (BI) have

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been identified as BrC components.11,16 While BI has some absorptivity in the visible range,

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the other has little.17 Most of the actual chromophores responsible for BrC color in these

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reactions have not been identified yet. Furthermore, IC can act as a photosensitizer, initiating

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further aerosol growth in the presence of gaseous volatile organic compounds.18,19 The molar

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absorptivity of BI at 280 nm is two orders of magnitude higher than that of IC, but the formation

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rate of IC is two orders of magnitude higher than that of BI.16

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Heterogeneous reactions between glyoxal and aqueous particles involve two important

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steps: (i) glyoxal uptake and (ii) subsequent reactions (Scheme S1). The effective Henry’s Law

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constant of glyoxal in salt particles is sensitive to the aerosol liquid water (ALW) and the

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molality of the salt, which are controlled by the relative humidity (RH).20,21 Glyoxal monomers

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react with unprotonated ammonia, NH3(aq), to form light-absorbing materials (e.g., imidazole,

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IC and BI), via a number of steps, including dehydration reactions.11 Glyoxal can also react

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with water through hydration and, in some cases (e.g., at concentrations greater than 1 M),

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undergo self-oligomerization to form highly viscous glyoxal oligomers.22

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The majority of aqueous phase reaction studies have used bulk solutions containing the

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reactants (e.g., ammonium sulfate and glyoxal).10,11,13,16,17,23,24 The formation rates of

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imidazoles and their derivatives have been found to be relatively low, suggesting that the

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contributions of these reactions to ambient particle mass may be negligible,11 while IC

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photosensitization may influence particle reactivity and/or aging. 19 On the other hand, De Haan

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et al.12 and Lee et al.25 have shown that evaporating droplets containing glyoxal and ammonium

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sulfate produce light-absorbing species on time scales orders of magnitude shorter than those

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observed in bulk solutions. In addition to the atmospheric studies, similar chemical reactions

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(i.e., Maillard reactions) have been found to be sensitive to the availability of water in food

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chemistry.26 The types of inorganic anions present (e.g., sulfate, nitrate, or chloride) also affect

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the formation rates of light-absorbing species.25 Experiments involving droplets containing

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NH3(aq) and glyoxal under controlled RH or ALW conditions are useful for elucidating the role

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of droplet evaporation in the accelerated production of light-absorbing species.27 However, few

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studies have explored the effects of RH on heterogeneous reactions.

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In this study, in-situ Raman analysis was used to characterize the uptake of glyoxal into

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ammonium salt droplets under controlled RH conditions (30, 45, 60, and 75%) and to semi-

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quantitatively determine the concentrations of glyoxal (Gly) and BI in the particle phase as a

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function of uptake time. The presence of BI and IC was verified by off-line UV-vis and

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fluorescence analysis and surface-enhanced Raman spectroscopy. Comparisons of relative BI

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formation rates under different RHs enable discussion of the effects of salt type (sulfate or

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nitrate) and RH or ALW on the BI formations.

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MATERIALS AND METHODS

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Sample preparation.

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Aqueous stock solutions of ammonium sulfate (AS, 30 wt%; 99.0%, Sigma-Aldrich),

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ammonium bisulfate (ABS, 30 wt%; 99.5%, Fluka), sodium sulfate (NaS, 12 wt%; 99.0%,

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Sigma-Aldrich), magnesium sulfate (MgS, 25 wt%; 99.0%, Uni-Chem), ammonium nitrate

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(AN, 50 wt%; 98.5%, Nacalai Tesque), and sodium nitrate (NaN, 45 wt%; 99.5%, Sigma-

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Aldrich) were prepared, as were aqueous solutions of reference compounds imidazole (16.2

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mM; 99%, Sigma-Aldrich) and IC (10.7 mM; 97%, Sigma-Aldrich). BI was synthesized using

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the methods of Mao et al.,28 and an 8.1 mM aqueous BI solution was prepared. All chemicals

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were used without further purification.

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Stock solutions of each salt were atomized using a piezoelectric particle generator

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(Model 201, Uni-Photon Inc.). The generated droplets were collected on a 200-nm gold coated

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Si wafer (100, N type, Y Mart, Inc.). The substrate, which contained supermicron particles (72

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± 14 μm), was introduced into a Teflon flow cell. The RH in the flow cell was controlled by

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mixing dry and wet nitrogen gas streams (N 2 > 99.995%; Fig. S1) and monitored with an RH

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sensor (Model SHT21, Sensirion); the RH measurement accuracy is  2% for 20-80% RH. The

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particles were allowed to equilibrate at a given RH for 30-60 min before each uptake

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experiment.

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Glyoxal uptake experiment.

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The Gly monomer was generated by heating glyoxal trimer dihydrate powder (>95%, Fluka)

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at 140 C.29 The gas concentration of Gly was estimated by weighing the mass of the powder

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before and after each uptake experiment. The average concentration of Gly in the gas phase

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throughout the experiments was estimated to be 18.2 ppmv; this value is 4 to 5 orders higher

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than the Gly concentrations reported in urban and rural areas. 30–32 Thus, relatively high Gly

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concentrations were used for in situ Raman analysis of supermicron particles in these

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experiments. For off-line chemical analyses, five substrates containing particles were prepared

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and the reacted particles were dissolved in 2 mL of pure water. The Gly uptake experiments

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were conducted for 20-24 hours.

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Off-line chemical analysis.

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Aqueous extracts of the reacted particles were characterized using UV-vis (UV-2600,

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SHIMADZU) and fluorescence (RF-5301PC, SHIMADZU) spectroscopy to measure the

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absorbance and fluorescence of the reaction products (e.g., BI). In addition, surface-enhanced

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Raman spectroscopy (SERS)33 was conducted to analyze the reaction product functional

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groups. An equal volume of silver colloidal suspension (1 mM) synthesized via the citrate-

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reduction method34 was added into the liquid sample. The Ag nanoparticles dispersed through

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the aqueous solution due to the presence of citrates, which served as stabilizers, adsorbed on

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the particles.34 Following Munro et al.,35 nitric acid (NA, 50 mM) was added to the samples of

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dissolved particles after reaction to destabilize the dispersion of the Ag nanoparticles in the

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liquid phase in order to promote the aggregation of Ag nanoparticles, which enhances the

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Raman signals of the analyte molecules adsorbed on the nanoparticles. 35–37

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In-situ Raman analysis.

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Raman spectra were acquired at 100-4000 cm-1 at a spectral resolution of 4 cm-1 using a Raman

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spectrometer (EnSpectr R532, EnSpectr) coupled with an optical microscope (CX41,

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Olympus). A 20-30 mW 532 nm laser and holographic diffraction grating with 1800 grooves

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mm-1 were used. An objective 10× lens with numerical aperture = 0.25 (PlanC N, Olympus)

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was used to focus the laser onto the sample. The laser spot size and the depth of the sensing

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volume were roughly 2.6 µm and 17 µm, respectively, and the nominal particle size was about

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70 µm. After subtracting the background spectrum from each raw spectrum, Gaussian fits were

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used to obtain the peak position and area for isolated peaks, such as sulfate and nitrate at ~980

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and ~1046 cm-1, respectively (Igor Pro, Wavemetrics). Gaussian fits were not used for the v(C-

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H) mode at 2900-3050 cm-1 resulting from Gly uptake, as this region overlaps with the

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ammonium and water peaks; instead, background-subtracted Raman spectra were directly

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integrated in the 2900-3050 cm-1 region to obtain the peak areas in the v(C-H) mode. Note that

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the v(C-H) mode can be attributed to glyoxal monomers as well as its hydrates.

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RESULTS AND DISCUSSION

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First, we discuss the results of off-line analyses (UV-vis, fluorescence, and SERS) of the

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extracted particles solutions to identify the IC and BI reaction products and establish the

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appropriateness of in-situ Raman analysis for determining particle phase Gly and BI

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concentrations. Then, the Henry’s Law constant of Gly in salt is calculated to predict how

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variations in RH alter the concentration of particle-phase Gly as a function of uptake time (i.e.,

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the Gly uptake rate). Next, the Gly uptake rate as a function of RH is determined based on the

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v(C-H) peak at 2900-3050 cm-1 in the Raman analysis. Lastly, based on the fluorescence

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background in the Raman measurements, we examine the effects of RH on the BI formation

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rate.

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Off-line chemical analysis of reaction products.

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Figure S2 shows the UV-vis absorption and fluorescence emission spectra of the aqueous

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extracts of AS, MgS, AN, and NaN particles after reaction with Gly (AS/Gly, MgS/Gly,

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AN/Gly, and NaN/Gly, respectively) at 60% RH; for comparison, spectra are also shown for

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unreacted AS and AN particles. The absorption spectra of both AS/Gly and AN/Gly exhibit a

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peak at ~287 nm due to the formation of IC and BI,11,17,25 whereas the MgS/Gly, NaN/Gly, AS,

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and AN spectra do not have any obvious peaks; bulk solution studies on reactions of both AS

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and AN with Gly have produced similar absorption spectra.10

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AS/Gly and AN/Gly particles have similar fluorescence emission spectra featuring a

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peak at ~332 nm (Figs. S2c and d). This peak has been previously identified as an indicator of

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IC and BI.17 No significant peaks were observed for the other particle types. These UV-vis and

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fluorescence spectroscopy results demonstrate that the presence of ammonium and/or ammonia

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in the particle phase initiates reactions that form light-absorbing materials, which is consistent

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with earlier work.11,25

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SERS analysis33 was performed after the addition of Ag nanoparticles (Ag) and nitric

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acid (NA) to enhance the Raman signals of analyte molecules (Appendix 1, Supporting

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Information (SI)). Figure S3 shows SERS spectra for Ag+AS/Gly+NA and Ag+AN/Gly+NA

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from particles after reaction at 60% RH and the other particles with and without added Ag

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nanoparticles and NA. The AS/Gly sample (Ag+AS/Gly+NA) shows peaks at 655, 1176, 1441,

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1480, and 1573 cm-1, and the AN/Gly sample shows peaks at 655, 1147, 1173, and 1504 cm-1;

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peaks at ~655, 1176, 1441, and 1480 cm -1 have been reported in SERS spectra for

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imidazole.38,39 The spectral signatures associated with imidazole and/or its derivatives are

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consistent with the absorption and fluorescence emission spectra results for the AS/Gly and

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AN/Gly samples. Together, UV-vis, fluorescence, and SERS analyses indicate with high

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confidence that imidazole derivatives (e.g., IC and BI) form during the reactive uptake of Gly

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by NH3(aq)-containing droplets.

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Model prediction of effective Henry’s Law constants for Gly in salt as a function of RH.

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To examine the effect of RH on the solubility of Gly in salt, we estimate the Henry’s Law

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constants of Gly in different solutions. The solubility of Gly increases with salt concentration,

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which is often called the “salting-in” effect. In general, the solubility of an organic species in

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a salt solution can be described by:20

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log⁡(𝑆0 /𝑆) = ⁡ 𝐾S 𝐶salt,RH

(1)

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where S0 and S are the solubility of the organic compound in pure water and the salt solution,

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respectively; KS is the salting constant; and Csalt,RH is the molality of salt solution (mol kg-1) at

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equilibrium at a particular RH. The S0/S can be rewritten in terms of the Henry’s Law constants

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as KH,w/KH,salt,RH, where KH,w and KH,salt,RH are the effective Henry’s Law constants of the

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organic in pure water and the salt solution at equilibrium at a particular RH, respectively.21

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After the substitution, Eq. (1) is rewritten as:

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log⁡(𝑆0 /𝑆) ⁡ = log⁡(𝐾H,w /𝐾H,salt,RH ) = 𝐾S 𝐶salt,RH

(2)

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In this study, the KH,salt,RH of Gly was predicted using Eq. (2) and the KH,w = 4.19 × 105 M atm-

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0.06 ± 0.02, and -0.065 ± 0.006 kg mol-1 for AS, AN, and NaN, respectively. Note that while

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the chemical composition of salt particle has an effect on the Gly uptake, 41 we used the KS

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values of AS for ABS, NaS, and MgS, since previous studies have shown that the nature of the

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cation does not affect the value of KS.20 The salt molality, Csalt,RH, was calculated using the

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Extended-Aerosol Inorganic Model (E-AIM)42,43 with an assumption that no precipitates were

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formed. Concentrations of NH3(aq) in the AS, ABS, and AN particles were also predicted as a

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function of RH using the E-AIM model. For MgS particles, the Aerosol Inorganic-Organic

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Mixtures Functional groups Activity Coefficients model (AIOMFAC model, available at

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http://www.aiomfac.caltech.edu) was used to predict the given properties.44,45

reported by Ip et al.40 Waxman et al.20 experimentally measured KS values of -0.16 ± 0.02, -

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Table S1 lists the molalities, predicted effective Henry’s Law constants, concentrations

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of NH3(aq) ([NH3]), and measured gas phase and particle phase Gly concentrations ([Gly]g and

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[Gly]p, respectively) as functions of RH. Overall, as RH decreases, the calculated KH,salt,RH

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increases as the salt molality increases. This suggests that the Gly uptake rate should increase

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with decreasing RH. The KH,AS,RH at 45% RH (KH, AS, 45%) is almost two orders of magnitude

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larger than KH,AS,75%. KH,AN,30% is 7.21 × 1015 M atm-1, or nine orders of magnitude larger than

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that at 75% RH. Although the “salting-in” effect is more efficient for AS than AN (i.e., Ks for

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AS > for AN), the predicted KH,AS,RH and KH,AN,RH are comparable at each RH because the

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value of CAN,RH is more than twice that of CAS,RH over the entire RH range studied. However,

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kinetic limitation may constrain the actual solubility at high Csalt (e.g., < 45% RH). Kampf et

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al.21 reported that KH,AS leveled off at CAS > 12 mol kg-1 and attributed this effect to the

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availability of fewer water molecules for reactions and/or increased particle viscosity in their

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experiments. The predicted values of KH,salt,RH will be used to discuss the RH-dependent Gly

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uptake rate and its kinetic limitation in the following sections.

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Raman spectral changes after Gly uptake.

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We describe first the Raman spectral signatures after Gly uptake, and then the use of in-situ

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Raman spectroscopy to estimate [Gly]p. In the next section, we present the Gly uptake rates of

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different non-NH3(aq)-containing salt particles, followed by those of AN, AS, and ABS. The

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role of the interactions of [Gly]p with sulfate and NH3(aq) are also discussed, along with Gly

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oligomerization.

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In the absence of NH3(aq), Gly reacts with water through hydration, followed by

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oligomerization (Scheme S1).22 In addition, the hydrated form of Gly strongly binds to

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sulfate.46 All of these reactions and binding interactions can potentially increase the KH,salt value

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as predicted by Eq. (2). Figure 1 shows the Raman spectra of NaS and MgS particles at 75 and

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60% RH, respectively, and NaN particles at 60% RH as a function of uptake time; NaS particle

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experiments were conducted at 75% RH because the efflorescence RH (ERH) of NaS particles

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is close to 60% RH.47 The Raman spectra of NaS/Gly at 75% RH show a small peak at 2900-

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3000 cm-1 due to the Gly v(C-H) mode (Figs. 1a and b), but no significant spectral features are

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observed for MgS/Gly at 60% RH (Fig. 1c). NaN particles (Figs. 1d and e) have a much

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stronger v(C-H) mode than do the NaS and MgS particles, which may be caused in part by the

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fact that the KH,NaN at 60% RH is slightly higher than the KH,NaS at 75% RH. Three peaks at

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770, 872, and 952 cm-1 emerge in addition to the v(C-H) mode as uptake proceeds (Fig. 1f);

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these peaks correspond to the in-plane deformation mode of O-C-O (δ(O-C-O)ring), the

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stretching mode of C-C (v(C-C)), and the stretching mode of the ring structure (v(ring)),

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respectively, and are attributed to the formation of Gly oligomers (Scheme S1) in the aqueous

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phase.22

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Figure 2 shows the Raman spectra of AS/Gly and AN/Gly particles at 60% RH as a

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function of uptake time. The v(C-H) peak appears in AN/Gly particles as uptake proceeds (Fig.

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2b), while this peak is not as clearly visible for AS particles. For ABS/Gly particles, the

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development of the v(C-H) and δ(O-C-O)ring peaks is obvious (see Fig. S4). Interestingly, an

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obvious broadband background increase occurs with uptake for both AS and AN particles, but

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not for ABS particles; this background increase is likely due to fluorescent emission from the

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light-absorbing products, which will be discussed in detail later.

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Gly uptake rate as a function of RH.

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Since sulfate48 and nitrate10 ions are thought to not participate directly in the reactions, we

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normalized the peak area of the v(C-H) mode at 2900-3050 cm-1 to the sulfate (∆A(C-H/SO42-))

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or nitrate (∆A(C-H/NO3-)) peak, as appropriate, in order to estimate [Gly]p. The sum of the

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v(SO42-) and v(HSO4-) modes was used to calculate ∆A(C-H/SO42-) for ABS particles. The

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calibration results of ∆A(C-H/SO42-) versus [Gly]p/[SO42-] and ∆A(C-H/NO3-) versus

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[Gly]p/[NO3-] are shown in Figs. S5 and S6, respectively. Figure 3 shows the changes in [Gly]p

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for non-NH3(aq)-containing NaS, MgS, and NaN particles and NH3(aq)-containing ABS, AS, and

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AN particles during the Gly uptake process at each RH. In NaS and MgS particles, the Gly

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uptake rates are less dependent on RH simply because of the limited RH ranges studied (60-

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85% RH) and relatively small KH,NaS and KH,MgS values (Table S1). NaS particles show slight

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increases in [Gly]p of 0.70 and 0.81 M at the end of the experiments at 75 and 85% RH,

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respectively, while NaS particles at 60% RH (not shown), which are in solid form, do not show

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significant increases. The linear uptake rates of [Gly]p, d[Gly]p/dt, for the NaS particles are

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estimated to be 4.75  10-4 and 5.99  10-4 M min-1 at 75 and 85% RH, respectively. The MgS

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particles show smaller increases in [Gly]p of 0.43 and 0.28 M at the end of the experiments at

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60 and 75% RH, respectively, with d[Gly]p/dt values of 1.76  10-4 and 2.12  10-4 M min-1.

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Although KH,MgS,60% and KH,MgS,75% are larger than and comparable to KH,NaS,75% (Table S1),

280

respectively, the Gly uptake rates for MgS particles are smaller than those for NaS particles. In

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general, the “salting-in” effect in sulfate-containing solutions is caused partially by decreases

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in the hydrated forms of free Gly. Quantum chemical calculations suggest that free Gly can

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bind to sulfate directly, so that additional Gly must partition further into the aqueous phase to

284

maintain the equilibrium.46 In MgS particles, direct-contact ion pairs form from Mg2+ and

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SO42-.49 In fact, a change in the v(SO42-) mode at ~980 cm-1 is discernible when the RH is

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decreased to 60% RH. The inset in Fig. 1c presenting MgS particles at 60% RH (dashed line)

287

shows a shoulder at approximately 995 cm-1, which is evidence for the presence of contact ion

288

pairs (MgSO4).50 The formation of contact ion pairs reduces the number of free sulfate ions

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available to bind with Gly, leading to smaller d[Gly] p/dt values for MgS particles than for SS

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particles.

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In NaN particles, d[Gly]p/dt exhibits a strong dependence on RH. The d[Gly]p/dt values

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are 2.36  10-3, 4.04  10-3, 3.87  10-3, and 5.67  10-4 M min-1 at 30, 45, 60, and 75% RH,

293

respectively. The predicted KH,NaN increases from 1.95  106 at 75% RH to 9.40  108 M atm-

294

1

295

and then decreases as RH continues to decrease from 45 to 30% RH. The increase in d[Gly]p/dt

296

as RH decreases from 75% to 45% is likely due to the “salting-in” effect. Further decreases in

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RH lead to low ALW (needed for hydration) and/or increased particle viscosity, resulting in

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kinetically limited Gly uptake.21 Avzianova and Brooks22 reported that Gly oligomers form at

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Gly concentrations of 1 M and higher. We observed δ(O-C-O)ring, v(C-C), and v(ring) modes,

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which are signatures of Gly oligomers, for NaN particles under all experimental conditions

at 30% RH (Table S1). However, d[Gly]p/dt increases as RH decreases from 75 to 45% RH,

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except the reaction at 75% RH with [Gly]p = 0.80 M. In this study, we believe that there may

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be no significant mass transport limitations on the reaction kinetics (Appendix 2, SI).

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As discussed above, Gly oligomers can form as part of the Gly uptake reactions.

304

However, Raman spectra of AS and AN particles do not show clear Gly oligomer spectral

305

signatures (Fig. 2). In contrast, the δ(O-C-O)ring peak is visible for ABS particles (Fig. S4b).

306

This difference can be explained by the consumption of [Gly]p through reaction with NH3(aq)

307

in AS and AN particles. In ABS particles, the concentration of NH 3(aq) is 6 to 8 orders of

308

magnitude lower than that in AS and AN particles (Table 1), leaving sufficient [Gly]p for the

309

formation of oligomers. Overall, d[Gly]p/dt increases with decreasing RH for all NH3(aq)-

310

containing particles. Among the non-NH3(aq)-containing particles, sulfate particles (NaS and

311

MgS) do not show a clear Gly uptake rate dependence on RH. However, in NaN particles, Gly

312

uptake shows a strong dependence on RH (larger KH,NaN at lower RH), although Gly uptake is

313

suppressed at 30% RH due to the formation of Gly oligomers.

314

Time-dependent [BI] as a function RH.

315

First, we describe changes in [BI] as function of RH. In the next section, we estimate the rate

316

of formation of BI, d[BI]/dt. d[BI]/dt is compared under different RH conditions to elucidate

317

the roles of the type of anions (sulfate or nitrate) and RH or ALW in BI formation. Finally, we

318

compare the experimentally determined rates with the theoretical rates, which are based on the

319

dependence of reactant concentrations on RH (i.e., Eq. (3)). The major results from these

320

sections are summarized in Table 1.

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After partitioning, aqueous Gly in the particle phase can further react with NH3(aq) to

322

produce imidazole and imidazole derivatives (Scheme S1).10,11 As shown in Fig. 2, broadband

323

backgrounds emerge for both AS and AN particles as the reaction goes forward. This

324

background enhancement is not observed for ABS particles due to the very low concentrations

325

of NH3(aq) under all RH conditions (Table S1), which is consistent with earlier works. 25

13



326

In the NH3/Gly chemical system, IC and BI have been found to fluoresce.17 However,

327

the fluorescence background enhancement at ~3000 cm-1 in the Raman results was found to be

328

attributed primarily to BI formation in the particle phase (Appendix 3, SI). Based on the

329

broadband fluorescence, we estimate [BI] as a function of uptake time (Appendix 4, SI). Figure

330

4 shows time-dependent [BI] for AS/Gly and AN/Gly particles at 30, 45, 60, and 75% RH.

331

Clear trends are observed in the RH dependence of [BI]. At 75% RH, both AS and AN particles

332

show mild increases in [BI] as uptake occurs. In contrast, as RH decreases, [BI] increases

333

rapidly after a time delay. Note that, for AS particles in crystalline form at 30% RH, no BI was

334

expected to form and hence no background enhancement was observed (Fig. S11d), reflecting

335

the importance of ALW in the heterogeneous NH3/Gly reactions that produce BI. Trainic et

336

al.29 reported that reactions mediated by a thin aqueous layer of surface-adsorbed water on

337

crystalline AS particles are possible. However, our Raman analysis could not capture such

338

interfacial reactions due to the technique detection limit.

339

BI formation rate as a function of RH.

340

As shown in Fig. 4, [BI] initially increases slowly with time, and then increases rapidly

341

in an approximately linear fashion near the end of each experiment (i.e., for the last 3-5 data

342

points); we estimate the BI formation rate, d[BI]/dt, using these linear relationships (Table 1).

343

At 75% RH, the linear regression was applied to all data points.

344

The rate increases from 2.04  10-7 M min-1 at 75% RH to 6.73  10-5 M min-1 at 45%

345

RH for AS particles and from 1.96  10-7 M min-1 at 75% RH to 8.97  10-5 M min-1 at 30%

346

RH for AN particles. The formation rates for AN particles are comparable to those for AS

347

particles, although the [NH3] in the AN particles was an order of magnitude lower than that in

348

the AS particles (Table S1). The Gly uptake rates for AN particles are larger than those for AS

349

particles under comparable RH conditions (Table 1); these larger uptake rates may offset the

350

lower [NH3] in AN particles to produce BI formation rates comparable to those in AS particles.

14


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351

A recent laboratory study of evaporating droplets containing NH3(aq) and Gly from bulk

352

solutions concluded that AN is less effective than AS at producing light-absorbing species.25

353

In contrast to the earlier work, our reactive uptake experiments, which consider glyoxal

354

partitioning, demonstrate that AN particles are as effective as AS particles in BI formation at a

355

given RH. This comparable effectiveness may be attributable to the comparable or larger

356

effective Henry’s law constants of AN with those of AS at < 75%RH, in addition to its much

357

lower ERH (Tables S1 and S2).

358

The BI formation rate increases dramatically as RH decreases for both AS and AN

359

particles (Table 1). Interestingly, the rates at 30 and 45% RH are two orders of magnitude

360

larger than those at 75% RH and that reported for a bulk reaction study (4.31  10-7 M min-1 in

361

a 3 M AS and 1.5 M Gly solution).16 Increases in the reactant concentrations, [NH3] and [Gly]p,

362

may explain this enhancement in formation rate as RH decreases. To determine whether

363

increased reactant concentrations are the only cause of this enhancement, we examined the

364

relative BI production rate at a particular RH compared to that at 75% RH via two ratios: (i)

365

Rsalt,RH/Rsalt,75%, which represents the relative production rate based on first and second order

366

reactions involving [NH3] and [Gly]p, respectively,11 taking into consideration the increased

367

reactant concentrations at lower RH and gas-particle ammonia equilibrium; and (ii)

368

R'salt,RH/R'salt,75%, which is based on actual experimental BI concentration data. Assuming

369

equilibrium between gas and particle phase ammonia and instantaneous mixing within a

370

particle, the BI formation rate based on reactant concentrations, Rsalt,RH, can be described by:

371 2

372

𝑅salt,RH = 𝑑[BI]⁄𝑑𝑡 ∝ 𝛾NH3 × [NH3 ] × 𝛾NH4 + × [NH4 + ] × [Gly]p (3)

373

[Gly]p = 𝐴salt,RH × 𝑡

(4)

374 375

where [NH3] is the concentration of NH3(aq); [NH4+] is the concentration of NH4+(aq); NH3 and

15



376

NH4+ are the activity coefficients of [NH3] and [NH4+], respectively; and Asalt,RH is the Gly

377

uptake rate (d[Gly]p/dt). We predicted the initial concentrations of NH3(aq) ([NH3]) and NH4+

378

([NH4+]) using the E-AIM model and assumed [NH3] and [NH4+] to be constant throughout the

379

uptake experiments, as, in earlier work, less than 6% of the reactants in a 0.17 M Gly and 3.3

380

M AS solution were consumed even after the formation of BI decreased substantially.11 Eq. (3)

381

can predict a slow increase in [BI] with time followed by the rapid increase (Fig. 4) since the

382

BI formation rate is a function of squared [Gly]p.

383

RAS,RH/RAS,75% is estimated to be 23.4 and 1.5 at 45 and 60% RH, respectively. The

384

increased value at 45% RH may be caused by the increased [NH3] and faster Gly uptake at that

385

RH (in comparison to those at 75% RH). RAN,RH/RAN,75% is estimated to be 17.0, 1.8, and 0.1 at

386

30, 45, and 60% RH, respectively. RAN,RH/RAN,75% is smaller than RAS,RH/RAS,75% at a given RH

387

because of the decrease in [NH3] in AN particles as RH decreases (Table S1). In contrast,

388

R'AS,RH/R'AS,75% was estimated to be 329.2 and 29.3 at 45 and 60% RH, respectively, and

389

R'AN,RH/R'AN,75% was 456.6, 208.4, and 64.5 at 30, 45, and 60% RH, respectively. These ratios

390

are at least an order of magnitude larger than the Rsalt,RH/Rsalt,75% obtained using Eq. (3). This

391

discrepancy may be explained by the reduced amount of water molecules in the particles at low

392

RH.51 The water-to-salt molar ratio for AS at 45% RH is 2.84, which is ~3 times smaller than

393

that at 75% RH, and the ratio for AN at 30% RH is an order of magnitude smaller than that at

394

75% RH. Furthermore, the abundance of water in a droplet is limited to the droplet volume,

395

while the abundance changes little in a bulk solution. The formation of imidazole and its

396

derivatives involves the nucleophilic attack of NH 3(aq) species at reactive carbonyl sites

397

followed by dehydration reactions that result in intermolecular cyclization to form heterocyclic

398

compounds.11 Recently, Aiona et al. also suggested that dehydration processes play an

399

important role in the formation of light-absorbing material via cyclization.52 Therefore, we

400

attribute the significant increase in the BI formation rate at low RH to both increased reactant

16


Page 16 of 28

Page 17 of 28


401

concentrations and the removal of water, which promotes the dehydration and cyclization

402

process. Even though high concentrations of [Gly] g at ppm levels were used in this study, BI

403

formation is expected to be still atmospherically relevant because [Gly] p measured (Fig. 3) is

404

in the same range as [Gly]p found in the earlier work21 where [Gly]g at ppb levels was used for

405

AS particles. Furthermore, while BI may not be abundant in mass concentrations, their impacts

406

on light absorption have been well documented.17

407

ATMOSPHERIC IMPLICATIONS

408

Bulk phase reaction studies have reported relatively low production rates for light-absorbing

409

species (e.g., imidazole, IC, and BI), suggesting that the contributions of these reactions to

410

ambient aerosol mass are likely insignificant.11,16,53 However, evaporating droplets containing

411

AS or amino acids and Gly have shown faster production rates.12,25 The present study reveals

412

that heterogeneous reactions involving Gly and ammonium salt droplets are sensitive to ALW.

413

As RH decreases from 75 to 30% RH (i.e., ALW decreases), AS and AN droplets undergoing

414

heterogeneous reactions with Gly have BI production rates much higher than those estimated

415

based on the increased reactant concentrations of NH 3(aq) and Gly alone assuming instantaneous

416

mixing within a particle (Fig. 4 and Table 1). These enhanced production rates are likely due

417

to a combination of increased reactant concentrations and reduced water availability, which

418

promotes the dehydration reactions that form heterocyclic compounds. Furthermore, no BI

419

formation is observed when AS particles are in crystalline form. This result confirms that

420

water-mediated reactions between NH3(aq) and Gly form BI. These findings may highlight the

421

importance of dehydration reactions in the formation of light-absorbing species, particularly

422

under intermediate or low RH conditions; this information may help reconcile the differences

423

found between bulk and droplet studies in the formation rates of light absorbing species and be

424

relevant to reactions between other dicarbonyls and aldehydes, such as methylglyoxal,

425

glutaraldehyde, 4-oxopentanal, and ketolimononaldehyde.15,23,52,54 In the atmosphere, regions

17



426

with high pH, high ammonium concentrations, and moderate or low RH conditions could favor

427

the formation of imidazole and imidazole derivatives.

428

This study used a flow cell coupled with in-situ Raman analysis to characterize Gly

429

uptake by six salt particle types and the subsequent reactions therein (i.e., self-oligomerization

430

and formation of light-absorbing material). The reacted particles were further analyzed by UV-

431

vis and fluorescence spectroscopy and SERS. Reactions in similar chemical systems (NH3 +

432

carbonyls) have been studied extensively in bulk phase solutions.10,11,13,16,17,23,24 However,

433

heterogeneous uptake experiments in which ammonium salt particles are exposed to gaseous

434

species (e.g., glyoxal) under controlled RH conditions (e.g., chamber and flow tube

435

experiments) are scarce.55,56 Flow cells with in-situ Raman spectroscopy provide a viable

436

alternative method for the study of heterogeneous uptake.

437 438

ACKNOWLEDGEMENTS

439

The authors gratefully acknowledge the startup fund of the City University of Hong Kong.

440 441

SUPPORTING INFORMATION

442

The supporting information contains the following information: Salt molalities, Csalt, predicted

443

effective Henry’s Law constants of Gly in salt solutions, KH, and concentrations of NH3(aq)

444

([NH3]), gas phase Gly ([Gly] g) and particle phase Gly ([Gly]p); Water solubility, efflorescence

445

RH (ERH), and deliquescence RH (DRH) of salts; Reaction scheme of Gly with ammonium

446

salt particles; Schematic of experimental setup; Absorption and fluorescence spectra of reacted

447

particles; SERS spectra of reacted particles; Raman spectra of ABS; Calibration curves for

448

quantifying [Gly]p; Fluorescence spectra of IC and BI; Illustration of an example calculation

449

of A(BI)t; Calibration curves for quantifying [BI]; Time-series background enhancements in

450

Raman results.

18


Page 18 of 28

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451

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645 646 647

Table 1. Gly uptake rates (Asalt,RH or d[Gly]p/dt), BI formation rates near the end of each

648

experiment (d[BI]/dt or Rsalt,RH), relative production rates based on reactant concentrations of

649

[NH3] and [Gly]p (Rsalt,RH/Rsalt,75%), and relative production rates based on our experimental

650

data concerning [BI] as a function of uptake time (R'salt,RH/R'salt,75%) for AS and AN particles

651

at 30, 45, 60, and 75% RH.

Salt

RH (%)

d[Gly]p/dt (M min-1)

d[BI]/dt (M min-1)a

Rsalt,RH/Rsalt,75%b

R'salt,RH/R'salt,75%c

75

(1.94  0.08)  10-3

(2.04  0.44)  10-7

1.0

1.0

60

(1.67  0.10)  10-3

(5.98  2.01)  10-6

1.5

29.3

45

(5.23  1.38)  10-3

(6.73  0.74)  10-5

23.4

329.2

30

N/A

N/A

N/A

N/A

75

(1.06  0.06)  10-3

(1.96  0.20)  10-7

1.0

1.0

60

(2.18  0.05)  10-3

(1.27  0.07)  10-5

0.3

64.5

45

(1.25  0.06)  10-2

(4.09  0.27)  10-5

1.8

208.4

30

(3.78  1.82)  10-2

(8.97  0.73)  10-5

17.0

456.6

AS

AN

a) The values of d[BI]/dt or Rsalt,RH were determined near the end of each experiment, except for those at 75% RH (see Fig. 4). b) The relative BI production rate compared to that at 75% RH based on Eq. (3), taking into consideration the change in reactant concentrations. c) The relative BI production rate compared to that at 75% RH obtained from our uptake experiments (Fig. 4).

652 653

23



Page 24 of 28

654 655 656 40000

NaS at 75% RH (a) v(SO42-)

1380 min 1200 min 960 min 720 min 0 min

30000

20000

80000

NaN at 60% RH (d)

1440 min 1260 min 1020 min 720 min 360 min 0 min

v(NO3-)

60000

40000

(b) 10000

20000

(e)

(f)

Intensity (a.u.)

0

20000

0

500 1000 1500 2000 2500 3000 3500 4000

NaS at 75% RH (b) v(O-H)

20000

500 1000 1500 2000 2500 3000 3500 4000

NaN at 60% RH (e)

v(O-H)

15000

15000

10000

10000

v(C-H) v(O-H)

v(C-H) 5000

5000

0 2600 2800 3000 3200 3400 3600 3800 4000

14000

MgS at 60% RH (c) v(SO42-)

12000 10000 8000

1440 min 1140 min 840 min 480 min 0 min

75% RH 60% RH

0 2600 2800 3000 3200 3400 3600 3800 4000 8000 7000

NaN at 60% RH (f) v(C-C)

6000 5000

δ(O-C-O)ring

v(ring)

6000 4000 4000

960 980 1000 1020 1040

3000

2000 0

500 1000 1500 2000 2500 3000 3500 4000

2000 700

800

900

1000

Raman shift (cm-1) 657 658

Figure 1. Time series of Raman spectra of (a, b) NaS particles at 75% RH at 100-4000 and

659

2600-4000 cm-1, respectively, (c) MgS particles at 60% RH at 100-4000 cm-1, and (d, e, f)

660

NaN particles at 60% RH at 100-4000, 2600-4000 and 700-1100 cm-1, respectively. The inset

661

of (c) is the v(SO42-) peak at 60 (dash) and 75% (solid) RH at t = 0 min. 24


1100

Page 25 of 28


662 663 664 665 666 667 668 669

100000

1380 min 1200 min 1080 min 960 min 840 min 720 min 600 min 480 min 0 min

(a) 80000

v(SO42-)

60000

Intensity (a.u.)

40000 20000 0

500 1000 1500 2000 2500 3000 3500 4000

100000

(b)

v(NO3-) v(C-H)

80000 60000 40000

1380 min 1260 min 1140 min 1020 min 900 min 780 min 660 min 540 min 0 min

20000 0

500 1000 1500 2000 2500 3000 3500 4000 Raman shift (cm-1)

670 671

Figure 2. Time series of raw Raman spectra for (a) AS/Gly and (b) AN/Gly particles at 60%

672

RH.

673 674 675 676 677 678 679 680 681 682 683 684 685

25



686 687 688 689 690

2

(a) NaS, MgS 1.5

Page 26 of 28

7

NaS 75% RH NaS 85% RH MgS 60% RH MgS 75% RH

(d) NaN

6 5

30% RH 45% RH 60% RH 75% RH

4

1

3 0.5

2 1

0

0 10

3

(b) AS

[Gly]p (M)

2.5

30% RH 45% RH 60% RH 75% RH

(e) AN 8

2 6

1.5

4

1 0.5

2

45% RH 60% RH 75% RH

0

0

7

(c) ABS

6

0

4

400

600

800 1000 1200 1400

Uptake time (min)

30% RH 45% RH 60% RH 75% RH

5

200

3 2 1 0 0

200

400

600

800 1000 1200 1400

Uptake time (min) 691 692

Figure 3. Changes in [Gly]p for (a) NaS and MgS, (b) AS, (c) ABS, (d) NaN, and (e) AN

693

particles as a function of uptake time at each RH. Note that NaS and AS particles are not

694

shown at 60 and 30% RH, respectively, as they form solids at these RHs.

695 696 697 698 699 700 26


Page 27 of 28


701 702 703 704 705 706 707 708 709

0.01

(a) 0.008 0.006 0.004 45% RH 60% RH 75% RH

[BI] (M)

0.002 0 0.01

(b) 0.008

30% RH 45% RH 60% RH 75% RH

0.006 0.004 0.002 0 0

200

400

600

800 1000 1200 1400

Uptake time (min) 710 711

Figure 4. Time-dependent [BI] for (a) AS/Gly and (b) AN/Gly particles at 30, 45, 60, and

712

75% RH. The solid lines are linear regressions using the last 3-5 data points at 30, 45, and

713

60% RH and all data points at 75% RH to estimate the BI formation rate (d[BI]/dt). Note that

714

data are not shown for AS particles at 30% RH, which are in solid form. Experiments were

715

discontinued earlier, at t = 300 min for AS particles at 45% RH and at t = 120 and 360 min

716

for AN particles at 30 and 45% RH, respectively, because the v(SO42-) and v(NO3-) peaks are

717

overwhelmed by the fluorescence background enhancement.

718 27



Page 28 of 28

719 720 721

O

722

OH

HO

OH

OH

HO

OH

O

723 724

O HO

O

HO

O

OH

HO

OH

HO

TOC image

45%

[BI] (M)

0.008

Low RH or ALW high brown carbon

4

0 0

200

400

600

HO

NH HN 800 1000 1200 1400

Uptake time (min)

(g) OH

HO

O O

O

OH

O

O

OH

-H2O O

OH

75%

OH

HO N

NH

N

OH

NH

N

NH

OH

2,2’-biimidazole (BI) HN O

725

N

726

NH

O

N

OH

HO

OH

OH

N

OH

HN O


N

N NH

HN

28

+

OH

O

low brown carbon N

OH O

NH

OH

NH High RH orNALW OH

0.002

OH

OH NH +

N

HO

0.004

OH

N

3(aq)

NH

HO O

O

OH NHHO

60%

N

O OH

N

0.006

O

OH AS HOor AN OH

O

30%

O

NH

Reactive Uptake of Glyoxal by Ammonium-Containing Salt Particles as

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