Reactive Uptake of Dimethylamine by Ammonium Sulfate and

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Reactive Uptake of Dimethylamine by Ammonium Sulfate and Ammonium Sulfate – Sucrose Mixed Particles Yangxi Chu, and Chak Keung Chan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10692 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry

Reactive Uptake of Dimethylamine by Ammonium Sulfate and Ammonium Sulfate – Sucrose Mixed Particles

Yangxi Chu† and Chak K. Chan†,‡,§,*

†

Division of Environment and

‡

Department of Chemical and Biomolecular

Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China §

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

Kowloon, Hong Kong, China

1

ACS Paragon Plus Environment


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Abstract

Short-chain alkyl amines can undergo gas-to-particle partitioning via the reactive uptake by ammonium salts, whose phases have been thought to largely influence the extent of amine uptake. Previous studies mainly focused on particles of single ammonium salt at either dry or wet conditions without any addition of organic compounds. Here we report the uptake of dimethylamine (DMA) by ammonium sulfate (AS) and AS – sucrose mixed particles at different relative humidities (RH) using an electrodynamic balance coupled with in situ Raman spectroscopy. DMA is selected as a representative of short-chain alkyl amines and sucrose is used as a surrogate of viscous and hydrophilic organics. Effective DMA uptake was observed for most cases, except for the water-limiting scenario at < 5% RH and the formation of an ultra-viscous sucrose coating at 10% RH and below. DMA uptake coefficients (γ) were estimated using the particle mass measurements during DMA uptake. Addition of sucrose can increase γ by absorbing water or inhibiting AS crystallization, and decrease γ by elevating particle viscosity and forming a coating layer. DMA uptake can be facilitated for crystalline AS, or retarded for aqueous AS, with hydrophilic viscous organics (e.g. secondary organic material formed via the oxidation of biogenic volatile organic compounds) present in aerosol particles.

2


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1. Introduction As the most important alkaline gas species besides ammonia (NH3) in the atmosphere, short-chain alkyl amines are emitted via both anthropogenic and biogenic sources.1 They can contribute to the formation of secondary organic aerosols (SOA) via oxidation by ozone, hydroxyl and nitrate radicals,2-4 aqueous phase reactions with carbonyl compounds,5,6 acid-base reactions3,7-10 and most recently found, reactive uptake by ammonium-containing salts.11-16 The so-formed products can significantly change the aerosol physical and chemical properties. In ambient environment, typical concentrations of short-chain alkyl amines are in the parts per trillion (ppt) level, 100 – 1000 times lower than that of NH3.1 However, both field measurements17-19 and thermodynamic modelling20 have revealed much higher alkyl aminium – to – ammonium (NH4+) molar ratios in the particles than the corresponding concentration ratios of alkyl amine to NH3 in the gas phase. Besides the thermodynamically preferable uptake of alkyl amines over NH3 by clusters of acidic compounds,7 the displacement of particulate NH4+ due to the reactive alkyl amine uptake also accounts for the enhanced alkyl aminium formation in the particle phase. Studies have revealed that the reactive uptake of alkyl amines relies largely on the intrinsic chemical reactivity and phase state of ammonium-containing salts. According to Qiu et al.14 and Liu et al.16, the uptake of three methylamines, namely monomethylamine (MMA), dimethylamine (DMA) and trimethylamine (TMA), onto solid ammonium sulfate (AS) featured a displacement mechanism, while that onto solid ammonium bisulfate (ABS) followed an acid-base reaction. Liu et al. also 3



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observed the displacement reactions between MMA and solid ammonium nitrate and chloride, with the products (CH3NH3NO3 and CH3NH3Cl) being more volatile than the sulfate counterparts.16 Chan and Chan revealed that the extent of amine – NH3 exchange reactions is largely dependent upon the initial particle phase state. Specifically, aqueous or amorphous particles would enhance the displacement of original cation (NH4+ or aminium), whereas those initially crystalline particles suffered from limited cation displacement.12 Such dependency has also been found in the amine – amine exchange reactions.21 Studies on the short-chain alkyl amine uptake by particles of ammonium salts, especially AS, have focused either under dry condition14,16 or at elevated RH where the AS is well in aqueous phase.11,12 Such investigation at RH below the AS crystallization point is also of great significance, since there can be adsorbed water on the crystal surfaces that triggers heterogeneous reactions without the necessity of deliquescence of corresponding salts.22-27 With the knowledge that initial AS particle phase state can significantly influence the alkyl amine uptake, we extend the scope of our study by investigating the effects of AS – organic mixed particles. Ambient aerosols are usually mixtures of inorganic and a myriad of organic compounds.28-30 Among the particulate organics, secondary organic matter (SOM) formed via the oxidation of volatile organic compounds (VOC) has recently caught much attention concerning its physical phase state. Virtanen et al. reported that particles of biogenic SOM can adopt an amorphous solid phase, which may prolong the timescale of heterogeneous reactions and hygroscopic growth.31 4


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Renbaum-Wolff et al. found the viscosity of α-pinene derived SOM varying from ~ 10 to > 1012 Pa s, which implies the SOM can behave as liquid, semi-solid or solid depending on the surrounding relative humidity (RH).32 For mixed particles of inorganic electrolyte and SOM, liquid-liquid phase separation may be induced during humidity cycles. Specifically, the organic-rich shell can retard the uptake of water as well as reactive gas molecules such as NH3 and N2O5 by the inorganic core.33,34 Additionally, those biogenic SOM has been shown to be slightly hygroscopic.35-37 In this study, sucrose, whose hygroscopic and mechanical properties are similar to those of SOM,38-40 was used as a surrogate for the SOM and was mixed with AS in the particle phase. In this paper, we first present the reactive uptake of DMA at different RH by particles of AS alone, which served as control experiments. Then, we report the DMA uptake by AS – sucrose mixed particles with different sucrose loadings at different RH. Effects of sucrose on the equilibrium particle-phase compositions as well as the DMA uptake kinetics are discussed. DMA is selected as the representative of short-chain alkyl amines due to its high abundance in both gas and particle phases.1,17,41 As the uptake of DMA by AS can result in the displacement of ammonium from the particles, we use the terms “DMA reactive uptake by ammonium-containing particles” and “DMA – NH3 exchange reaction” interchangeably in the following.

2. Experimental 2.1. Generation of single particles 5



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AS (≥ 99.0%, Sigma-Aldrich) and sucrose (ultrapure, Affymetrix USB Products), used as received without further purification, were dissolved in Milli-Q water (18.2 MΩ cm). Concentrations of the prepared bulk AS or AS – sucrose mixed solutions were ~ 25 wt%. Single particle generation from each individual solution and levitation in the electrodynamic balance (EDB) followed those as described by Chan and Chan.12 In this work, the diameter of the dried particles was 37 – 71 µm.

2.2. Control of relative humidity and generation of dimethylamine vapor A schematic of the flow system is shown in Figure S1 (see supporting information (SI)). The relative humidity (RH) of the feeding stream was adjusted by mixing a dry nitrogen flow with another nitrogen flow humidified by bubbling through a water bottle and monitored with a digital humidity sensor (Sensirion, Model SHT21). Typical uncertainty associated with the humidity sensor is ± 2% for 20 – 80% RH and slightly larger at < 20% or > 80% RH with ± 3% as RH approaching 0% or 100%. The humidity sensor was calibrated against the deliquescence RH of AS.42-44 DMA vapor was generated from a diffusion vial containing ~ 20 wt% DMA aqueous solution immersed in a 10 oC circulating bath (PolyScience, Model MX07R). The commercially available DMA stock solution (~ 40 wt% in water, Sigma-Aldrich) was diluted by Milli-Q water prior to being transferred into the diffusion vial with a microliter syringe. The gas-phase DMA concentration was controlled by passing a RH-controlled nitrogen stream using mass flow controllers (MKS, Model 247C and GE50A) over the diffusion vial. In all experiments, the gas-phase DMA 6


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concentrations were estimated at 1 ppm, by measuring the mass difference of the diffusion vial before and after each experiment. Such estimation, in fact, served as an upper limit of the DMA vapor concentration in this study, since the adherence of DMA onto the inner surfaces of tubing, fittings, EDB housing and electrodes is very likely.45 All experiments were performed under room temperature of 24 ± 1oC.

2.3. Measurement of particle mass using the EDB The principle of EDB has been well documented46,47 and only a brief description is given here. Assuming no loss of charges on the particle, the particle mass is directly proportional to the balancing DC voltage. In this work, the mass of AS, sucrose or AS – sucrose particles equilibrated at specific RH before the DMA vapor was introduced was used as the reference to calculate the relative particle mass changes during the DMA reactive uptake process.

2.4. Raman spectral characterization of single particles An EDB-coupled Raman spectroscopic system, similar to that in Chan and Chan,12 was applied to detect the changes in chemical composition and phase state of the single particle levitated in the EDB. Minor modifications were made and presented here. A 532 nm long-pass Raman edge filter (OptoSigma, Model RSF–25C–532RU) was placed in the laser (532 nm, 150 mW) path to remove the ultra-intensive Rayleigh scattering peak. For each Raman spectrum scanning, an integration time of 60 seconds (60 frames, with an accumulation time of 1 second each) was applied. The 7



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assignments of Raman peaks followed Meinander et al.,48 Lin-Vien et al.,49 Brizuela et al.50 and Chan and Chan.11,12

3. Results and discussion In this section, we first report the reactive uptake of DMA gas by AS particles at relative humidity (RH) of 70%, 30%, 20%, 10% and below 5%. Then we investigate the DMA reactive uptake by AS – sucrose mixed particles with different sucrose loadings. Detailed analysis on the kinetics of DMA uptake is provided at last.

3.1. DMA uptake by ammonium sulfate particles alone Table 1 lists all the experimental conditions, initial particle compositions and phase states of AS particles (experiments #1 – 5). Particle mass changes, as measured by the EDB, were attributed to the DMA – NH3 exchange reaction and the resulting variations in condensed water contents. Figure 1 shows that mass increased for AS particles at all RH as a result of DMA uptake. The more rapid particle mass increase at 70% RH is consistent with our previous findings that alkyl amine uptake by aqueous AS droplets is enhanced over that by crystalline AS particles.12 More importantly, DMA uptake was also effective at RH below the crystallization RH (CRH) of AS (37 – 40%),42,51 although it took place at a lower rate. In our EDB experiments, the single levitated AS particle crystallized when it was firstly equilibrated at 30% RH and below, as supported by the irregular 90o Mie scattering pattern11 and the small full-width-at-half-height (FWHH) value of the sulfate bending 8


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peak, locating at ~ 450 cm–1 in the Raman spectrum (also see text below).52,53 We speculate that the DMA reactive uptake at RH below 30% was triggered by the surface adsorbed water (SAW) on the crystalline AS particle,22,24,26 although the amount of SAW may not be detectable in mass or spectral changes by our EDB – Raman system. The detection limit of water-induced mass change using the EDB – Raman system is 3%, making it not sufficiently sensitive to SAW, which has been reported to form sub-monolayer at ≤ 30% RH.54 Once DMA displaced NH4+, dimethylaminium sulfate (DMAS) salt was formed. Being a non-crystallizing compound at low RH,12,55 DMAS enhanced the particulate water content, thus further promoting the reactive uptake of DMA till the depletion of NH4+. At < 5% RH, the driest condition in our study, there was a discernable particle mass increase of 3%, likely due to the limited amount of SAW on the particle surface. The DMA – NH3 exchange reactions may be confined to the surface defects only14 and DMA molecules are unable to penetrate through the particle bulk. Changes in particle-phase chemical compositions during the reactions were also recorded in the Raman spectra. For instance, Figure 2a shows the Raman spectral changes of an initially crystalline AS particle (which ultimately turned into an aqueous DMAS droplet as a result of DMA – NH3 exchange reaction, see text below) during the course of DMA uptake at 30% RH. Raman signals from DMA, i.e., C–H stretching peaks at 2970 and 3035 cm–1, C–H bending peak at 1465 cm–1 and C–N–C stretching peak at 890 cm–1, appeared and strengthened as the reaction proceeded. Simultaneously, the N–H stretching band at 3100 – 3200 cm–1 and bending peaks at 9



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1413 and 1660 cm–1 from particle-phase NH4+ gradually weakened, which confirms the exchange reaction between DMA and NH3. In addition, the envelope at 3400 – 3600 cm–1 containing individual O–H stretching peaks53,56 strengthened as a result of increased particulate water contents due to the higher hygroscopicity of DMAS. Furthermore, we also observed a gradual change in the sharpness (i.e., FWHH) of the sulfate bending peak at ~ 450 cm–1 as the reaction proceeded, represented by the blue squares in Figure 2b. We have previously observed abrupt changes in FWHH of specific Raman peaks as a result of the phase transition of corresponding compounds.43,52,53,57 However, in this particular case the RH was controlled at constant value during each reaction. Therefore, the increase in FWHH was induced by the gradual changes in chemical composition (AS to DMAS) during the DMA – NH3 exchange

reactions

rather

than

the

phase

transition

of

any

individual

sulfate-containing component. The particle phase gradually transformed from crystalline into liquid as the amount of DMAS accumulated and that of AS decreased. This is in agreement with the transition in 90o Mie scattering (from irregular before DMA uptake to highly ordered after 465 min of reaction) patterns shown in Figure 2a. Such changes in particle phase state was also observed during the DMA uptake at 20% and 10% RH, but neither at 70% where AS has already been in aqueous phase initially nor at < 5% RH where the particle-phase composition was only slightly altered (see Figure 2b and text below). It is worth pointing out that the relative intensities of the C–H stretching peaks at 2970 and 3035 cm–1 changed during the reaction, especially from 240 to 465 min, although we do not have an exact explanation for this trend 10


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currently. The neutralized salt of DMAS (i.e. with aminium-to-sulfate molar ratio at 2.0) is unstable and will equilibrate to compositions with the ratio ranging from 1.5 to 2.0 depending on the surrounding RH conditions.53 As the DMA – NH3 exchange reaction approached completion, the DMA re-partitioning was very likely when the EDB-feeding stream was stopped in order to take particle mass and Raman spectral measurements. At the end of each experiment, the supply of DMA vapor was terminated and the levitated particle was dried at < 5% RH until its mass became constant within experimental uncertainty. The particle mass ratio of the reacted particle to the corresponding initial particle on a dry basis was used to calculate the NH4+ displacement percentage, and the results are listed in Table 1. For RH of 10% and above (experiments #1 – 4), the mass ratios matched the composition with aminium-to-sulfate molar ratio of 1.5,12,53 and more than 94% of the original NH4+ was displaced. At < 5% RH (experiment #5), the driest condition in our study, 8 – 12% of the original NH4+ was displaced by DMA, likely due to the limited amount of SAW as discussed earlier. Existence of DMA in the particle phase was well evidenced by the C–H and C–N–C stretching peaks in the Raman spectra (Figure 3). Our findings at < 5% RH are consistent with Qiu et al.14 and Liu et al.,16 who reported limited irreversible alkyl amines absorption/adsorption on solid AS particles. In this study, we found that DMA uptake by aqueous AS droplets was almost complete but uptake by crystalline AS particles at < 5% RH was very limited, consistent with Chan and Chan.12 More importantly, as long as there is sufficient 11



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moisture in the gas phase, i.e., at 10%, 20% and 30% RH, even initially crystalline AS particles can react with DMA effectively due to the presence of SAW and that DMAS is more hygroscopic than AS. Dawson et al. observed an incomplete amine-amine exchange for solid aminium-methane sulfonate particles at ~ 10% RH even after overnight reaction.21 The discrepancy between our results (complete displacement of NH4+ by DMA at 10% RH) and theirs is likely due to the originally different particle-phase chemical compositions.

3.2. DMA uptake by ammonium sulfate – sucrose mixed particles In this section, the reactive uptake of DMA by mixed AS – sucrose particle is presented. The experimental conditions, particle-phase compositions and phase states of AS prior to DMA uptake are summarized in Table 1 (experiments #6 – 20). In discussions below, the molar fraction of sucrose on a dry basis (Fsu) is calculated as Fsu = n(su) / [n(su) + n(AS)], where n(su) and n(AS) represent the moles of sucrose and AS in the stock solutions from which the particles were generated. Because DMA – NH3 exchange reactions do not change the total molar amount of sulfate (including bisulfate or letovicite) in the particle phase, Fsu remained constant and n(AS) is equal to the total moles of sulfate-containing salts during DMA uptake in each experiment. Figures 4 and 5 show the mass changes of AS – sucrose mixed particles with low (Fsu = 0.15) and medium (Fsu = 0.28) sucrose loadings during DMA uptake. All studied particles exhibited mass increase before reaching equilibrium (as indicated by the plateau at the end of each experiment). We also estimated the displacement percentage 12


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of NH4+, as listed in Table 1, based on the fact that sucrose is inert to DMA vapor (see Figure S2 and Section S1, SI). Similar to the case where AS is initially the sole solute in the particle phase, the NH4+ in those original AS – sucrose particles (Fsu = 0.15, experiments #6 – 9 and Fsu = 0.28, experiments #11 – 14) were completely displaced at 10% RH or above. At < 5% RH, the NH4+ displacement percentage for the initial AS – sucrose (Fsu = 0.15) particle was ~ 23% (experiment #10), higher than that (~ 10%) for the initial pure AS particle (experiment #5). The non-crystallizing sucrose absorbs water,39,40 which facilitated the partitioning of DMA into the particle phase. For the initial AS – sucrose particle at Fsu = 0.28, much less NH4+ displacement (~ 4%) was observed at < 5% RH (experiment #15), likely because sucrose might have formed a highly viscous coating that hindered the uptake. The little impact of sucrose at Fsu = 0.15 may be due to a thinner coating or incomplete coverage over the particle surface. The initial phase of AS could be altered as the concentration of sucrose increased. In the following, we describe the phase state of AS in those AS – sucrose particles originally equilibrated at 70% RH as “aqueous” and that at 30% RH and below, where crystallization of AS was absent, as “amorphous”. At Fsu = 0.15, AS was always crystalline at 30% RH and below. When Fsu increased to 0.28, AS crystallization was suppressed in experiments #12a, 14 and 15. However, in experiments #12b and 13, crystalline AS was still present. It has been reported that in submicron particles containing equal mass of AS and sucrose (identical composition with Fsu = 0.28), crystallization of AS is suppressed.58,59 We speculate that the presence of impurities in 13



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the particles in experiments #12b and #13 may have triggered the crystallization of AS. Nevertheless, it is clear that equilibrium particle compositions were not largely affected by the initial phase state of AS. For instance, both initially crystalline and amorphous (non-crystalline) AS resulted in full NH4+ displacement at 30% RH (experiments #12a and #12b in Table 1). The effects of AS phase state and the sucrose coating on the reaction kinetics will be discussed in Section 3.3. Figure 6 shows the mass change of AS – sucrose mixed particles at Fsu = 0.50. Crystallization of AS was well suppressed at all RH studied. DMA uptake was effective, albeit with decreased rate as RH decreased from 70% down to 20%. Significantly different from the above situations, negligible DMA uptake was observed at 10% and < 5% RH, even though AS was amorphous. Freedman et al. indicated the formation of a sucrose shell after drying the droplets generated from the miscible AS – sucrose aqueous solution, with comparable Fsu values (0.54).60 The existence of the sucrose-rich coating layer was also confirmed by capturing the vertical Raman profile of the particle.61,62 Stronger C–H stretching Raman signals were observed near the top and bottom of the particles at < 5% RH than at 70% RH (see Section S2 and Figure S3, SI). According to our experimental results, the sucrose-rich coating layer is inferred to have turned glassy at < 5% and 10% RH prior to DMA uptake. Similar to the ultraslow diffusion of water molecules in glassy sucrose particles at low RH,38-40,63 the sucrose-rich coating layer in our study could massively retard the diffusion of DMA molecules so that particle mass increase was not detected within the timescale of our experiments. However, as RH increased to 14


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20%, there was possibly a moisture-induced glass transition, where the sucrose-rich coating layer, transformed from glassy into viscous liquid state.40,64 The amount of condensed water, which acted as a plasticizer, increased and it reduced the particle viscosity, thus allowing detectable DMA uptake at 20% RH.

Generally, in terms of final equilibrium particle-phase compositions, the DMA – NH3 exchange reactions were divided into three categories. I. At 10% RH or above (i.e., 20%, 30% and 70% RH in our experiments) for almost all Fsu studied, except at Fsu = 0.50 at 10% RH, complete NH4+ displacement took place for particles with AS initially being aqueous (experiments #1, #6, #11 and #16) and amorphous (#12a, #14 and #17 – 18). For those particles with AS originally being crystalline (experiments #2 – 4, #7 – 9, #12b – 13), water adsorbed at the surface of AS crystals or absorbed by sucrose triggered the DMA uptake. The formed DMAS increased the particulate water content, thus promoting further NH4+ displacement till completion. II. At < 5% RH and Fsu = 0, 0.15, and 0.28, limited (2 – 25%) NH4+ displacement (experiments #5, #10 and #15) occurred, possibly due to insufficient water to facilitate DMA uptake. III. At 10% RH or below (i.e., < 5% in our experiments) and Fsu = 0.50, negligible NH4+ displacement (experiments #19 – 20) was observed, likely due to the presence of an ultra-viscous sucrose coating. The sucrose layer could have turned glassy, effectively hindering the diffusion of DMA molecules so that no particle mass change 15



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was detected within the timescale of our experiments. More importantly, no clear dependence of the NH4+ displacement percentage on the initial phase state of AS (before any DMA uptake took place) has been observed. It is worth noting that our experiments involved a continuous flow with 1 ppm DMA over the levitated single particle to ensure that the measurements of uptake are not limited by the availability of DMA in the system. Besides, the displacement mechanism between alkyl amines and NH4+ has been well validated in previous work where gas-phase amine concentrations ranged from 1 ppb to a few hundred ppm.11-15,65 Hence, we believe there is little influence of using 1 ppm DMA concentration on demonstrating the equilibrium particle-phase compositions.

3.3. Kinetics of DMA uptake The net uptake reaction is expressed as (CH3)2NH (g) + NH4+ (aq) → (CH3)2NH2+ (aq) + NH3 (g)

(1)

Since the gas-phase concentration of DMA was controlled at constant level and in great excess over what was actually taken up by the single particle levitated in the EDB, a pseudo-first-order reaction mechanism can be assumed: +

d[ NH 4 ] + + − = k 2 ⋅ [ NH 4 ] ⋅ [ DMA] = k1 ⋅ [ NH 4 ] dt

(2)

where t is the reaction time; [NH4+] is the particle-phase NH4+ concentration (molecule cm–3); [DMA] is the gas-phase concentration of DMA molecules (molecule cm–3); k2 is the second-order reaction rate constant (cm3 molecule–1 s–1); k1 is the pseudo-first-order reaction rate constant (s–1) with k1 = k2 · [DMA]. 16


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From Eq. (2), one can get +

ln

[ NH 4 ]0 +

[ NH 4 ] t

= −k1 ⋅ t

(3)

where [NH4+]t is the remaining NH4+ concentration in the particle at time t (molecule cm–3), and [NH4+]0 is the initial NH4+ concentration in the particle (molecule cm–3) that can be estimated from the initial particle density and diameter at < 5% RH. In this paper, we assume all particles are spherical when calculating volume based on the particle diameter measured using an optical microscope (see Figure S4, SI). The geometric mean of the major and minor axis lengths was taken as the apparent diameter for ellipsoidal particles. For the determination of [NH4+]t, the moles of NH4+ in the particle phase [n(NH4+)] at time t is necessary. An approach to estimate n(NH4+) during the DMA uptake process is given below. Assuming DMAS exists in neutralized form (aminium-to-sulfate molar ratio = 2) with 1 ppm gaseous DMA present, each component (AS, DMAS and sucrose) absorbs water individually and the water equilibrium between gas and particle is instantaneously reached, the mass of particulate water (mw) can be calculated using the Zdanovskii-Stokes-Robinson (ZSR) equation:66 mw =

Fsu n ( AS ) n ( DMAS ) + + m 0 ( AS ) m 0 ( DMAS ) m 0 (su )

(4)

where n(AS) and n(DMAS) are the moles of AS and DMAS in a multi-component aqueous droplet with a water activity of aw. m0(AS), m0(DMAS) and m0(su) are the molalities (mol kg–1) of binary AS, DMAS and sucrose aqueous droplets at the same aw, respectively. Molalities of AS, DMAS and sucrose are obtained from the Extended 17



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AIM

Aerosol

Thermodynamic

model

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(E–AIM,

available

at

http://www.aim.env.uea.ac.uk/aim/aim.php),67 Sauerwein et al.68 and the Aerosol Inorganic-Organic

Mixtures

Functional

groups

Activity

Coefficients

model

(AIOMFAC, available at http://www.aiomfac.caltech.edu),69,70 respectively. Here we suppose there is 1 mol of total solutes, i.e., the molar amount of sucrose equals Fsu and n(AS) + n(DMAS) = 1 – Fsu. Hence, total (solute and water) mass of the droplet is given below. mtotal = n(AS) MAS + n(DMAS) MDMAS + Fsu Msu + mw

(5)

where MAS, MDMAS and Msu are the molar mass of AS, DMAS and sucrose, respectively. Similarly, when the particle is equilibrated at specific RH (corresponding to aw in the particle) prior to DMA uptake, the initial water mass (mw,0) and total mass (mtotal,0) can be estimated: m w,0 =

1 − Fsu Fsu + m0 ( AS) m0 (su )

(6)

mtotal,0 = (1 – Fsu) MAS + Fsu Msu + mw,0 .

(7)

If AS crystallizes, the first term on the right-hand side in Eqs. (4) and (6) will be removed. At this point, n(AS) and n(NH4+) can be estimated from the EDB-measured particle mass ratios during DMA uptake, i.e., mtotal / mtotal,0. k1 is determined experimentally from Eq (3). An example is shown in Figure S5 (see SI). The uptake coefficient (γ) can be determined from k1:15,23,71,72 +

+

γ=

k 1 ⋅ [ NH 4 ]0 1 [ DMA] ⋅ ω DMA 4

S  ⋅ a   V p

=

2k1 ⋅ [ NH 4 ]0 ⋅d p

(8)

3[ DMA] ⋅ ω DMA

where dp is the particle diameter (cm) and (Sa/V)p is the particle surface 18


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area-to-volume ratio (cm–1) given by 6/dp; ωDMA is the mean thermal velocity of DMA molecules (cm s–1), given by ωDMA = 100 · (8RT/πMDMA)1/2; R is the ideal gas constant (8.314 J mol–1 K–1); T is the temperature (K); MDMA is the molar mass of DMA (kg mol–1). Note in Eq. (8), we used the initial NH4+ concentration, i.e., the one prior to DMA uptake, so that we can compare the reaction kinetics with a commonly known reference. During the DMA uptake, the extent of decrease in [NH4+] varies and it is not straightforward to draw any comparison on kinetics from the data. Regardless, k1 was averaged over the first 3 – 7 [NH4+] data points during the DMA uptake. Fewer data points were used at higher RH, where DMA uptake was faster. The γ uncertainties were 20 – 40%, including the error in estimates of n(NH4+) using the ZSR equation (those assumptions mentioned above may not hold at high Fsu and low RH), the linear regression to derive k1, particle mass and diameter measurements and the calibration of DMA vapor concentration. The relative uncertainties with γ at < 5% RH were significantly larger, because the constant error (2 µm) in particle diameter measurements has a large effect on the ratio of [NH4+]t / [NH4+]0. Therefore, the γ values should only be interpreted as orders of magnitude estimates. Figure 7a – 7i shows the γ of DMA onto AS and AS – sucrose particles at specified Fsu and RH. Available literature data on uptake coefficient of DMA by AS is rather limited, and our result at < 5% RH is slightly lower than that reported by Qiu et al.14 (see Figure 7i). This is possibly due to different experimental methods used. We estimated γ based on particle mass variations while Qiu et al. used the decay in gas-phase DMA concentrations.14 Nevertheless, the order of magnitude (10–4) of γ we 19



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estimated matched well with that in Qiu et al.14 Figure 7a shows the relationship between γ and RH for AS particles. The γ decreased as RH decreased, especially at 30% RH and below. It can be explained by the less amount of SAW when RH decreased.25 Less efficient accommodation and hydration of DMA molecules at the particle surface accounts for the decrease in γ. Similar trends were also observed for AS – sucrose particles (Figures 7b – 7d), where both SAW and particle viscosity play a role on reaction kinetics. A number of studies have revealed a monotonically increasing viscosity73,74 or decreasing diffusion coefficient of water38,40 in droplets/solutions of sucrose or sucrose – inorganic electrolyte mixtures as RH decreases, with drastic changes near the glass transition point.63 Hence, the diffusion of DMA molecules in those AS – sucrose mixed particles was hindered as RH decreased, decelerating the uptake kinetics. For the AS – sucrose (Fsu = 0.50) particles at < 5% and 10% RH where DMA uptake was undetectable within the timescale of our experiments (see Figure 6), the upper limits of γ were estimated at 1.5 × 10–4 and 1.4 × 10–4, respectively, and indicated as the horizontal bars in Figures 7d, 7h and 7i. A moisture-induced glass transition may explain the abrupt change in γ between 10% and 20% RH (see Section 3.2). The dilution effect with more absorbed water at higher RH may not be dominant within the studied RH range; otherwise a decrease in γ would have been observed as RH increased. Next, we present the relationship between γ and Fsu at fixed RH. At 70% RH (Figure 7e), the addition of sucrose did not lower γ significantly until Fsu reached 0.28. γ further decreased by almost 50% when Fsu increased to 0.50. Although 20


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sucrose instantaneously achieves water equilibrium at RH as high as 70%,40 the viscosities of aqueous droplets/solutions of sucrose are still 2 – 3 orders higher than those of inorganic salts74 and can considerably slow down the diffusion of DMA molecules within the particles. Our results were in agreement with the expectation that particle viscosity positively correlates with the concentration of sucrose. At 30% RH (Figure 7f), γ at different Fsu were mostly comparable, except for that of AS – sucrose (Fsu = 0.28) particles without AS crystallization (indicated as open symbol), which had substantially higher γ than the other estimates even considering the experimental uncertainties. As Fsu reached 0.50, AS crystallization was well suppressed but γ became similar to those of particles with crystalline AS, likely because the enhancement due to an amorphous phase was offset by the hindrance due to elevated particle viscosity. At 20% and 10% RH (Figures 7g and 7h), it is interesting to note that γ became larger as Fsu increased to 0.15. Although such slight sucrose addition did not alter the initial crystalline phase of AS, it increased the particulate water content over the SAW associated with AS alone, thus accelerating the DMA uptake. Such enhancement at 30% RH may also exist but not as obvious (see Figure 7f), likely due to the presence of sufficient SAW. As Fsu further increased, the decrease in γ was remarkable. We note that such decrease was greater than at 70% and 30% RH, especially when Fsu reached beyond 0.28.This is because for the same increase in Fsu, the particle viscosity increases by much larger extents at lower RH.74 At < 5% RH (Figure 7i), with the addition of sucrose, γ decreased significantly to 21



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values below 2 × 10–4 due to the formation of ultra-viscous sucrose coating to hinder the DMA uptake. In this study, we focus on the roles of RH and the addition of particulate sucrose on the reactive uptake of DMA. The effect of DMA vapor concentration on DMA uptake kinetics is beyond the scope of this paper.

4. Conclusion and atmospheric implication We investigated the DMA reactive uptake by particles of AS and AS – sucrose mixtures at 70%, 30%, 20%, 10% and below 5% RH using a coupled EDB – Raman system. The equilibrium particle-phase compositions and NH4+ displacement percentages were not dependent upon the initial phase states of AS, but considerably affected by particulate water and sucrose contents instead. In most cases, the original NH4+ was fully displaced as long as there was sufficient moisture, such as at 10% RH and higher, in the environment. Such phenomenon was observed for particles with AS originally not only in aqueous phase,11,12 but also in crystalline and amorphous phases. The condensed water adsorbed at crystalline AS surface or absorbed by the non-crystallizing sucrose triggered the uptake, forming DMAS, which significantly increased the particulate water content and thus promoted further DMA uptake till NH4+ depletion. This may explain observations that SOM formed via the oxidation of biogenic VOC promotes gas-to-particle partitioning of DMA in the semi-arid urban Tucson area in Arizona.17 At the driest condition in our study (RH < 5%), water was limiting and the displacement percentage of NH4+ was 22


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confined to values below 25%. With Fsu as high as 0.50, a thick, ultra-viscous sucrose coating is likely present over the AS core, resulting in negligible DMA uptake at 10% RH and below. Uptake coefficients of DMA onto AS and AS – sucrose particles were estimated using the particle mass measurements during the course of DMA uptake. On one hand, for all studied initial particle compositions, γ always decreased as RH decreased. On the other hand, at any fixed RH, the addition of sucrose can accelerate DMA uptake by inhibiting the AS crystallization and increasing particle-phase water contents, and hinder DMA uptake by forming a viscous coating over the inorganic-rich particle core. Conventionally, the sources of short-chain alkyl amines are often the sources of NH3,1 but it has recently been pointed out that vegetation could be a distinctive alkyl amines source which is NH3-deficient.75 Although the gas-phase DMA concentrations used in this study may only be detected in proximity to DMA emission sources, our results suggest the feasibility of reactive DMA uptake by AS and mixed AS – viscous SOM aerosol particles. Kinetically, DMA uptake could be facilitated for crystalline AS, or retarded for aqueous AS, with viscous SOM in the aerosol particles. DMA uptake by mixed AS – hydrophobic organics is underway in our laboratory to understand the role of organics on the heterogeneous DMA uptake more thoroughly.

Supporting information Schematic of the flow system, reactivity of sucrose particles against DMA and an 23



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example of the estimation on the pseudo-first order reaction rate constant (k1). This material is available free of charge via the Internet at http://pubs.acs.org.

Author information *

Corresponding author: Chak K. Chan

E–mail:

[email protected],

Phone:

+(852)-3442-5593,

Fax:

+(852)-3442-0688. Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the Research Grants Council (RGC) of the Hong Kong Special Administrative Region (HKSAR), China (GRF 16300214). The grant from Hong Kong RGC PhD Fellowship Scheme (HKPFS) is also gratefully acknowledged.

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Tables

Table 1. Summary of experimental conditions, initial particle compositions, AS phase states and percentages of NH4+ displaced

a

experiment #

Fsua

RH (%)

initial AS phase state

1 2 3 4 5 6 7 8 9 10 11 12a 12b 13 14 15 16 17 18 19 20

0 0 0 0 0 0.15 0.15 0.15 0.15 0.15 0.28 0.28 0.28 0.28 0.28 0.28 0.50 0.50 0.50 0.50 0.50

70 30 20 10 96 10 ± 2 > 97 > 96 > 96 > 97 23 ± 2 > 96 > 99 > 99 > 95 > 98 4±2 > 99 > 96 > 91

Reactive Uptake of Dimethylamine by Ammonium Sulfate and

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