Real-Time Investigation of Chemical Compositions and Hygroscopic

Dec 6, 2016 - Xue Li, Dhrubajyoti Gupta, Jisoo Lee, Geonhee Park, and Chul-Un Ro. Department of Chemistry, Inha University, Incheon, 402-751, Republic...
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Real-Time Investigation of Chemical Compositions and Hygroscopic Properties of Aerosols Generated from NaCl and Malonic Acid Mixture Solutions Using in Situ Raman Microspectrometry Xue Li, Dhrubajyoti Gupta, Jisoo Lee, Geonhee Park, and Chul-Un Ro* Department of Chemistry, Inha University, Incheon, 402-751, Republic of Korea S Supporting Information *

ABSTRACT: Recently, ambient sea spray aerosols (SSAs) have been reported to undergo reactions with dicarboxylic acids (DCAs). Several studies have examined the hygroscopic behavior and chemical reactivity of aerosols generated from NaCl−DCA mixture solutions, but the results have varied, especially for the NaCl−malonic acid (NaCl−MA) mixture system. In this work, in situ Raman microspectrometry (RMS) was used to simultaneously monitor the change in chemical composition, size, and phase as a function of the relative humidity, for individual aerosols generated from NaCl−MA solutions, during two hygroscopic measurement cycles, which were performed first through the dehydration process, followed by a humidification process, in each cycle. In situ RMS analysis for the aerosols showed that the chemical reaction between NaCl and MA occurred rapidly in the time scale of 1 h and considerably in the aqueous phase, mostly during the first dehydration process, and the chemical reaction occurs more rapidly when MA is more enriched in the aerosols. For example, the reaction between NaCl and MA for aerosols generated from solutions of NaCl:MA = 2:1 and 1:2 occurred by 81% and 100% at RH = 42% and 45%, respectively, during the first dehydration process. The aerosols generated from the solution of NaCl:MA = 2:1 revealed single efflorescence and deliquescence transitions repeatedly during two hygroscopic cycles. The aerosols from NaCl:MA = 1:1 and 1:2 solutions showed just an efflorescence transition during the first dehydration process and no efflorescence and deliquescence transition during the hygroscopic cycles, respectively. The observed different hygroscopic behavior was due to the different contents of NaCl, MA, and monosodium malonate in the aerosols, which were monitored real-time by in situ RMS. For aged SSAs collected during a field study in central California, it was observed that sulfate and nitrate could not fully account for chloride depletion, suggesting the substantial contribution of organic acids to the chloride depletion in the aged SSAs.22 In addition, it was reported that chloride was depleted for laboratory-generated NaCl aerosols mixed with dicarboxylic acids (DCAs) and different chemical reactivity was observed for different DCAs.22 This new observation for the aging of SSAs indicates an additional source of gaseous HCl in the air and an additional sink for gaseous organic acids as resultant nonvolatile organic salts will remain in aerosol phase.23 It was also reported that nitrate in laboratorygenerated NaNO3 aerosols mixed with DCAs could be replaced by DCAs which was driven by the liberation of gaseous HNO3, suggesting the presence of a recycling pathway of gaseous HNO3 in the air.24

1. INTRODUCTION Airborne sea spray aerosols (SSAs), comprising a large proportion of the atmospheric particulate mass,1,2 become increasingly complex in their chemical compositions when they experience long-distance transport, aging, and interactions with various gas-phase species in air. For example, nascent SSAs were reported to undergo chloride (Cl) depletion, which was attributed mainly to the reactions of NaCl/MgCl2 with inorganic NOx/HNO3 and SOx species.3−9 When the chemical compositions of nascent SSAs are modified by aging and heterogeneous reactions, their hygroscopic properties are altered accordingly,10−15 which in turn alters their optical properties and cloud-droplet nucleation efficiency.16−20 Although chloride depletion by the reactions of SSAs with inorganic NOx/HNO3 and SOx species has long been recognized, ambient SSAs were also claimed to react with water-soluble organic acids, where the reaction was driven by the liberation of gaseous HCl.21,22 For supermicron SSAs collected at a site near the Arctic ocean, it was reported that sulfate and nitrate were main contributors to chloride depletion, followed by methanesulfonate and oxalate ions, and malonate and succinate ions were minor contributors.21 © XXXX American Chemical Society

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August 27, 2016 December 1, 2016 December 6, 2016 December 6, 2016 DOI: 10.1021/acs.est.6b04356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

dimensional (2-D) size, and phase as a function of the RH, during two hygroscopic measurement cycles that were performed first through a dehydration process, followed by a humidification process, in each cycle. The in situ RMS analysis of the individual aerosols generated from solutions with different NaCl to MA mixing ratios clearly showed that the chemical reaction between NaCl and MA occurs rapidly in the time scale of 1 h and considerably in the aqueous phase, mostly during the first dehydration process. The reaction produces MSM; the chemical reaction occurs more rapidly when MA is more enriched in the aerosols; and the controlling factor for the reactivity of the aerosols is the availability of H+ ions dissociated from MA.

Low-molecular weight (LMW) DCAs are significantly present in marine aerosols.25−27 In remote ocean, it was reported that LWM DCAs and related compounds contribute to more than 10% of total carbon (TC) and 3.6−23% (14% on average) of water-soluble organic carbon (WSOC) in marine aerosols.25,27 Among LMW DCAs, oxalic acid is predominant, followed by malonic and succinic acids, where oxalic acid accounts for 47−70% (58% on average) of DCAs and malonic and succinic acid levels are ∼30% and ∼14% of oxalic acid level, respectively.27 As LMW DCAs are significant in marine aerosols, up until now, several studies on the hygroscopic properties and chemical reactivity of NaCl−DCAs mixture aerosol particles as surrogates for SSAs and organic acids mixture aerosols have been reported,28−33 including the aerosol system nebulized from aqueous solutions of NaCl−malonic acid (NaCl−MA) mixtures, for which somewhat inconsistent observations have been reported, as summarized in Table S1 of the Supporting Information. For aerosols generated from equimolar NaCl−MA solution, some studies reported somewhat deviating deliquescence relative humidity (DRH)28−30 and efflorescence RHs (ERHs),28,29 which were in the range of 57−70% and ∼31− 52%, respectively, whereas the continuous uptake and shrinkage without any phase transitions (no DRH and ERH) during humidification and dehydration processes were observed in another work.31 When aerosols generated from solutions with different NaCl to MA mixing ratios were investigated, aerosols from NaCl-rich solutions showed deviating DRHs31−33 and ERHs,31,32 whereas those from MA-rich solutions did not show DRHs and ERHs.31−33 Two studies28,29 that investigated aerosols nebulized from equimolar NaCl−MA solution without using a diffusion dryer, provided different DRHs and ERHs (Table S1). All the other studies30−33 investigated dry aerosols generated by passing through a diffusion dryer during the nebulization from NaCl−MA solutions, which might have different chemical compositions from the NaCl−MA solutions due to HCl evaporation during the quenching process in the diffusion dryer.22 One study claimed that a chemical reaction occurred during the dehydration process for NaCl−MA mixture aerosols, probably with the release of gaseous HCl and the formation of monosodium malonate (MSM).31 Laskina et al.32 reported chloride depletion or the occurrence of a reaction both in submicron (100 nm) and supermicron (3−12 μm) NaCl−MA mixture particles after the hygroscopic measurements using a hygroscopic tandem differential mobility analyzer (HTDMA) and Raman microspectrometry (RMS), respectively. On the other hand, no chemical reaction was claimed to take place for supermicron aerosol particles generated from equimolar NaCl− MA solution.30 In their work, they assumed that the reactions between NaCl and MA would lead to the formation of disodium malonate (DSM), whereby both Raman and infrared (IR) spectra did not show the DSM signatures in the NaCl− MA mixture particles after hygroscopic measurements. To clearly explain the deviating DRHs, ERHs, and chemical reactivity of the aerosols generated from the NaCl−MA solutions, it is essential to investigate the hygroscopic behavior and chemical reactivity of the aerosols at the same time as the different hygroscopic behavior observed for aerosols generated from the NaCl−MA mixture solutions might be due to chemical compositional modification of the aerosols during the hygroscopic measurements. In this study, in situ RMS was used to monitor the changes in the chemical compositions, 2-

2. EXPERIMENTAL SECTION 2.1. Sample preparation. Pure 1.0 M aqueous solutions of NaCl (>99.9% purity, Sigma-Aldrich), MA (99%, SigmaAldrich), and DSM (>98%, Sigma-Aldrich) were prepared. A 1.0 M MSM solution was obtained by mixing the same volumes of 2.0 M NaOH (>99.9% purity, Sigma-Aldrich) and MA solutions. NaCl−MA solutions with molar mixing ratios of NaCl:MA = 2:1, 1:1, and 1:2; and MA−MSM solutions with MA:MSM = 3:1, 1:1, and 1:3 were prepared. The aerosols were nebulized from the solutions using a single jet atomizer (HCT4810), deposited on bare Si wafer (MTI Corporation, 99.999% purity), and examined for their hygroscopic behavior and chemical reactivity. 2.2. In situ Raman microspectrometry (RMS). In situ RMS measurements were carried out to examine the hygroscopic behavior and chemical compositional variations of the aerosols generated from NaCl−MA solutions under controlled RHs. The experimental setup, which is shown schematically in Figure S1 in the Supporting Information, is composed of three parts: (A) a sample cell, (B) humidity controlling system, and (C) Raman microscope/spectrometer. The Si wafer substrate onto which the aerosols would be deposited by the nebulization was mounted on the plate in the sample cell. The RH inside the sample cell was controlled by mixing dry and wet (saturated with water vapor) N2 gases. The wet N2 gas was obtained by bubbling through two deionized water reservoirs. The flow rates of the dry and wet N2 gases were controlled by two mass flow controllers to obtain the desired RH in the range, ∼3−90%, which was monitored by a digital hygrometer (Testo 645). The digital hygrometer was calibrated using a dew-point hygrometer (M2 Plus-RH, GE), providing RH readings with ±0.5% reproducibility. A more detailed discussion of the sample cell and humidity controlling system can be found elsewhere.34,35 The in situ Raman spectra of the aerosols deposited on the Si wafer substrate were recorded using a confocal micro-Raman spectrometer (XploRA, Horiba Jobin Yvon) equipped with a 50×, 0.5 numerical aperture objective (Olympus). An excitation laser with a wavelength of 532 nm and 12 mW power was used and the scattered Raman signals were detected using an air cooled multichannel charge-coupled device (CCD) detector. The data acquisition time was 90 s for each Raman measurement. The spectral resolution was 5.7 cm−1 using a 600 gr/mm grating. Optical images of the aerosols on the substrate were obtained using a video camera to measure their 2-D size. The optical image size was 908 × 680 pixels. The spectra and images were acquired using Labspec6 software and the images were processed using image analysis software (Matrox, Inspector v9.0). A more detailed discussion of the RMS measurement B

DOI: 10.1021/acs.est.6b04356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 1. (A) Optical images of the aerosols generated from a solution with a mixing ratio of NaCl:MA = 2:1, obtained during the first dehydration (↓) and humidification (↑) cycle; (B) 2-D area ratio plot as a function of RH for an aerosol during two dehydration−humidification cycles; (C) in situ Raman spectra of the aerosol obtained at different RHs during the first dehydration process; and (D) Raman spectra of pure MSM and MA aerosols together with the mixed aerosol at RH = 82.2% and 9.1%.

aqueous droplets generated from the solution with a mixing ratio of NaCl:MA = 2:1 on the image field decreased gradually in size, until an abrupt decrease in size was observed at RH = 44.8−39.4% for different droplets, which are their ERHs. Thereafter, as RH was decreased to ∼5%, no further decrease in size was observed. During the humidification process, all particles on the image field began to absorb moisture at RH = 71.1(±0.3) % (image (h) of Figure 1(A)) and dissolved completely at RH = 73.2(±0.3) % (image (i) of Figure 1(A)), which are their measured DRHs, and the homogeneous droplets underwent hygroscopic growth, as shown in image (j) of Figure 1(A). Figure 1(B) shows the 2-D area ratio plot of an aerosol on the optical image field of Figure 1(A) as a function of the RH for two hygroscopic measurement cycles. The dehydration and humidification curves are represented as the area ratio (A/A0), where the 2-D projected aerosol area at a given RH (A) is divided by that of the aerosol at the end of the first dehydration process (A0). During the first and second dehydration processes, single efflorescence transitions were observed at ERH = 44.8% and 41.8%, respectively, for this aerosol, which were attributed to homogeneous crystallization of the NaCl moiety.14,35 During the first and second humidification processes, single deliquescence transitions were observed at DRH = 73.2% and 71.5%, respectively. The DRH and ERH of the aerosol for the second hygroscopic cycle are lower than those for the first one. As deliquescence is a thermodynamic process, the DRHs of aerosols with the same chemical compositions should be reproducible (within ±0.5% in our experimental setup),11,14,35 strongly suggesting that the chemical compositions of the aerosol were altered during the hygroscopic measurements. A thermodynamic model predicted two deliquescence transitions for the NaCl−MA mixture

conditions for single particle analysis can be found elsewhere.36−39

3. RESULTS AND DISCUSSION 3.1. In Situ Raman Analysis of Aerosols Generated from NaCl−MA Mixture Solutions. Figure 1 presents (A) optical images of aerosols generated from a solution with a molar mixing ratio of NaCl:MA = 2:1 on an image field obtained at different RHs during the first dehydrationhumidification cycle; (B) 2-D area ratio plot as a function of the RH for an aerosol during two dehydration-humidification cycles; (C) Raman spectra of the aerosol obtained at different RHs during the first dehydration process; and (D) Raman spectra of pure MA and MSM aerosols together with the mixed aerosol. Although the hygroscopic behavior and chemical compositional variation of the aerosols generated from solutions with mixing ratios of NaCl:MA = 2:1, 1:1, and 1:2 were investigated under controlled RHs in this work, the example data shown in Figure 1 can be helpful for understanding overall features of the hygroscopic behavior and chemical reactivity of the NaCl−MA aerosol system. In the in situ Raman experiments, NaCl−MA solutions with different mixing ratios were nebulized to deposit aerosols on Si wafer substrates while maintaining the entire hygroscopic measurement system at RH > 90%. Optical images with an image field containing 8−10 aerosols, ∼12−15 μm in diameter at RH = ∼90%, and Raman spectra of the aerosols on the image field were obtained at specific RHs, by decreasing the RH from ∼90% to 5% (dehydration process) and then increasing the RH from ∼5% to 90% (humidification process), which were repeated twice. Images (a)−(e) and (f)−(j) of Figure 1(A) were recorded during the first dehydration and humidification processes, respectively. During the dehydration process, C

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Figure 2. Raman spectra of (A) MA, (B) MSM, and (C) DSM aerosols, obtained at high and low RHs, together with those of their crystalline powders.

system, including a mutual DRH (MDRH) at RH = 59.9%,40 which is not the case for the aerosol in Figure 1(B), suggesting that the aerosol does not belong to the NaCl−MA binary mixture system. Instead, the hygroscopic behavior of the aerosol in Figure 1(B) during the second hygroscopic cycle closely resembles that of the aerosols with a mixing ratio of NaCl:MSM = 1:1 (single deliquescence and efflorescence transitions at DRH = 71.1(±0.3) % and ERH = 43.2−40.0%) (a typical hygroscopic curve is shown in Figure S2(A)), suggesting that the aerosol generated from the NaCl:MA = 2:1 solution has a similar chemical composition to that of the NaCl:MSM = 1:1 aerosol during the second hygroscopic cycle. Figure 1(C) shows the Raman spectra of the aerosol obtained at different RHs during the first dehydration process. As NaCl is not Raman active, the Raman spectra can only provide information on the organic species and water in the aerosol. As the RH was decreased, a peak at 1375 cm−1 (CO stretching/bending vibration of COO− group41) was increased relatively, indicating an increase in the content of the COO− group related to the malonate moiety, and the free water peak at 3450 cm−1 (OH stretching vibration42) was decreased, even though overall peak patterns of the spectra appear similar. Figure 2 shows the Raman spectra of MA, MSM, and DSM aerosols, obtained at high and low RHs, together with their crystalline powdery particles. A comparison of the Raman spectra of the aerosol in Figure 1(C) and those of MA, MSM, and DSM in Figure 2 suggests that the Raman spectra in Figure 1(C) obtained at different RHs are for a MA−MSM mixed aerosol. In addition, the Raman spectra in Figure 1(C) do not resemble those of crystalline MA and MSM powders, indicating that the aerosols generated from the NaCl:MA = 2:1 solution are in amorphous form at very low RH = 9.1%. Figure 1(D) shows the Raman spectra of a pure MA aerosol obtained at RH = 90%, the aerosol (generated from a solution of NaCl:MA = 2:1) at RH = 82.2% and 9.1%, and a pure MSM aerosol at RH = 10%, which are normalized to a peak at 1730 cm−1 (CO stretching vibration of COOH group41). The Raman spectrum of the mixed aerosol at RH = 82.2% shows a much stronger free water peak at 3450 cm−1 than that of the pure MA aerosol at RH = 90% due to the presence of more hygroscopic NaCl moiety (Figure S3 shows that NaCl is more hygroscopic than MA, MSM, and DSM). When the free water peaks, of which the intensities strongly depend on the RH, are excluded from Raman spectral analysis, the Raman spectrum of the aerosol at RH = 82.2% is comparable to that of the pure MA aerosol at RH = 90%, whereas that of the aerosol at RH =

9.1% is comparable to that of the pure MSM aerosol at RH = 10%, indicating that the aerosol generated from the solution of NaCl:MA = 2:1 is mostly a NaCl-MA mixture at a high RH and a NaCl-MSM at a low RH during the first dehydration process. As stated above, the hygroscopic behavior of the aerosol generated from a solution of NaCl:MA = 2:1 during the second hygroscopic cycle is similar to that of the aerosols with the mixing ratio of NaCl:MSM = 1:1, which is consistent with the observation by the in situ Raman measurements, strongly suggesting that MA was converted to MSM during the hygroscopic measurements. In this study, the aerosols generated from the solution of NaCl:MA = 2:1 produced MSM as a reaction product. As the first acid dissociation constant of MA (pKa1 = 2.8543) is ∼3 orders of magnitude larger than the second one (pKa2 = 5.70), the mole fraction of (COOH)CH2COO− dissociated from MA in an aqueous solution should be much larger than that of CH2(COO)22−, which is also supported by the extended aerosols inorganic model (E-AIM) calculation for the NaCl− MA mixture system (http://www.aim.env.uea.ac.uk/aim/aim. php),44,45 so that the chemical reaction between NaCl and MA would occur in aqueous aerosols to produce MSM as below. (COOH)CH 2COO−(aq) + H+(aq) + 2Na +(aq) + 2Cl−(aq) → (COOH)CH 2COO−(aq) + 2Na +(aq) + Cl−(aq) + HCl(g)( ↑ ) → (COOH)CH 2COONa(s) + NaCl(s)(after efflorescence)

3.2. Real-Time Monitoring of Chemical Compositional Modification and Hygroscopic Behavior of Aerosols Generated from NaCl−MA Mixture Solutions. To clearly understand the hygroscopic behavior and chemical reactivity of the NaCl−MA mixture system, in situ RMS measurements were performed for aerosols generated from solutions with molar mixing ratios of NaCl:MA = 2:1, 1:1, and 1:2. In this study, MA was converted to MSM for those aerosols during the hygroscopic measurements (Raman analysis of the aerosols from NaCl:MA = 1:1 and 1:2 solutions is not shown here because it is similar to that of the NaCl:MA = 2:1 aerosols stated above). The Raman spectra, obtained at different RHs, of standard aerosols generated from MA−MSM solutions with mixing ratios of MA:MSM = 0:1, 1:3, 1:1, 3:1, and 1:0 (XMSM = 1.0, 0.75, 0.5, 0.25, and 0.0, respectively, where XMSM = [MSM]/([MA]+[MSM]) (defined as the mole fraction of MSM for RMS analysis)) were used to produce the calibration D

DOI: 10.1021/acs.est.6b04356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 3. (A) Raman spectra of the standard aerosols generated from pure MA and MSM solutions as well as MA−MSM mixture solutions with mixing ratios of 1:3, 1:1, and 3:1, normalized to a Raman peak at 1730 cm−1 and (B) calibration curve data based on the intensity ratios of two peaks at 1406 and 1730 cm−1 (I1406/I1730) as a function of RH for MA, MSM, and MA−MSM mixture aerosols.

Figure 4. Plots of 2-D area ratio (A/A0) and estimated mole fractions of MSM (XMSM), obtained during two dehydration−humidification cycles, for aerosols generated from NaCl−MA solutions with mixing ratios of NaCl:MA = 2:1 (A and D), 1:1 (B and E), and 1:2 (C and F).

increased with decreasing RH, which is because the CO stretching vibrations of free COOH group (at 1730 cm−1)46,47 and intramolecular hydrogen bonded COOH group (at 1680 cm−1)46,47 in aqueous solution were weaker and stronger, respectively, as shown in Figure 2, when MA and MSM were more supersaturated. Therefore, the intensity of the peak at 1730 cm−1 is relatively smaller than that at 1406 cm−1 as RH decreases. Nevertheless, the linearity between XMSM and I1406/I1730 at each RH was observed to be sufficiently consistent. In Figure 4, plots of the 2-D area ratio (A/A0) and mole fractions of MSM (XMSM), obtained during two dehydrationhumidification cycles as a function of RH, are shown for the aerosols generated from the NaCl−MA solutions. The hygroscopic curves were obtained based on the optical images of the individual aerosols and the mole fractions of MSM were determined using the calibration curves based on the Raman intensity ratios (I1406/I1730). As shown in Figures 4(D)-(F), the XMSM values, which are expressed as a range of values (bars),

curves for the determination of the relative MA and MSM contents in aerosols generated from the NaCl−MA solutions at specific RHs. Figure 3(A) shows the Raman spectra of MA, MSM, and MA−MSM mixture aerosols obtained at RH = 90% and normalized to a peak at 1730 cm−1. The Raman intensities of the CH2 bending vibration at 1406 cm−1 would be similar for MA and MSM as both have a single CH2 group and those of the CO stretching vibration of COOH group at 1730 cm−1 would be smaller for MSM having just one COOH group than for MA having two. Therefore, the intensity ratio of the two peaks at 1406 and 1730 cm−1 (I1406/I1730) increases with increasing MSM content, as shown in Figure 3(A). Indeed, Figure 3(B) shows good linearity at each RH between the molar fractions of MSM (XMSM) and the Raman intensity ratios (I1406/I1730), which was utilized as the calibration data, where the data points and error-bars represent the mean values and range of values, respectively, obtained for ∼10 aerosols of each standard sample. The I1406/I1730 ratios for standard aerosols E

DOI: 10.1021/acs.est.6b04356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology have uncertainties in the range of ±0.03−0.05, due mainly to statistical variations in the Raman peak intensities caused by the baseline correction procedure and the uncertainties involved in calibration measurements using standard aerosols. 3.2.1. In Situ RMS Analysis of Aerosols Generated from NaCl:MA = 2:1 Solution. For the aerosols generated from a solution with a mixing ratio of NaCl:MA = 2:1, the DRHs obtained for nine aerosol particles on an image field decreased from 73.2(±0.2) % to 71.5(±0.2) %, and the ERHs were comparable as they were 41.9(±1.7) % and 41.8(±0.8) % when the dehydration and humidification processes were repeated twice. The lower DRH observed in the second hygroscopic cycle appears to be due to the lower NaCl content as the conversion of MA to MSM mostly occurred during the first hygroscopic cycle. As shown in Figure 4(A), the 2-D area ratios are larger at RHs before efflorescence during the first dehydration process than the other processes because NaCl is present more in the first dehydration process, which is also observed for aerosols generated from solutions with mixing ratios of NaCl:MA = 1:1 and 1:2 (Figure 4(B),(C)). As stated above, NaCl is more hygroscopic than MA and MSM. As shown in Figure 4(D), the mole fraction of MSM increased rapidly from XMSM = 0.37(±0.03) at RH = 87% to XMSM = 0.81(±0.03) at RH = 42% during the first dehydration process. Even at high RH = 87%, ∼37% of MA already reacted with NaCl to produce MSM, and ∼81% of MA reacted at RH = 42%. As the in situ RMS measurement was performed for ∼10 min at each RH, the chemical reaction between MA and NaCl occurred rapidly in the aqueous aerosols during the first dehydration process. After the NaCl moiety effloresced at RH = 44.8%, a further RH decrease until RH = 10% did not change XMSM at 0.78(±0.04), suggesting that no further reaction took place after efflorescence of the NaCl moiety because aqueous Cl− ions for the HCl liberation were unavailable in the effloresced NaCl solids. During the second dehydration and humidification processes, X MSM remained constant at 0.86(±0.03), even though XMSM had increased somewhat at RH > 70% (after deliquescence) during the first humidification process (from XMSM = 0.78(±0.04) at RH < 70% to XMSM = 0.86(±0.03) at RH ≥ 70% (Figure 4(D)). This suggests that most of the reaction took place in the aqueous phase during the first dehydration process when the considerable amounts of aqueous H+ from MA and Cl− ions from NaCl were available for HCl liberation and that the reaction would slow down or stop when the available MA (and thus H+) for the reaction became small and/or the aerosols were in the solid phase. When the dehydration and humidification processes were repeated twice, the reaction product, MSM, dominated (XMSM = 0.86(±0.03)) with NaCl and MSM as the major constituents, i.e., most MA was consumed during the first dehydration process. The DRH of 71.5(±0.2) % measured in the second humidification process was similar, DRH = 71.1(±0.3) %, for laboratory generated standard aerosols with a mixing ratio of NaCl:MSM = 1:1 (Figure S2(A)), strongly supporting that the aerosols from the NaCl:MA = 2:1 solution has a similar chemical composition to that of the NaCl:MSM = 1:1 standard aerosols. Laskina et al.32 also reported a DRH of 71.8(±5.3) % for aerosols generated from a NaCl:MA = 2:1 solution. 3.2.2. In Situ RMS Analysis of Aerosols Generated from NaCl:MA = 1:1 Solution. For the aerosols generated from a solution with a mixing ratio of NaCl:MA = 1:1, only an ERH at RH = 30.9(±2.6) % was observed during the first dehydration process for ten aerosol particles on an image field. As shown in

Figure 4(E), XMSM at RH = 32% during the first dehydration process was 0.88(±0.04), which means that the molar fraction of MA is 0.12(±0.04). As the contents of unreacted MA and NaCl are the same and the NaCl molar content is considerably small at the time of efflorescence (i.e., NaCl:MA:MSM = 0.10:0.10:0.80), the observed ERH value and the decrease in size during the efflorescence transition are lower and smaller than those of the aerosols generated from the NaCl:MA = 2:1 solution, respectively (see Figure 4(A)). This indicates that the efflorescence is due mainly to crystallization of the NaCl moiety in the aerosols. During the first humidification and the second dehydration and humidification processes, the aerosols did not show any clear deliquescence and efflorescence, indicating the absence of considerable amounts of NaCl and MA in the aerosols (NaCl and MA have DRHs and ERHs, but MSM does not (see Figure S3)). As shown in Figure 4(E), the mole fraction of MSM increased rapidly from XMSM = 0.34(±0.04) at RH = 89% to XMSM = 0.86(±0.02) at RH = 42% during the first dehydration process. For aerosols from a NaCl:MA = 1:1 solution, the chemical reaction of MA with NaCl also occurred rapidly in the aqueous aerosols during the first dehydration process. After the NaCl and MSM moieties effloresced at RH = 32.5%, a further decrease of RH until RH = 9% did not change XMSM = 0.88(±0.04), suggesting no further reaction after efflorescence. During the first humidification process, XMSM increased somewhat at RH > 35% (from XMSM = 0.89(±0.03) at RH < 35% to XMSM = 0.95(±0.05) at RH ≥ 35%), where the aerosol grew continuously (Figure 4(B)), suggesting that the aerosol absorbed water continuously and the chemical reaction, albeit to a small extent, occurred at RH > 35%. During the second dehydration−humidification cycle, the XMSM values are somewhat constant at 1.02(±0.06), regardless of the RH, meaning that all MA reacted completely with NaCl to become MSM. The chemical reaction between MA and NaCl occurred completely, probably because the aerosols from the NaCl:MA = 1:1 solution have relatively more MA (and thus more H+ ions) available for the reaction than those from NaCl:MA = 2:1. It was reported that the aerosols generated from the NaCl:MA = 1:1 solution exhibited gradual water uptake−evaporation during the humidification−dehydration processes.31 On the basis of the current observation, it appears that a complete reaction already occurred during their aerosol generation process before the hygroscopic measurements. In addition, the first and second humidification processes are similar (Figure 4(B)), indicating that the chemical compositions of the aerosols are comparable during the first and second humidification processes, and the hygroscopic behavior of the aerosols from the NaCl:MA = 1:1 solution after the first dehydration process is close to that of the pure MSM aerosols (Figure S3(C)). 3.2.3. In Situ RMS Analysis of Aerosols Generated from NaCl:MA = 1:2 Solution. As shown in Figure 4(C), the eight aerosols generated from a solution with a mixing ratio of NaCl:MA = 1:2 did not show any ERH and DRH, strongly suggesting that a considerable amount of the NaCl moiety is not present to exhibit efflorescence and/or deliquescence phenomena during the hygroscopic cycles. As shown in Figure 4(F), the mole fraction of MSM increased rapidly from XMSM = 0.12(±0.04) at RH = 85% to XMSM = 0.51(±0.03) at RH = 45% during the first dehydration process. During the first humidification and the second dehydration processes, X MSM values are constant at 0.53(±0.06), regardless of the RH. As the aerosols were F

DOI: 10.1021/acs.est.6b04356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

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generated from the NaCl:MA = 1:2 solution, the complete chemical reaction between NaCl and MA results in aerosols with XMSM = 0.50 and no remaining NaCl. That is, the reaction was completed at RH = ∼45% during the first dehydration process. The hygroscopic curves obtained during the first humidification and second dehydration processes (Figure 4(C)) are similar to those of the aerosols with a mixing ratio of MA:MSM = 1:1 with having no ERH and DRH (Figure S2(B)), also supporting the occurrence of a complete reaction during the first dehydration process. When MA in the aerosols generated from the NaCl and MA mixture solutions is enriched further, the chemical reaction occurred more rapidly. That is, for the aerosols from the NaCl:MA = 2:1, 1:1, and 1:2 solutions, the chemical reaction was not complete even after two hygroscopic cycles, was completed after the first dehydration and humidification processes, and was completed before the end of the first dehydration process, respectively, suggesting that the chemical reaction, which is driven by the liberation of HCl in an open system (like in ambient atmosphere), is controlled mainly by the available MA in the aqueous aerosols. The Cl− ions in the aqueous aerosols are always available for the reaction until complete consumption. Therefore, H+ ions dissociated from the weak acid, MA, in the aqueous aerosols need to be available for the continuation of the reaction. The inconsistent observations on the hygroscopic behavior by previous studies can be explained better if the real-time monitoring of chemical compositions can be performed during the hygroscopic measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b04356. Literature summary of the hygroscopic properties and chemical reactivity of NaCl−MA mixture aerosol particles (Table S1); and schematic diagram of the in situ RMS measurement system (Figure S1); plots of 2-D area ratio (A/A0) of aerosols with the mixing ratios of NaCl:MSM = 1:1 and MA:MSM = 1:1 (Figure S2); and plots of 2-D area ratio (A/A0) of NaCl, MA, MSM, and DSM aerosols (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 32 860 7676; fax.: +82 32 867 5604; e-mail: curo@ inha.ac.kr (C.-U. Ro). ORCID

Chul-Un Ro: 0000-0001-9357-1854 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Basic Science Research Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2015R1A2A1A09003573).

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DOI: 10.1021/acs.est.6b04356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.est.6b04356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX