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1 Environmental Science Processes & Impacts PAPER View Article Online View Journal View Issue Cite this: Environ. Sci.: Processes Impacts, 2018,20, 1069 Received 5th February 2018 Accepted 27th May 2018 DOI: /c8em00053k rsc.li/espi Environmental signi cance Investigation on the hygroscopicity of oxalic acid and atmospherically relevant oxalate salts under sub- and supersaturated conditions Suresh K. R. Boreddy and Kimitaka Kawamura * Oxalic acid (OxA) is an end product in the oxidation of many organic compounds, and therefore is ubiquitous in the atmosphere and is often the most abundant organic species in ambient aerosols. To better understand the hygroscopic properties of OxA under sub- and supersaturated conditions in the atmosphere, we investigated the hygroscopic growth and cloud condensation nuclei (CCN) activation ability of pure OxA and its salts using a hygroscopic tandem differential mobility analyzer (HTDMA) and cloud condensation nuclei counter (CCNC), respectively. OxA particles absorb water under >45% RH, suggesting that the initial phase state might be an amorphous solid. The measured hygroscopic growth factor (HGF) of OxA at 90% RH was We found that the HGF of ammonium oxalate (NH 4 -Ox) was larger than that of OxA, whereas HGFs of sodium, calcium, and magnesium oxalates (Na-Ox, Ca-Ox, and Mg-Ox) were smaller than that of OxA particles. Potassium oxalate (K-Ox) behaved like a typical watersoluble inorganic salt, exhibiting deliquescence and efflorescence transitions at around 85% and 50% RH, respectively. Na-Ox exhibited strong activation capabilities among all the investigated salts, followed by NH 4 -Ox and K-Ox as inferred from the activation ratios (CCN/CN) against supersaturations (SS). On the other hand, Ca-Ox showed moderate activation ability and Mg-Ox showed poor CCN activation ability. We also observed significantly higher k CCN values compared to k HTDMA for pure OxA and its salts (NH 4 - Ox and Na-Ox), suggesting that the condensation of OxA into the aqueous phase occurs during water uptake. These findings improve the fundamental understanding of hygroscopic behaviors and phase states of oxalic acid and its salts under sub- and supersaturated conditions in the atmosphere and impacts of hygroscopicity on the direct and indirect effects of aerosol particles. Understanding the hygroscopic properties of aerosol particles is very important for assessing their atmospheric impacts (direct and indirect) because hygroscopicity is related to optical properties, such as the scattering and absorption of aerosols, which affects not only the visibility but also the direct radiative forcing. In addition, the presence of hygroscopic electrolytes in ne aerosols may in uence the deposition of particles on biological tissues and posing a toxicity risk to human health. The cloud condensation nuclei (CCN) activation efficiency of atmospheric aerosols is related to the cloud microphysics and precipitation processes and thus affects the indirect radiative forcing, which is highly uncertain and not well understood. Oxalic acid (OxA) is typically the dominant watersoluble organic compound in aerosols and ubiquitous in the atmosphere, and thus, these data in the present study are highly relevant to aerosol cloud interactions in the atmosphere. Our ndings will improve the fundamental understanding of hygroscopic behavior and phase states of oxalic acid and its oxalates under sub- (RH < 100%) and supersaturated (RH > 100%) atmospheric conditions. 1. Introduction Oxalic acid (OxA) is known as the dominant dicarboxylic acid in the atmosphere 1 3 and has attracted great interest due to its potential impact on cloud condensation nuclei (CCN) activities of ambient particles and thus on climate change. 4 8 OxA is a major contributor to the total dicarboxylic acid mass in Institute of Low Temperature Science, Hokkaido University, Sapporo , Japan. boreddy@pop.lowtem.hokudai.ac.jp Present address: Chubu Institute for Advanced Studies, Chubu University, Kasugai , Japan,kkawamura@isc.chubu.ac.jp ambient organic aerosol particles. 1,3,9 12 In addition, mineral dust is o en enriched with oxalic acid OxA is a likely end product of photochemical oxidation of many hydrocarbons in water mediated aerosols. 16 Due to its high oxidation state and strong ability to interact with inorganic elements, its effects on aerosol chemical composition and microphysical properties are unique and complex, whose uncertainty in climate sciences is large and not well understood. Reactions of OxA with monovalent (Na +,NH + 4, and K + ) and divalent (Ca 2+ and Mg 2+ ) cations are ubiquitous and important in atmospheric aerosols. 4 Formation of ammonium and sodium oxalates (NH 4 -Ox and Na-Ox) is o en in urban and marine This journal is The Royal Society of Chemistry 2018 Environ. Sci.: Processes Impacts, 2018,20,

2 aerosols, respectively. 17 Reid et al. 18 reported that ammonium is highly correlated with oxalate and other organic species rather than sulfate in regional hazes dominated by biomass burning smoke in Brazil. 19 Similar results were reported in East African biomass-burning aerosols. 20 Reactions between oxalic acid and metal components are frequently observed in mineral dusts, which may modify the physicochemical properties of aerosols and, hence, affect the cloud formation capabilities. 13,14,21 27 Understanding the hygroscopic properties of aerosol particles is very important for assessing their atmospheric impacts on radiative forcing (direct and indirect) because hygroscopicity is related to the optical properties of aerosol particles via scattering and absorption of radiations. Hygroscopicity affects not only the visibility but also direct radiative forcing. Hygroscopicity also determines the number concentrations of CCN and thus impacts cloud properties such as their lifetime. 28 The CCN activation efficiency of atmospheric aerosols is related to the cloud microphysical and precipitation processes and thus affects the indirect radiative forcing, which is highly uncertain and not well understood because of the complexity and number of atmospheric interactions involved, as well as the wide range of scales on which these interactions occur. 29 Heterogeneous reactions in water mediated aerosols may also depend on the hygroscopicity of particles. 30 In addition, the presence of hygroscopic electrolytes in ne aerosols may in uence the deposition of particles on biological tissues, posing a toxicity risk to human health. 30 The hygroscopic behavior of OxA and other dicarboxylic acids has been investigated widely in the literature using different measurement techniques. 5,6,8,31 40 All these studies have focused on the hygroscopicity of mixtures composed of dicarboxylic acids and ammonium sulfate or sodium chloride or both. A few studies have focused on the hygroscopic behavior of mineral dust 13,25,41 and biomass burning particles. 20,42 44 However, there is no systematic observation on the hygroscopic behaviour of OxA and its salts under subsaturated and supersaturated conditions, particularly for salts such as Na-Ox and K-Ox. The present study addresses the hygroscopic growth and CCN activities of oxalic acid and its mono and divalent salts with various inorganic species, such as Na, NH 4, K, Ca and Mg under subsaturated (relative humidity, RH < 100%) and supersaturated (RH > 100%) conditions using a hygroscopic tandem differential mobility analyzer (HTDMA) and cloud condensation nuclei counter (CCNC), respectively. We discuss the deliquescence RH, efflorescence RH, and critical supersaturations (SS crit ) of all the investigated salts and compare those with the literature studies. We also discuss the atmospheric implications of this study based on our ndings. 2. Experimental section 2.1. Reagents Oxalic acid (OxA, H 2 C 2 O 4 $2H 2 O) and its oxalate salts, such as ammonium oxalate (NH 4 -Ox, (NH 4 ) 2 C 2 O 4 $H 2 O), sodium oxalate (Na-Ox, Na 2 C 2 O 4 ), potassium oxalate (K-Ox, K 2 C 2 O 4 $H 2 O), calcium oxalate (Ca-Ox, CaC 2 O 4 $H 2 O), and magnesium oxalate (Mg-Ox, MgC 2 O 4 $H 2 O), were purchased from Wako Pure Chemical Industries Ltd. Table 1 summarizes the chemical and physical properties of investigated salts at room temperature (298.1 K) Preparation of aqueous solutions Aqueous solutions with 0.1 wt% of OxA and its oxalate salts were prepared in the laboratory by dissolving the relevant reagents in organic-free ultrapure water (resistivity of >18.2 MU cm, Sartorius arium 611 UV) and were used immediately for the experiment. The aerosol particles were generated using an atomizer from aqueous solutions and dried to RH < 5% by owing through a series of diffusion dryers, silica gels and molecular sieves, and were introduced into the HTDMA-CCNC system as shown in Fig Hygroscopic growth factor measurements To measure the hygroscopic growth factors (HGF), a HTDMA system used in this study has been described in our previous studies 20,21 and here is a brief description as follows. The dried (RH < 5%) polydispersed aerosol particles were neutralized using an Am-241 radioactive source and passed into DMA1, to select the desired quasi-monodispersed particles. DMA1 was operated at a constant ow of 0.3 and 3 l min 1 for the polydispersed aerosol and sheath air ( ltered dry air), respectively. A er size selection (100 nm), particles went through a humidi- cation section, which was composed of a Na on tube, to set RH in the range of 5 95% or through a pre-humidi er in dehumidi cation mode, where RH was about 95%. The size distribution of humidi ed particles was measured using DMA2 coupled with a condensation particle counter (CPC1). For both DMAs (DMA1 and DMA2), the sheath-to-aerosol ow ratio was held at 10 : 1 and all experiments were performed at room temperature ( K). Lognormal Gaussian curve tting was used to correct the size distribution of humidi ed particles. 45 The measured HGF is de ned as the ratio of particle diameter at elevated RH (DMA2) to that of the initial dry particle diameter (DMA1) at 5% RH. gðrhþ ¼ DðRHÞ D 0 (1) where g(rh) is the HGF, D 0 is the initial dry particle mobility diameter at an RH of <5% and D(RH) is the diameter at an elevated RH. The HGF of a particle depends on its chemical composition and size (Kelvin effect). The equilibrium RH over the surface of an aqueous droplet is associated with the water activity of the solution (a w ) and the Kelvin factor as described by Köhler theory using the following equation: a w ¼ RH 1 A exp (2) 100 gðrhþd 0 where a w is water activity and g(rh) is the respective growth factor at 90% RH. A is the constant ( m) that describes the Kelvin effect, as de ned below: 1070 Environ. Sci.: Processes Impacts, 2018,20, This journal is The Royal Society of Chemistry 2018

3 Table 1 Chemical properties of the investigated salts at room temperature Chemical compound Molecular formula Molar mass, g mol 1 Density, g cm 3 Solubility, mg ml 1 Purity a (%) Oxalic acid dihydrate (Ox-A) C 2 H 2 O 4 $2H 2 O Ammonium oxalate monohydrate (NH 4 ) 2 C 2 O 4 $H 2 O (NH 4 -Ox) Sodium oxalate (Na-Ox) (COONa) Potassium oxalate monohydrate (KO-x) (COOK) 2 $H 2 O Calcium oxalate monohydrate (Ca-Ox) CaC 2 O 4 $H 2 O Magnesium oxalate Monohydrate (Mg-Ox) MgC 2 O 4 $H 2 O g/100g H 2 O 90 a Manufacturer: WAKO PURE CHEMICAL INDUSTRIES, Ltd. A ¼ 4s s=am w r w RT where s s/a is the surface tension of the solution air interface, M w is the molar mass of water (18.02 g mol 1 ), r w is the density of water (998 kg m 3 ), R is the universal gas constant ( J mol 1 K 1 ), T is the temperature in Kelvin ( K), and D 0 is the droplet diameter (or wet particle diameter). In this study, the effect of solutes on the surface tension of the droplet was not obvious, and thus the surface tension of pure water (0.072 J m 2 ) was used for estimation using eqn (3). Further detailed information on the assumed values of s s /a and T was described elsewhere To simplify the representation of aerosol hygroscopicity in k- Köhler theory, Petter and Kreidenweis 46 proposed the use of a single hygroscopicity parameter, kappa (k), to describe the composition-dependent hygroscopicity. Assuming that the surface tension of the aqueous solution corresponding to particles equilibrated at 90% RH is equal to that of pure water, the water activities can be computed using eqn (2) and then k can be obtained from the measured HGF values using the following equation: gðrhþ 3 1 ð1 a w Þ k HTDMA ¼ (4) a w (3) 2.4. Cloud condensation nuclei activity measurements To measure the activation ratios of number concentration of CCN to that of condensation nuclei (CN) (N CCN /N CN ) of monodispersed aerosols in the supersaturation (SS) range of 0.1 to 1.0, we used a CCNC-CPC system that was operated in parallel to a HTDMA system (see Fig. 2). Particles were generated from aqueous solutions and dried as described above. The CCNC used in this work was a commercial model (CCN-100, Droplet Measurement Technologies) equipped with a single growth column and an optical particle counter (OPC). SS was achieved with a vertical thermal gradient along the constantly wetted column. The CCNC was operated with the principle that the diffusion of heat in the air is slower than the diffusion of water vapour from the walls to the centre of the cylinder, resulting in a quasi-uniform supersaturation. 49 The SS in the CCNC was ramped from 0.1 to 1.0%, while keeping the selected dry size constant. The activated fraction as a function of SS, generally referred to as the S-scan, was tted with a sigmoidal curve in order to determine the critical supersaturation (SS crit ). The SS crit is de ned as the SS at which 50% of the particles with a selected dry diameter are activated into cloud droplets (CCN). Effects of multiple charges were accounted for by taking the 50% cut of the sigmoid representing the singly charged particles. More details on data evaluation and analysis were described elsewhere. 50 Fig. 1 Schematic representation of the experimental setup used in this study. This journal is The Royal Society of Chemistry 2018 Environ. Sci.: Processes Impacts, 2018,20,

4 Fig. 2 Hygroscopic growth factors of (a) oxalic acid and (b f) its salts as a function of relative humidity (RH) in hydration mode. Initial dry particle mobility diameter is 100 nm. Predictions from the ideal solution model are also included (solid red line). The predicted HGF of Na-Ox below 80% RH is not shown in c due to the restructuring (shrinking) of the particles. Being similar to the subsaturated hygroscopicity parameter (k HTDMA ) as mentioned above, the hygroscopicity parameter under supersaturated conditions (k CCNC ) was also calculated from the measured SS crit for the initial dry size studied and is given by, 46 k CCNC ¼ where D 0 is the dry particle diameter. 4A 3 27D 0 3 ln 2 SS crit (5) 2.5. Quality control Ammonium sulfate (AS) particles (99.999% purity, Sigma- Aldrich) were used to calibrate HTDMA and CCNC systems. The measured HGFs at 90% RH for 100 nm particles of AS were compared with modeled HGFs using the aerosol inorganic model (AIM), 51 which showed a good agreement with an uncertainty of <2% in the HGF. The RH sensors of HTDMA were calibrated based on the deliquescence RH (DRH) of AS salt. The measured uncertainty in the DRH of AS salt showed good 1072 Environ. Sci.: Processes Impacts, 2018,20, This journal is The Royal Society of Chemistry 2018

5 agreement with literature studies. 27,45,52 The relative difference in RH at the inlet and outlet of DMA2 was <1%. Therefore, the variability of RH within our system is less than 1%. Correspondingly, this uncertainty results in a relative uncertainty of around 2.5% for HGFs of AS particles measured at 90% RH. The size selection of two DMAs (1 and 2) was adjusted by selecting AS particles with DMA1 and by measuring the mode diameter with DMA2 and the error was less than 1%. Ammonium sulfate particles were introduced into the CCNC to calibrate the SS conditions before and a er the experiment. More information about the calibration of CCNC used in this study was described elsewhere. 50, Ideal solution model The hygroscopic growth of soluble particles as a function of RH is described by the Köhler theory. 54 For an ideal solution, the equilibrium RH above a at surface is equal to the mole fraction of water in the solution (Raoult's law). Previous investigations have suggested that the ideal solution model was the best possible approach to predict the hygroscopic growth of atmospherically relevant organics such as humic-like substances, biomass burning related organics, and oxalic acid and its mixtures. 6,45,55 57 The activity of water (a w ) is given by its mole fraction (X w ) (Raoult's law): X w ¼ n w n w þ n s ¼ a w ¼ X w (6) GF 3 1 GF 3 1 þ M w r w (7) r s M s where n w and n s are the mole numbers (amount of substance) of water and solute in the droplet, respectively. r w and r s are the density of water and solute, respectively. M w and M s are the molar mass of water and solute, respectively. 3. Results and discussion The measured HGF and CCN activation ratios as a function of RH (range: 5 95%) and SS (range: %) for all the investigated salts in this study are shown in Fig. 2 and 3, respectively. The ideal solution model is applied to estimate the hygroscopic growth of all the investigated salts in this study and has also been shown in Fig. 2. As seen from Fig. 2, measured HGF curves of all the investigated salts (except Na-Ox and Mg-Ox) showed good agreement with the predicted curves, indicating that the water activity of particles is almost equal to the mole fraction of water. Therefore, we believe that the ideal solution model is applicable to compare the measured HGF in this study. The measured HGF (at 90% RH) and SS crit values are reported in Table 2 together with the calculated k values under sub- (k HTDMA ) and supersaturated (k CCNC ) conditions. vapor slightly, resulting in a gradual size growth with increasing RH until 80% RH. Over 80% RH, OxA particles absorb more water and become completely deliquescent with an abrupt increase in HGF. The plausible explanation for these observations is a deliquescence transition that involves the formation and swelling of gel-like structures in the highly concentrated aqueous OxA droplets, which is consistent with the tendency of OxA to form hydrogen bonds and hydrates. 58,59 Similarly, Prenni et al. 8 and Mikhailov et al. 56 reported the substantial water uptake of OxA particles above 40% RH in the HTDMA measurements and also explained that the initial phase state of OxA particles in their study was an amorphous solid, thus resulting in a continuous water uptake. 56 The phase transition from anhydrous (<5% RH) to dehydrate form (>11% RH) of OxA particles during hydration has been found in the study of Baxter and Lansing. 60 In this study, the measured HGF of OxA at 90% RH was 1.47 during a hydration experiment and consistent with previous studies. 8,56 We also found good agreement between the measured and modeled hygroscopic behavior of OxA (Fig. 2a). However, recently, Jing et al. 6 reported no signi cant size growth or deliquescent behavior for OxA particles in the range of 5 90% RH in the HTDMA studies. A similar deliquescence behavior of OxA particles has been reported in many previous studies using EDB and other techniques. 31,38,61,62 Further, thermodynamic models and other bulk-solution measurements also predicted that the deliquescence RH of OxA particles was greater than 97%. 31,61,63 The plausible reason for this inconsistency is that the initial phase state of generated OxA particles may be crystalline in their studies because of dry conditions and electrical charge effects, 56 which cannot account for a signi cant water uptake. Therefore, the phase state (crystalline or amorphous) of particles may strongly in uence the HGF of aerosol particles. 36 On the other hand, we couldn't nd evaporation losses (no difference between the dry particle size selected by DMA1 and that measured by DMA2) for OxA, although it has a high vapor pressure ( mmhg, 25 C, 64 ), and this point is consistent with previous HTDMA studies. 6,8,31,38,56 This result suggests that the generated particles a er the drying process exist as stable anhydrous OxA with low vapor pressure in this study. The CCN activity presented as CCN/CN ratios of particles that activate as cloud droplets as a function of SS. Fig. 3a shows the activation behavior of generated OxA particles with a sigmoidal t to the data shown as a solid line. OxA particles exhibit a good CCN activation ability with broad CCN spectra where the value of measured SS crit is about 0.13% (Fig. 3a). The measured SS crit of OxA in the present study is lower than theoretical SS (0.20%) calculated using Köhler theory, 65 and the literature values 0.176% (ref. 66) and 0.44%. 8 The inconsistency of SS crit among these studies might be due to the initial phase state (amorphous solid or crystalline), shape, solubility, volatility and other factors, which may control the CCN activity of OxA particles OxA As seen in Fig. 2a, the HGF of OxA showed no obvious change below 45% RH, a er which the particles start to take water 3.2. NH 4 -Ox and Na-Ox Being similar to OxA, the HGF of NH 4 -Ox showed no particle growth below 45% RH (Fig. 2b), which is consistent with the This journal is The Royal Society of Chemistry 2018 Environ. Sci.: Processes Impacts, 2018,20,

6 Fig. 3 Cloud condensation nuclei activation capabilities of (a) oxalic acid and oxalate salts (b f) as a function of supersaturations (SS). Initial dry particle mobility diameter is 100 nm. Critical supersaturation (SS crit ) value of each salt was also included. previous study by Wu et al., 67 who reported that no hygroscopic growth was observed below 30% RH for diammonium sulfate particles using a HTDMA. We found a signi cant growth of NH 4 -Ox particles above 72% RH with a continuous increase of RH. The observed HGF of NH 4 -Ox at 90% RH was 1.57 during the hydration experiment. At 90% RH, the measured growth factor of NH 4 -Ox is higher than that of OxA but lower than that of ammonium sulfate particles. In contrast, the HGF was constant and about 1.05 a er 30% RH for diammonium oxalate particles. 67 Similarly, using a HTDMA, Mensah et al. 68 reported that NH 4 -Ox didn't take up water below 90% RH and they also showed that the DRH of NH 4 -Ox was about 93%, although they didn't nd efflorescence RH. On the other hand, the experimental SS for NH 4 -Ox is low, exhibiting good CCN activation ability with an SS crit of 0.152%, as shown in Fig. 3b. This observation is consistent with the study of Hori et al., 69 who 1074 Environ. Sci.: Processes Impacts, 2018,20, This journal is The Royal Society of Chemistry 2018

7 Table 2 Hygroscopic growth factor (HGF), critical supersaturations (SS crit ) and hygroscopicity parameter (k) for the investigated salts Salt Measured parameters Hygroscopicity parameter HGF at 90% RH SS crit (%) k HTDMA k CCN OxA NH 4 -Ox Na-Ox K-Ox Ca-Ox Mg-Ox observed quite lower SS and higher CCN activation capability for NH 4 -Ox particles. Na-Ox has a low solubility of only mol kg 1 at 25 C. 70 The growth factor curve showed that no growth was observed below 40% RH (Fig. 2c). Above this RH, the HGF increased very slightly with increasing RH. The observed HGF of Na-Ox particles at 90% RH was 1.10 during the hydration experiment. The slight increase in the HGF suggests weak water absorption on the surface of Na-Ox particles. Similarly, Peng and Chan 71 observed no hygroscopic growth below 93% RH using the EDB technique, while Mensah et al. 68 reported that Na-Ox particles start to take up water vapor above 93% RH using a HTDMA. Similarly, Wu et al. 67 observed no hygroscopic growth below 90% RH. On the other hand, Na-Ox showed a strong CCN activation ability with an SS crit of 0.140% as shown in Fig. 3c. It is of interest to note that Na-Ox particles with such a lower solubility could show high CCN activation ability among all the oxalates. Solubility is one of the key properties governing the hygroscopic growth and CCN activation of aerosol particles as described by the Köhler theory. 72,73 Together with aqueous-phase activity coefficients, surface tension, density and dry mass of the particle, water solubility affects the aerosol particle water content in thermodynamic equilibrium However, this is not the case for Na-Ox particles and this interesting feature can be explained by the following hypothesis. It is well known that when the air is cooled down to the dew point and the RH increases to 100%, water vapor condenses on small atmospheric particles, called as condensation nuclei. Above 100% RH, water vapor molecules condense around the CCN particle to form a water droplet. There are two types of cloud condensation nuclei or condensation nuclei: hygroscopic and hydrophobic. The hygroscopic CCN allows water uptake below 100% RH (e.g., ammonium sulfate), whereas condensation will only occur around hydrophobic nuclei when RH is above 100% (e.g., dust particles). In this study, Na-Ox particles may have shown hydrophobic CCN activity. It is traditionally documented that high solubility particles show quite lower SS (or strong CCN activity) in the atmosphere, however, it is not true in the case of Na-Ox particles. This hypothesis is further supported by the study of Ovadnevaite et al., 77 who studied the physicochemical properties of primary marine organic seaspray aerosols using a HTDMA-CCNC system. They observed an apparent dichotomous behavior in terms of water uptake with low hygroscopic and high CCN activity for sea-spray aerosols enriched with the primary organic matter under clean marine conditions, and hypothesized that marine hydrogels during high biological activity are responsible for this dichotomous behavior. However, they didn't discuss Na-Ox particles, although the formation of Na-Ox has o en occurred in the marine atmosphere. 17 The possibility of non-spherical particles is an important consideration for the salts studied in the present study. Using an optical microscope, Hori et al. 69 observed a very ne needleshaped structure for NH 4 -Ox under 30% RH. Estillore et al. 78 observed the linking between hygroscopicity and the surface microstructure of inorganic salts and sea-spray aerosols using atomic force microscopy (AFM) with hygroscopic growth measurements in order to provide important insights into the particle hygroscopicity and surface microstructure. They observed that when NaCl (or sea-spray aerosols) is mixed with the organics, the particles adopt a core shell morphology with a cubic NaCl core and the organics forming a shell. The observation of such core shell morphologies is found to be highly variable depending on the salt to organic ratio and on the nature and solubility of organic components. 78 However, we didn't observe any restructuring (particle shrinking) for the salts studied, although the modeled growth factor of Na-Ox (Fig. 2c) showed signi cant reduction below 80% RH (Fig. 2c), suggesting that generated solution droplets from the salts may be spherical. Schroeder and Beyer 79 determined the deliquescence relative humidities (DRH) for several atmospherically relevant salts including NH 4 -Ox, Na-Ox, etc., as a function of temperature using humidity controlled thermogravimetric analysis (HTGA). They reported that the average DRH of NH 4 -Ox and Na-Ox was >95% and 75%, respectively, and was constant with temperature varying from 288 to 308 K. However, we did not observe DRH for Na-Ox particles although NH 4 -Ox showed deliquesce over 70% RH in this study as explained above K-Ox The hygroscopic behavior of K-Ox is similar to that of inorganic salts with strong deliquescence and efflorescence transitions. The hygroscopic growth curve showed no obvious change below 85% RH (Fig. 2d), indicating that initial particles are fully dried and there is no evaporation loss. We found an obvious deliquescence transition for K-Ox particles around 86% RH, which was higher than those of ammonium sulfate (80%), sodium chloride (76%), sodium sulfate (84%), ammonium nitrate (62%), ammonium chloride (78%), and calcium nitrate (47%) but lower than those of potassium sulfate (97%) and potassium nitrate (90%) However, the DRH of K-Ox particles may be comparable to that of potassium chloride (84%) salts. 83 On the other hand, K-Ox showed an efflorescence point around 56% RH during the dehydration experiment (not shown as a gure). Mensah et al. 68 reported that the DRH and ERH of K-Ox particles are and , respectively, supporting our results on the deliquescence of K- This journal is The Royal Society of Chemistry 2018 Environ. Sci.: Processes Impacts, 2018,20,

8 Ox particles. The observed HGF of K-Ox particle at 90% RH was about 1.85, which is higher than that of ammonium sulfate particles (1.67). The CCN activation of K-Ox particles occurred at lower SS levels with an SS crit of 0.145% (Fig. 3d) Ca-Ox and Mg-Ox It is well known that metal oxalates such as Ca-Ox and Mg-Ox are hardly soluble in water, whose water solubilities are very poor at room temperature with values of mg ml 1 and g/100 g H 2 O, respectively. The measured HGF of Ca-Ox at 90% was 1.44, whereas CCN activation occurred at lower SS with SS crit about 0.144% (Fig. 3e). The particles may contain impurities from the stock solution (purity level 95%), which might be the cause of the enhancement of the hygroscopic growth of Ca- Ox particles. On the other hand, Mg-Ox particles showed a low hygroscopic growth factor (1.15) at 90% RH and a poor CCN activation ability (Fig. 3f) due to poor solubility nature. As seen from Fig. 2f, the measured HGF curve starts to deviate from the theoretical prediction at 60% and higher RH although this is a good overlapping below 60% RH. This substantial deviation between predicted and measured HGFs can be attributed to the mass transfer limitation in the aerosol particles. 84 Aerosol acidity or ph is an important characteristic of the aqueous phase of aerosols, affecting the heterogeneous chemistry in the atmosphere that has a profound impact on cloud formation, atmospheric composition, etc For example, biomass burning aerosol presented the highest values of ph in the submicron fraction and the lowest values in total water mass concentration 86 or CCN activity or hygroscopic growth factors. 88 However, we did not focus on ph measurements of investigated salts due to the unavailability of spectroscopic instruments in this study Hygroscopicity (k) under subsaturated and supersaturated conditions The hygroscopicity parameters (k) derived from HTDMA (subsaturated, k HTDMA ) and CCNC (supersaturated, k CCN ) measurements for all the investigated salts in this study are reported in Table 2. We found that OxA, NH-Ox, Na-Ox, and Ca- Ox salts showed higher k CCN and their abundances were 3.4, 1.8, 17, and 3.0 times those of k HTDMA, respectively, while K-Ox and Mg-Ox salts showed relatively similar k CCN values to k HTDMA probably due to the variations in the equilibrium vapor pressure of oxalic acid in the above-mentioned salts. Condensation of oxalic acid into the aqueous phase during water uptake might in uence these differences in kappa values. For example, pure OxA has a high vapor pressure ( ) 89 and thus there is a large pool of oxalic acid vapors in the gas phase. The effective equilibrium vapor pressure of OxA drops as the particles take up water, which drives OxA from vapor phase to particle phase. 75,90 This partitioning to the particle phase with increasing water enhances the apparent hygroscopicity and kappa values derived from HTDMA and CCNC measurements because there is much more solute in the wet particle relative to the dry particle. In fact, it might be expected that CCNC allows more added oxalic acid due to partition into the activating droplet. Thus, the results show a signi cantly higher k CCN than k HTDMA values than those for OxA, NH 4 -Ox, Na-Ox, and Ca-Ox salts. Therefore, condensation of oxalic acid into the aqueous phase during water uptake may be the potentially important factor that may in uence the hygroscopicity and kappa values under subsaturated and supersaturated conditions, respectively, in the atmosphere. 4. Atmospheric implications and conclusion In this study, we investigated the hygroscopic growth and CCN activation ability of oxalic acid and oxalate salts using HTDMA and CCNC systems, respectively. Hygroscopic properties of OxA showed continuous water uptake behavior a er 40% RH, suggesting that the initial phase state might be an amorphous solid in the present study. The phase state of particles is an important factor in determining the particle's physicochemical properties and climate effects. 91 Due to its signi cance, the comprehensive understanding of the particle phase in aerosols with varying components could improve the predictive ability of climate models Na-Ox particles exhibit high CCN activation ability, although they have a lower solubility or HGF under subsaturated conditions, implying that the water uptake properties of Na-Ox particles under subsaturated and supersaturated conditions may be different. This dichotomous behavior of Na-Ox should be considered in the thermodynamic modeling calculations in order to reduce uncertainty in climate forcing calculations. K-Ox particles showed strong deliquescence and efflorescence transitions around 85% and 56% RH, respectively. The CCN activation of K-Ox particles occurs at lower SS. On the other hand, the water-soluble portion of Ca-Ox particles showed moderate CCN activation abilities due to impurities existing in the stock solution while Mg-Ox showed poor hygroscopic growth and CCN activation ability. All these results are important because oxalic acid is a signi cant contributor to atmospheric organic aerosols. 2 Our ndings will improve the fundamental understanding of hygroscopic (CCN activity) behaviors and phase states of oxalic acid and its salts under sub- and supersaturated atmospheric conditions and impacts of hygroscopicity on the direct and indirect effects of aerosol particles. Conflicts of interest There are no con icts to declare. Acknowledgements This study was in part supported by a grant-in-aid ( ) from a Japan Society for the Promotion of Science (JSPS). We appreciate the nancial support of a JSPS fellowship to SKR Boreddy (ID No. PU16905). The authors would like to thank Dr Mochida for his support for the assembly of the HTDMA system and Dr Kawana for the CCN data analysis in this study Environ. Sci.: Processes Impacts, 2018,20, This journal is The Royal Society of Chemistry 2018

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