Effects of Relative Humidity and Particle Phase Water on the

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Effects of Relative Humidity and Particle Phase Water on the Heterogeneous OH Oxidation of 2-Methylglutaric Acid Aqueous Droplets Man Mei Chim, Chun Yin Chow, James F. Davies, and ManNin Chan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11606 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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The Journal of Physical Chemistry

Effects of Relative Humidity and Particle Phase Water on the Heterogeneous OH Oxidation of 2-Methylglutaric Acid Aqueous Droplets Man Mei Chim a, Chun Yin Chow a, James F. Davies b, Man Nin Chan a, c* a

Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong,

Hong Kong, China b

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

c

The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong,

Hong Kong, China *To whom correspondence should be addressed. M.N. Chan - Tel: (852) 3943-9863 E-mail: [email protected]

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ACS Paragon Plus Environment


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Abstract Organic aerosols can exist as aqueous droplets, with variable water content depending on their composition and environmental conditions (e.g. relative humidity (RH)). Recent laboratory studies have revealed that oxidation kinetics in highly concentrated droplets can be much slower from those in dilute solutions. However, it remains unclear whether aerosol phase water affects the formation of reaction products physically and/or chemically. In this work, we investigate the role of aerosol phase water on the heterogeneous chemistry of aqueous organic droplets consisting of 2-methylglutaric acid (2-MGA), measuring the reaction kinetics and the reaction products upon heterogeneous OH oxidation over a range of RH. An atmospheric pressure soft ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer is used to obtain real-time molecular information of the reaction products. Aerosol mass spectra show that the same reaction products are formed at all measured RH. At a given reaction extent of the parent 2-MGA, the aerosol composition is independent of RH. These results suggest the aerosol phase water does not alter reaction mechanisms significantly. Kinetic measurements find that the effective OH uptake coefficient, γeff, decreases with decreasing RH below 72%. Isotopic exchange measurements performed using an aerosol optical tweezers reveal water diffusion coefficients in the 2-MGA droplets to be 3.0 × 10-13 to 8.0 × 10-13 m2 s-1 over the RH range of 47% to 58%. These values are comparable to that of other viscous organic aerosols (e.g. citric acid), indicating that 2-MGA droplets are likely to be viscous at low humidity. Smaller γeff at low RH is likely attributed to the slower diffusion of reactants within the droplets. Taken together, the observed relationship between the γeff and RH are likely attributed to changes in aerosol viscosity rather than changes in reaction mechanisms.

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1. Introduction Organic compounds contribute a significant mass fraction (about 50%) of atmospheric aerosols1-3 and are subjected to oxidation initiated at the aerosol surface by gas-phase oxidants such as hydroxyl (OH) radicals, ozone (O3), and nitrate radicals.4-6 These heterogeneous oxidation processes have been found to continuously alter the composition and the properties (e.g. light absorption, hygroscopicity, and cloud condensation nuclei activity) of aerosols.7-13 Recent laboratory studies have shown that the physical state of organic aerosols plays an important role in the heterogeneous reactivity.14-16 Depending on the composition and environmental conditions such as temperature and relative humidity (RH), an aerosol can exist in subsaturated or supersaturated states based on the availability of water. For example, an aerosol that initially consists of aqueous droplets at high RH will lose water by evaporation when the RH decreases. Due to the lack of heterogeneous nucleation sites, crystallization typically does not occur at the deliquescence point, leading to droplets that are supersaturated in solute with respect to bulk solubility limits. At a sufficiently large supersaturated concentration, efflorescence is usually observed. This hysteresis phenomenon is commonly observed for many organic compounds (e.g. oxalic acid, succinic acid and glutaric acid).17-19 Alternatively, some organic aerosols (e.g. levoglucosan, malonic acid, tartaric acid, and citric acid) do not exhibit distinct crystallization or deliquescence, instead forming amorphous states consisting of small quantities of water at very low RH.20, 21 An understanding of the hygroscopicity and humidity history of aerosols is thus essential to interpret the effect of RH and particle phase water on their heterogeneous reactivity. While aqueous droplets are present in the atmosphere,22, 23 laboratory studies have shown that oxidative kinetics observed in concentrated droplets can differ significantly from those in 3



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dilute droplets during oxidation.24-28 For example, Davies and Wilson25 have observed that the effective uptake coefficient, γeff, in citric acid aerosol decreases from 0.157 to 0.052 when the RH drops from 60% to 20%. These observations revealed that oxidation was limited to a region close to the surface of the particles due to slow diffusion of condensed phase reactant molecules at low RH. Under these low RH conditions, the diffusion coefficient of water molecules in the bulk is estimated to be on the order of 10−13 to 10−14 m2 s−1, while the diffusion coefficient of citric acid molecules is expected to be significantly smaller due to its greater molecular size.29 While the RH can control the reactivity by regulating the aerosol viscosity23, it can also influence the particle phase water content and the concentration of the reactants, which may lead to alternativee chemical reaction pathways. Previous studies have primarily focused on measuring the decay of parent molecules in order to explore the reaction kinetics. Obtaining molecular information about reaction products can provide more insights into how the RH affects heterogeneous reactivity either physically and/or chemically. In this work, we investigate the effects of the RH on the molecular transformation of aqueous organic droplets upon heterogeneous OH oxidation. The OH-initiated oxidation of aqueous 2-MGA droplets is carried out using an aerosol flow tube reactor over a range of RH. The composition of the aerosols before and after oxidation is obtained by a atmospheric pressure soft ionization source (Direct Analysis in Real Time, DART) coupled with a high resolution mass spectrometer. Additionally, to obtain information about how the aerosol viscosity affects the diffusion of molecules in the bulk and the heterogeneous reactivity, the hygroscopicity of 2-MGA aerosols and water diffusion coefficients in 2-MGA droplets prior to oxidation are measured using aerosol optical tweezers and an isotope exchange measurement (D2O/H2O). The effects of RH and viscosity on the decay of 2-MGA and the kinetic evolution of reaction products are examined based on the aerosol mass

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spectra, hygroscopic data, and water diffusion coefficients. 2-MGA is one of the most abundant branched dicarboxylic acids detected in atmospheric aerosols30-32 and is chosen here as a model compound to investigate the heterogeneous OH reaction mechanisms of small branched dicarboxylic acids (Table 1).

2. Experimental approach 2.1 Heterogeneous oxidation The heterogeneous OH oxidation of 2-MGA aerosols was carried out in an aerosol flow tube reactor over a range of RH at 20oC. The experimental methods have been described in detail previously.33,

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Briefly, aqueous droplets were generated by atomizing an aqueous 2-MGA

solution using a constant output atomizer. Before introduction into the reactor, poly-disperse aqueous droplets were mixed with humidified nitrogen (N2), oxygen (O2), ozone (O3), and hexane. Inside the reactor, 2-MGA aerosols were oxidized by gas-phase OH radicals, generated by the photolysis of O3 under ultraviolet light at 254 nm. The OH concentration was regulated by changing the O3 concentration and was determined by measuring the decay of hexane, a gas phase OH tracer, using gas chromatography coupled with a flame ionization detector.35 The OH exposure, defined as the product of OH concentration and the aerosol residence time, t, is determined by measuring the concentration of the hexane before and after OH reaction:

OH exposure = −

= OHdt

(Eqn. 1)

where [Hex]0 is the initial hexane concentration (~100 ppb) entering the reactor, [Hex] is the hexane concentration leaving the reactor, and kHex is the OH reaction rate coefficient with hexane (5.2×10−12 cm3 molecule−1 s−1). The aerosol residence time was determined to be 1.3 minutes, 5



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and the OH exposure was varied from 0 to 2.73×1011 to 3.20×1011 molecule cm−3 s over the RH range. The aerosol stream leaving the reactor was passed through an annular Carulite catalyst denuder and an activated charcoal denuder to remove O3 and gas-phase species from the aerosol stream, respectively. A portion of the aerosol stream was sampled by a scanning mobility particle sizer (SMPS) to measure the aerosol size distribution. The remaining flow was directed into a stainless steel tube heater at 250oC, where the aerosol was vaporized. The resultant gas-phase species were directed into the atmospheric pressure ionization region, an open space between the DART ionization source (IonSense: DART SVP) and the inlet of the high resolution mass spectrometer (ThermoFisher, Q Exactive Orbitrap), for real-time chemical characterization. The DART ionization source was operated in negative ionization mode with helium (He) as an ionizing gas. In the ionization region, the electrons produced through Penning ionization of the metastable He atom in DART ionization source are captured by atmospheric O2 molecules to form anionic oxygen (O2−) ions (Eqn. 2). The O2− ions react with the gas-phase molecules (M) by proton abstraction to form the deprotonated molecular ion, [M−H]− (Eqn. 3). 36, 37 + →

(Eqn. 2)

+ → − +

(Eqn. 3)

Previous studies have shown that the acidic proton of the carboxylic acid group can be abstracted by the O2− ions to generate the [M−H]−, which is the dominant ion observed in the mass spectra.16, 38, 39 Proton abstraction from the carboxyl group of 2-MGA and its reaction products is likely to occur to produce the [M−H]−. These ions were sampled by the high resolution mass spectrometer. Mass spectra were collected over a scan range from m/z 70 – 700. Each mass spectrum was averaged for 5 minutes at a mass resolution of about 140,000. Mass calibration

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was performed with standard solutions before experiments. The mass spectra were analyzed using the Xcalibur software (Xcalibur Software, Inc., Herndon, VA, USA). 2.2 Hygroscopicity Measurements The hygroscopicity of 2-MGA aerosols was measured using an aerosol optical tweezers (Biral AOT 100). First, a single micron-sized droplet (~5 µm in diameter) was held in an optical trap and exposed to a controlled humidity ranging from 20% to 82% RH. The droplet size and refractive index response were monitored using the wavelength position of cavity enhanced resonances in the Raman spectrum and the sizing algorithms of Preston and Reid40. The refractive indices are reported in the Supporting Information. A calibration correlating mass fraction of solute (mfs) to the refractive index was performed offline using bulk solutions and a digital refractometer (Atago PAL-RI). The refractive index determined in the AOT was corrected to the wavelength of the calibration, using the dispersion parameters reported in the sizing algorithm, and mfs was inferred at each measurement RH. 2.3 Water Diffusion Coefficient Measurements Isotope exchange measurements (D2O/H2O), following the procedure described by Davies and Wilson29, were performed on aqueous 2-MGA droplets to quantify the diffusion rate of water molecules within the droplet. Briefly, a single-micron sized droplet was held in the aerosol optical tweezers (Biral AOT 100) and equilibrated at a known RH with H2O at 20oC. The water source generating the RH was swapped for D2O, and a flow maintained at the same RH was introduced into the chamber. The exchange of D2O for H2O was monitored using Raman spectroscopy and the water diffusion coefficient was inferred from the exchange timescale using Fick’s second law.

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3. Results and Discussions 3.1 Hygroscopicity of 2-MGA Aerosols Figure 1 shows the hygroscopicity of 2-MGA aerosols (mfs as a function of RH). When the RH increases or decreases, 2-MGA aerosols absorb or desorb water reversibly. The droplets remain spherical across the experimental RH range (>20% RH), indicating that crystallization does not occur. At low RH, the mfs is close to one, suggesting that 2-MGA aerosols likely exist in an anhydrous state. The experimental data is fit to an empirical parameterization of water activity, aw (=RH/100) based on an offset stretched exponential function: !"# = −$ exp(100 × ,- . + 1 + $

(Eqn. 4)

with parameters A = 0.624, B = 4.054, and k = 0.592. Thus, based on the hygroscopic data, 2MGA aerosols are likely aqueous droplets prior to oxidation at all measured RH. 3.2 Aerosol Mass Spectra at Different RH Figure 2 shows the aerosol mass spectra obtained at three different RH (i.e. RH = 53.0%, 72.0%, and 82.5%). The same reaction products are observed at all measured RH, with insignificant changes between the data at different RH. For instance, before oxidation, the single dominant peak is the deprotonated molecular ion of 2-MGA. After oxidation, a few major product peaks evolve, corresponding to two functionalization products (C6H10O5 and C6H8O5) and three C5 fragmentation products (C5H8O2, C5H8O3, and C5H8O4). Some minor product peaks are also observed, and each of these peaks contributes less than 3% of the total ion signal. At the maximum OH exposure, the contribution of all minor products is less than 18% of the total ion signal for all measured RH. It is noted that to quantify the aerosol composition from the analysis of DART mass spectra, a correction is required for the ionization efficiency of the parent 2-MGA

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and reaction products. While standards are not available for many reaction products, as a first approximation, we assume that the ionization efficiencies of 2-MGA and the reaction products are the same. We also acknowledge that some peroxides (e.g. organic peroxides and oligomers) could possibly be formed by reactions between peroxy radicals and/or hydroperoxy radicals. These peroxides may thermally decompose in the DART ionization source and may not be detected efficiently in the mass spectra generated by the DART. We do not find evidence in the mass spectra of fragment ions originated from the thermal decomposition of these peroxides. In the following sections, the effect of RH and viscosity on the oxidation kinetics and the formation of reaction products will be discussed. Reaction mechanisms are proposed to explain the formation of major reaction products detected in the mass spectra and are given in the Supporting Information. 3.2.1 Effect of RH on Oxidation Kinetics Figure 3A shows the normalized decay of 2-MGA as a function of OH exposure at different RH’s. It can be seen that 2-MGA decays faster at higher RH. For example, at a very similar maximum OH exposure (2.73×1011 – 3.20×1011 molecule cm-3 s), ~ 75% of 2-MGA is oxidized at the highest RH (82.5% RH), while only ~ 60% is reacted away at the lowest RH (33.8% RH). The OH-initiated decay of 2-MGA exhibits an exponential trend at all measured RH, and can be fit with an exponential function to obtain a second order reaction rate constant (k2-MGA): /0

1

1

= −(234 OH5

(Eqn. 5)

where I is the ion signal of 2-MGA at a given OH exposure, Io is the ion signal of 2-MGA before oxidation. [OH] is the concentration of gas-phase OH radicals and t is the reaction time. As shown in Figure 3B, the k2-MGA increases from 3.01×10–12 to 5.42×10–12 cm3 molecule–1 s–1 when

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the RH increases from 33.8% to 82.5% (Table 2). Using the fitted k2-MGA, the effective OH uptake coefficient, γeff, defined as the fraction of OH collisions that yields a reaction of a 2-MGA molecule, is computed 25: γ

677

=

8 9 mfs :; AAAAAA < 2= > ?@

(234

(Eqn. 6)

where D0, mfs, and ρ are the mean surface weighted diameter, the mass fraction of solute, and the density of the droplets before oxidation, respectively. Mw is the molecular weight of 2-MGA, NA is Avogadro’s number, and BAAAAA CD is the mean speed of gas-phase OH radical. Prior to oxidation, the droplet diameters are measured to be approximately 112.1 nm to 118.7 nm over the RH range. The composition of the droplets (i.e. mfs) at different RH is determined from the hygroscopicity measurements reported in Section 3.1 and Figure 1. The density of the 2-MGA droplets over the RH range investigated is not known. The droplet density is estimated using the volume additivity rule with a known density of water and 2-MGA (1.246 g cm-3) with an uncertainty of 20-30%. As shown in Figure 3C, γeff increases from 1.93 to 2.63 when the RH increases from 33.8% to 72.0%. A slight decrease in γeff is observed at the highest RH (82.5%). This observation could be explained by that when the RH increases, the droplets become more dilute and less viscous. 2MGA molecules diffuse more rapidly from the bulk of the droplets to the aerosol surface for oxidation, leading to higher overall reaction rates.24,

25, 28, 41, 42

At high RH above 72%, the

droplets are dilute well-mixed within the timescale of the oxidation reaction, and the reaction is no longer limited by the diffusion of the species within the bulk.43-45 As there are more water molecules near the aerosol surface under the high RH condition than under low RH condition, the OH radicals may have a lower probability to collide with unreacted 2-MGA molecules at the surface.25 This dilution effect may explain the slight decrease in γeff observed at 82.5% RH. It is also noted that the γeff is observed to be larger than unity, suggesting secondary chemistry. One 10


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likely explanation is the alkoxy radicals, resulted from the reactions of two peroxy radicals, can abstract hydrogen atoms from neighboring molecules.

3.2.2 Effects of RH on Water Diffusion Coefficients in 2-MGA droplets To gain more insight into how the aerosol viscosity, and thus molecular diffusion, affects the reactivity observed in Figure 3C, the water diffusion coefficient, Dw, in 2-MGA droplet before oxidation was measured using an aerosol optical tweezers with D2O/H2O isotope exchange. Due to the very low hygroscopicity of 2-MGA (Figure 1), measurements below 47% RH were not possible due to the negligibly small integrated intensity of the OH band from water molecules. Furthermore, measurements above 58% RH yield isotope exchange timescales on the same order of the instrument response. Thus, Dw within the RH range of approximately 47% 58% are reported. As shown in Figure 4, the measured values suggest a small decrease in Dw from 8.0 ×10–13 m2 s–1 to 3.0 × 10–13 m2 s–1 when RH decreases from 58% to 47%. These values are smaller than those of citric acid solution droplets ( Dw = 10–11 – 10–12 m2 s–1) by about one to two orders of magnitudes over the same RH range.29 For the OH oxidation of citric acid aerosols, the effects of diffusion on reaction rates were only observed below 50% RH, where Dw was smaller than 5 ×10–12 m2 s–1.25 Dw represents an upper limit of the diffusion coefficients expected for the larger organic molecules (e.g. 2-MGA and other reaction products), and the small Dw values measured at low humidity suggest that 2-MGA droplets are likely to be viscous. These data suggest that the decrease in γeff at low RH, shown in Figure 3C, may be rationalized by a decrease in the rate of molecular diffusion of species in the bulk. While Dw data over the whole RH range are not accessible to this technique, in order to extrapolate the measurements across the RH range investigated in this work, a simple Vignes fit, 11



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which is commonly used to parametrize the relationship between composition and diffusion coefficient, is applied 29, 46: J E-,GHI = E-,-

H=

J E-,KLM

NH=

(Eqn. 7)

J J where E-,and E-,KLM are diffusion coefficients of water molecules in pure water (2×10–9 m2 s–1)

and organic solution, respectively, and xw is the mole fraction of water obtained from the J hygroscopic growth measurements (Figure 1). E-,KLM was varied to achieve a best fit in the

dependence of ln(Dw) versus xw, yielding a value of 8.51 × 10–14 m2 s–1. Using the Vignes fit, the dependence of Dw on RH and aerosol composition is established, suggesting that the 2-MGA droplets become more viscous as the RH decreases. At the lowest RH, the Dw is estimated to be 1.44 ×10–13 m2 s–1. At the highest RH, the Dw increases to 9.03 ×10–11 m2 s–1. This supports the hypothesis that diffusion limitations do not affect the overall reactivity because of rapid water diffusion within the droplets at high RH; and changes in γeff are likely determined by the surface composition of the droplets. It is important to address that while the focus of this discussion is the diffusion of water molecules, the diffusion of the organic components is the key factor in regulating chemistry in viscous media. A thorough analysis of the transport properties of 2-MGA and reaction products in the 2-MGA + OH system is needed; however, there exists no robust experimental methods to access these data. To gain more insight into the kinetic limitations of 2-MGA molecules, a characteristic timescale analysis is performed to examine whether the observed decrease in γeff at low RH is due to the slow diffusion of 2-MGA. The timescale for diffusive mixing of the 2MGA throughout a spherical droplet can be estimated 47, 48:

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O8 = R

PQ

Q QSTU; V

Formatted: Font: (Default) Times New Roman, 12 pt, Font color: Text 1

(Eqn. 8)

where r is the radius of the droplet and D2-MGA is the diffusion coefficient of the 2-MGA in the droplet. Using the measured water diffusion coefficients, Dw as a proxy for 2-MGA, the diffusive mixing timescale is very small. It decreases from 2.69 ms to 3.69 µs when the RH increases from 33.8% to 82.5%. However, slow diffusion of the reactants over the timescale of the reaction will lead to inhomogeneity of the particles, and thus changes in the kinetics relative to the assumption of well-mixed behavior. We can estimate the time between reaction events from the collision frequency of OH: W>KXX =

B Y$ ≈ 450 # N 4

(Eqn. 9)

where B̅ is the mean speed of OH molecules, with concentration [OH], and A is the particle surface area. This gives a time of approximately 0.002 s between reactive collision events for the mean size and OH concentration in this study (assuming unit reactive uptake coefficient for OH). This is on the order of the mixing timescales based on the water diffusion coefficient at the lowest RH, and indicates than even at higher RH’s, the assumed slower diffusion of 2-MGA will lead to incomplete mixing on the reaction timescales. This explanation is consistent with the observed decrease in the uptake coefficient, indicating slower kinetics. However, a molecular description of the role of limited diffusion on uptake kinetics is not available, and further quantitative analysis is not possible.

Additionally, the changes in aerosol viscosity and hygroscopicity following oxidation are not known. In the above analysis, it is assumed that the changes in the aerosol water content and

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the molecular distribution of reaction products upon oxidation do not significantly alter the aerosol viscosity.

3.2.3 Effects of RH on the Formation of Reaction Products Figure 5 shows the evolution of major reaction products as a function of RH (The kinetic evolution of minor reaction products at different RH is shown in the Supporting Material). At the same OH exposure, the abundance of major reaction products increases when the RH increases. This can be explained by the faster rate of decay of 2-MGA at higher RH, yielding more reaction products. Since the reaction rates vary with RH (Figure 3), different masses of 2-MGA are oxidized at the same OH exposure. To better understand the effect of RH on the formation of reaction products, it is more useful to examine the aerosol speciation data at the same oxidation lifetime, which is the product of the heterogeneous OH reaction rate constant and the OH exposure. At the same oxidation lifetime, equal mass of 2-MGA are reacted away. As shown in Figure 6, when the abundance of major reaction products is plotted against the oxidation lifetime, the data collected at different RH overlap with each other at the same oxidation lifetime. At a given reaction extent of the parent 2-MGA, the aerosol composition is independent of RH, with the same result also observed for minor reaction products (Supporting Material). Moreover, the same reaction products are observed at all measured RHs. The analysis of reaction products suggests that the reaction mechanisms are not strongly influenced by the RH. The observed relationship between the γeff and RH is likely attributed to the regulation of aerosol viscosity by RH rather than reaction mechanisms.

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4. Conclusions To better understand how the RH and viscosity controls the heterogeneous reactivity of aqueous organic droplets, we investigate the oxidative kinetics and molecular transformations of aqueous 2-methylglutaric acid (2-MGA) droplets upon heterogeneous OH oxidation over a range of RH. Hygroscopic measurements reveal that 2-MGA aerosols are aqueous droplets and do not exhibit crystallization when the RH is above 20%. Aerosol speciation data show that the same reaction products are formed at all measured RH. At the same oxidation lifetime, the aerosol composition does not depend on RH significantly. These results indicate that the changes in aerosol water content and the concentration of reactants do not alter the reaction mechanisms chemically. Water diffusion coefficient measurements reveal that the aerosol viscosity is sensitive to the RH and the aerosol composition. The measured water diffusion coefficients in 2MGA droplets are comparable with other viscous organic aerosols at low RH. These observations suggest the overall oxidation rates may be limited by the molecular diffusion of the species in the bulk. Our aerosol speciation data and water diffusion coefficient data support the hypothesis that RH and water likely influence the heterogeneous OH reactivity of aqueous 2MGA droplets physically by varying the aerosol viscosity.

Supporting Information. (1) Description of the proposed reaction mechanisms and scheme of the heterogeneous oxidation of 2-methylglutaric acid, (2) Table of experimental data obtained from the hygroscopicity measurements and the isotope exchange measurements, (3) The kinetic evolution of minor reaction products in the heterogeneous OH oxidation of 2-methylglutaric acid aerosols against OH exposure and oxidation lifetime at different relative humidity.

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Acknowledgements M. M. Chim, C. Y. Chow, and M. N. Chan are supported by the Direct Grant for Research (4053089) and One-Time Funding Allocation of Direct Grant (3132765), The Chinese University of Hong Kong. J. F. Davies is supported by the Department of Energy, Office of Science Early Career Award and the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We acknowledge Kevin R. Wilson for his technical support when we carried out the experiments in the LBNL.

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48. Steimer, S. S.; Berkemeier, T.; Gilgen, A.; Krieger, U. K.; Peter, T.; Shiraiwa, M.; Ammann, M. Shikimic acid ozonolysis kinetics of the transition from liquid aqueous solution to highly viscous glass. Phys. Chem. Chem. Phys. 2015, 17 (46), 31101-31109.

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Table 1. The properties of 2-methylglutaric acid (2-MGA). CH3 HO

Chemical structure

OH O

Chemical formula

O

C6H10O4

Carbon oxidation state, OSC

-0.33

O:C ratio

0.67

H:C ratio

1.67

Carbon number, NC

6

Table 2. The heterogeneous rate constant (k2-MGA) and effective OH uptake coefficient (γeff) of 2methylglutaric acid at different relative humidity (RH). RH (%)

k2-MGA (10-12 cm3 molecule-1 s-1)

γeff

33.8

3.01 ± 0.09

1.93 ± 0.14

43.4

3.64 ± 0.08

2.27 ± 0.17

53.0

4.07 ± 0.03

2.43 ± 0.17

63.3

4.44 ± 0.10

2.59 ± 0.19

72.0

4.76 ± 0.09

2.62 ± 0.19

82.5

5.24 ± 0.13

2.43 ± 0.18

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Figure 1. The hygroscopicity of 2-MGA aerosols measured by an aerosol optical tweezers.

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A

B

C

Figure 2. Aerosol mass spectra of the heterogeneous OH oxidation of 2-MGA aerosols at different relative humidity: (a) 53.0%, (b) 72.0% and (c) 82.5% before (upper row) and after (lower row) oxidation.

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Figure 3. (a) Normalized parent decay, (b) Heterogeneous OH reaction constant (k2-MGA) and (c) Effective OH uptake coefficient (γeff) are plotted against different relative humidity, ranging from 33.8% to 82.5%, for the heterogeneous OH oxidation of 2-MGA aerosols.

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Figure 4. Water diffusion coefficient, Dw, of 2-MGA aerosols before oxidation, measured by isotope exchange measurements using an aerosol optical tweezers. The Dw of citric acid aerosols is shown for comparison.

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Figure 5. The kinetic evolution of major reaction products in the heterogeneous OH oxidation of 2-MGA aerosols against OH exposure at different relative humidity.

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Figure 6. The kinetic evolution of major reaction products in the heterogeneous OH oxidation of 2-MGA aerosols against oxidation lifetime at different relative humidity.

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TOC Image

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Effects of Relative Humidity and Particle Phase Water on the

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