Article pubs.acs.org/JPCC
Thermochemical, Cloud Condensation Nucleation Ability, and Optical Properties of Alkyl Aminium Sulfate Aerosols Avi Lavi,† Nir Bluvshtein,† Enrico Segre,‡ Lior Segev,† Michel Flores,† and Yinon Rudich*,† †
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, 76100 Israel Physical Services, Weizmann Institute of Science, Rehovot, 76100 Israel
‡
ABSTRACT: Alkyl aminium sulfates have been postulated to constitute important components of nucleation and accumulation mode atmospheric aerosols. In this study we present laboratory data on the thermochemical, cloud condensation nuclei (CCN) activity, and optical properties of selected aminium sulfate compounds of atmospheric relevance (monomethyl aminium sulfate (MMAS), dimethyaminium sulfate (DMAS), trimethylaminium sulfate, monoethylaminium sulfate (MEAS), diethylaminium sulfate (DEAS), and triethylaminium sulfate (TEAS)). We found that the vapor pressure of these aminium salts is 1−3 orders of magnitude lower than that of ammonium sulfate and as such they can contribute to new aerosols and secondary aerosols formation. We infer that these species have very high CCN activity, with hygroscopicity parameter that is similar to that ammonium sulfate. Finally, between 360 and 420 nm, these aminium sulfate salts scatter light less efficiently than ammonium sulfate, and do not absorb light. These derived parameters can contribute to the better understanding and characterization of the role that these compounds play in atmospheric chemical reactions, gas−solid partitioning and their possible contribution to the microphysical and radiative effects of atmospheric aerosols.
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INTRODUCTION Short aliphatic alkyl amines (C1−C2) are atmospherically ubiquitous, highly volatile organic compounds (VOCs), originating from various anthropogenic and biogenic sources.1 The estimated annual flux of mono,di- and trimethylamine is ∼300Gg N a−1, from which more than 50% is trimethylamine.1 The contribution of alkyl amines to aerosol mass was previously neglected because of their high vapor pressure, ranging from 5 to hundreds of kilopascals.2 However, field measurements followed by laboratory experiments showed that ambient aerosols contain substantial amounts of organic and inorganic amine compounds.3−10 The measured ratio between particulate aminium and ammonium ions in atmospheric aerosols is usually 0.1 or lower11,12 but may reach up to 0.2 in an urban area,13 suggesting that aminium ions may constitute a substantial fraction of atmospheric aerosols. These observations suggest that in the atmosphere, alkyl amines efficiently partition to the aerosol phase and contribute to the physic-chemical properties of atmospheric aerosols. Several routes for atmospheric alkyl amine degradation have been suggested based on laboratory experiments.9,14 Photooxidation of alkyl amine by atmospheric oxidants, such as ozone and OH radicals, followed by gas to particle partitioning, produce various imines, nitro compounds and trialkyl-Noxides.7−10 In addition, condensation reactions with carboxylic and dicarboxylic acids in the condensed phase lead to the formation of amides and macromolecules.15,16 Finally, due to their high solubility in water, alkyl amines can undergo acid− base reactions with aerosol-phase acids (HNO3, HCl, H2SO4) © 2013 American Chemical Society
to form dissolved alkyl aminium salts, in a manner analogous to ammonium sulfate formation.7,9,10 The increased interest in the formation of atmospheric alkylaminium salts and their possible roles in the atmosphere emerged recently due to observations of high correlation between new particle formation events and the concentration of dimethylaminium ion (the cationic form of dimethylamine) in freshly nucleated particles in remote boreal forests and in urban environments.17,18 Theoretical thermodynamic calculations showed that alkyl amine dimers and (ionic and neutral) clusters of alkyl amine with sulfuric acid are stable enough to overcome the mass balance gradient caused by the low concentration of alkyl amines relative to ammonia.19 In addition, the displacement reaction of ammonia by alkylamine promote particle growth and the formation of submicrometer particles.20 The presence of aliphatic amine in nucleation mode aerosols indicates that they have sufficiently low vapor pressures to partition to the aerosol phase. Therefore, their vapor pressure and heat of evaporation (ΔHvap) are important parameters in detailed aerosol formation mechanisms. Aerosols in the troposphere act as cloud condensation nuclei (CCN) and hence can modify cloud properties.21 In addition, aerosols scatter and absorb incoming solar radiation.22 As such, aerosols play an important role in the climate system. Hence, Special Issue: Ron Naaman Festschrift Received: March 31, 2013 Revised: June 11, 2013 Published: June 11, 2013 22412
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Table 1. Physical Properties for the Aminium Sulfate Salts Measured in this Work compound AS MMAS DMAS TMAS MEAS DEAS TEAS
MW(g mol−1) 132.1 160.2 188.3 216.3 188.3 244.4 300.5
density (g cm−3) e
1.77 1.438j 1.351j 1.307j 1.313l 1.211j 1.138j
Vac (cm3 mol−1)
γb (J m−2)
f
g
248.4 332.88f 491.36f 514.75f 496.62f 724.82f 972.65f
0.04 0.04g 0.1k 0.1k 0.1k 0.1k 0.1k
σiic (Å)
εii/kBd (K)
h
5.19 5.72h 6.74h 6.86h 6.66h 7.55h 8.33h
966.05i 898.85i 642.12i 653.59i 721.18i 696.93i 740.29i
a Critical volume. bSurface free energy. cThe interparticle distance at which the potential between particles is zero. dThe potential well depth. eTaken from ref 39. fEstimated following Valderrama et al.40 gCohen et al.41 hEstimated following Bird et al.42 iEstimated following Bird et al.42 jQiu et al.27 k Estimated. lThis work, AMS-method, assuming a shape factor of 1.
atmospheric relevance. These include CCN activity, optical properties, and thermochemical data. This data is needed for better understanding and characterization of the role of these compounds in atmospheric chemical reactions, gas−solid partitioning, and their possible contribution to the radiative effect of aerosols.
quantitative understanding of aerosols’ effects on climate requires a better characterization of their physical and chemical properties. Specifically, sulfate or ammonium sulfate aerosols have a major role in perturbation of the global radiative forcing23 via their microphysical (indirect effect) and radiative effects.23,24 The properties of aminium salts as possible CCN are not well characterized to date. Dinar et al.25 showed that the formation of aminium carboxylic salts by uptake of ammonia onto slightly soluble organic acid particles results in a significant increase of their hygroscopic growth and CCN activity since the uptake of ammonia decreases the water vapor saturation required for a particle to become a CCN nuclei.25 The hygroscopic growth of alkyl aminium salts such as monomethyl aminium acetate has been recently studied.26 Their growth factor at relative humidity (RH) = 90% is lower than that of ammonium sulfate (AS). By contrast, short alkyl aminium sulfate salts are highly hygroscopic. Measurements by hygroscopic tandem differential mobility analyzer found that unlike AS, alkylaminium sulfate particles showed a continuous water uptake as the RH increased from 10% to 90%, without a clear deliquescence point.27 The addition of 10 wt % alkylaminium sulfate to AS aerosols decreased the deliquescence point of AS from 79% to 71%. At 1:1 wt % ratio, the deliquescence point of AS diminished.27 In addition, the CCN activity of dry amino acids depends mainly on their water solubility. Amino acids with solubility of 10g L−1 or higher are effective CCN compounds.28 These observations suggest that alkylamminium sulfate salts may be active components in the CCN activity of atmospheric aerosols. The radiative effect of aerosols is caused by scattering and absorption of incoming shortwave and outgoing long wave radiation.22,29 Sulfate aerosols mainly scatter light back to space. The net radiative effect caused by aerosols in general, and sulfate aerosols in particular, depends on the wavelength of the incident light, the aerosols’ vertical, horizontal and size distributions, lifetime and their complex refractive index (RI; m = n + ik).30 The extent of scattering and absorption of light by the aerosols, noted by the real (n) and imaginary (k) parts of the RI respectively, is determined by their chemical composition and mixing state. As it is becoming evident that heterogeneous reactions between gaseous alkylamines and ammonium sulfate aerosols may alter the composition of sulfate aerosols in the atmosphere, it is necessary to quantify the RI of alkyl aminium sulfate salts and its wavelength dependence. Their contribution to the effective RI of mixed aerosols can then be assessed using various mixing rules calculations.31−35 In this study we present laboratory data on the physiochemical properties of selected alkylaminium sulfate salts of
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EXPERIMENTAL METHODS Reagents. Ammonium sulfate >99.5%, methylamine 45% solution in water, dimethylamine 40% solution in water, trimethylamine, ethylamine solution in water, diethylamine, triethylamine, and 1 M volumetric solution of sulfuric acid were purchased from Sigma and used as is. Nanopure water (18.2 MΩ) was taken from an in-house system. Synthesis. Monomethyl aminium sulfate (MMAS), dimethyaminium sulfate (DMAS), trimethylaminium sulfate, monoethylaminium sulfate (MEAS), diethylaminium sulfate (DEAS) and triethylaminium sulfate (TEAS) were prepared from their alkylamine precursors by slow addition of the required alkylamine to a stirred sulfuric acid solution until it became alkaline (pH = 10−11). The alkylamine excess was removed by gentle heating of the solution to 60 °C until the solution was slightly acidic (pH ≈ 6). After cooling to room temperature, the solution was diluted to a final known concentration. Volatility Measurements. Assuming non equilibrium conditions, the vapor pressure of a particle can be obtained from the change in particle diameter using eq 1:36 p0 = −
ρRT 4DM Δt
∫D
Df
Dpe(−4γM / DpρRT )f −1 (Kn, α) dDp
i
(1)
Here p0 is the saturation vapor pressure, ρ is the density of the aerosol, R is the gas constant, T is the temperature (K), D is the diffusivity of the vapors in air, M is the molecular weight of the aerosol. Dp,i and Dp,f are the initial and final particle diameters, respectively. γ is the particle surface free energy and Δt is the residence time in the thermal denuder taking into account the parabolic shape of the flow and neglecting thermophoresis losses. The basis of the method and its basic assumptions are described elsewhere.37 The Fuchs and Sutugin correction term ( f(Kn,α)) is a correction factor for particle evaporation in the noncontinuous flow regime given by eq 2:38 f (Kn, α) =
1 + Kn 1 + 0.3773 × Kn + 1.33 × Kn ×
(1 + Kn) α
(2)
where Kn is the Knudsen number, and α is the evaporation coefficient, describing the ratio between the molecular flux from 22413
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the particle and the theoretical flux predicted by the kinetic theory of gases. In this work, α was taken as 1. The parameters used for the vapor pressure calculations are summarized in Table 1. A home-built volatility tandem differential mobility analyzer (VTDMA) was used to quantify the volatility of the aminium sulfate salts, i.e., its vapor pressure in a selected temperature range. The experimental setup consists of four major units: aerosol generation, size selection, volatilization, and size measurement. The alkylaminium sulfate salts were diluted from the stock solution to a final concentration of ∼0.1−1 μg L−1. A polydisperse aerosol population was generated using a constant output atomizer (TSI 3076). The aerosol was dried by two silica diffusion driers. The dried aerosol (Vaisala hmp-113, RH 130 °C, the enthalpy was derived assuming the Clausius−Clapeyron relationship, i.e., a linear dependence of log p0 on T−1. The enthalpy of vaporization and the saturation vapor pressure (ΔHvap and Psat) at room temperature were extrapolated using linear regression and are summarized in Table 2. In cases where a small decrease of the median particle diameter was observed between room temperature and 100 °C, the calculations were based on the reduced initial diameter as was measured above 80 °C. Ammonium Sulfate. the thermogram of AS showed a minor decrease in VFR between RT and 100 °C, most likely due to water layer desorption from the outer particle surface35 or solid state rearrangement resulting in higher density of the particle.27 The temperature range in which AS showed rapid decomposition (140−180 °C) is in good agreement with previously reported thermal stability and volatility measurements.27,59,60 This temperature range of vaporization as reported in the literature slightly varies as a result of the differences in the experimental setup and the initial particle diameter due to the Kelvin effect.26,27 Scott and Cattel showed that the thermal decomposition of ammonium sulfate in an open system proceeds through the sublimation of ammonia and the formation of ammonium bisulfate (eq 6a).57 The ammonium bisulfate product is unstable at this temperature and reacts to produce ammonium sulfate and sulfuric acid residue in the condensed phase (eq 6b).57
(5)
Figure 2 shows the sigmoidal characteristic curve of VFR dependence on the temperature, for AS, DEAS, and TEAS.
Figure 2. The VFR versus the temperature for DEAS, DMAS, and AS.
The enthalpy of vaporization at 298 K can be calculated from the vapor pressure at different temperatures using the Clausius−Clapeyron relationship by the extrapolation of the linear dependence of log p0 versus T−1 to room temperature. All the alkylaminium sulfate salts showed high linear dependence. Figure 3 depicts the linear relationship for some of the compounds measured in this study. The median particle diameter as a function of the temperature in the TD was measured for AS, MMAS, DMAS, TMAS, MEAS, DEAS, and TEAS. The VFR of some of the alkylaminium sulfates salts showed a slight (up to a few
(NH4)2 SO4(s) → NH3(g) + NH4HSO4(s)
(6a)
2NH4HSO4(s) → H 2SO4(s) + (NH4)2 SO4(s)
(6b)
Ammonium sulfate decomposed almost to completeness (VFR at 180 °C is lower than 0.2) in the temperature range employed in our experiments, and only one linear slope is observed between 150 and 175 °C (Figure 2). This profile indicates that ammonium sulfate decomposition is the result of a single reaction. It is expected that if a more stable salt (for example (NH4)3H(SO4)2) would have been formed, the final VFR would have been higher. Therefore, we suggest that the evaporation of AS particles is the result of ammonia sublimation that leads to almost complete evaporation of the particle. The calculated enthalpy of ammonia sublimation measured in this experiment is 114 ± 2 kJ·mol−1. This result agrees well with previously published results57 and the current thermochemical data (see Table 2).39 The current knowledge on the vapor pressure over solid ammonium sulfate particles is limited, and the published results vary over orders of magnitude.61 The saturation vapor pressure over AS particles calculated in this study (2.7 × 10−9 Pa) is within the range reported by Scott and Cattel,57 and within the error range of the vapor pressure of ammonia over AS calculated from the measurements of sulfuric acid over AS by Marti et al.62 Alkylaminium Sulfate Salts. The vapor pressure and enthalpies of vaporization for a series of alkylaminium sulfate salts were measured for the first time in this work. An irregular pattern in which the derived vapor pressure of the salts at room temperature, prepared from tertiary alkyl amines (TMA and
Figure 3. The temperature dependence of the saturation vapor pressures of AS, DMAS, and DEAS. 22415
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Table 2. Calculated Vapor Pressure and Enthalpy of Vaporization this study compound AS MMAS DMAS TMAS MEAS DEAS TEAS
literature value
p0 (298 K, Pa)
ΔHvap (kJ mol−1)
T (K)
p0 (Pa)
ΔHsub (kJ mol−1)
−9 2.7+1.4 ‑0.8 × 10
114 ± 2
418−446
4.53 × 10−9a 8.1 × 10−9c -
125a, 142c, 108b
−10 5.4+2.5 ‑1.7 × 10 −11 +6 2.5‑2 × 10 −9 1.1+0.7 ‑0.5 × 10 −10 4.0+3.0 × 10 ‑1.0 −12 1.8+11.0 ‑1.6 × 10 −10 +3.5 3.5‑1.8 × 10
125 146 114 143 168 137
± ± ± ± ± ±
2 4 2 2 5 2
415−448 418−443 424−453 424−454 423−448 423−453
-
a Scott and Cattell.57 bEnthalpy was calculated based on the proposed mechanism.39 cKiyoura and Urano.58 The values of p0 and Hvap for each substance, with their 95% confidence intervals, were obtained by robust fit (Matlab, MathWorks) of the experimental data to eq 1.
TEA) was significantly higher than that of the salts prepared from primary or secondary alkylamines is seen in Figure 4.
3. Evaporation and dissociation from ionic solid salt into gaseous ions. The thermal decomposition and evaporation mechanism of 1:1 ammonium salts have been studied in detail due to their industrial and environmental importance. For example, the vapor composition above ammonium nitrate showed that the gas phase consists of: ammonia, nitric acid and also ammonium nitrate as a neutral molecule.64 These products are the result of proton transfer reactions from the ammonium ion to the conjugated base of the strong acid. The question whether this proton transfer reaction occurs in the gas phase or solid phase was studied for some salts by quantum mechanical calculations.65−67 It was shown that the first step in vaporization or decomposition of 1:1 salts is a solid state transformation from the crystalline state to the amorphous state.65−67 This solid state transformation is followed by concerted proton-transfer and desorption reactions of the neutral acid−base complex from the solid surface.65−67 Further decomposition of the neutral structure to its precursors (ammonia and acid) is also an endothermic reaction.65−67 For example, the mechanism of thermal decomposition of ammonium chloride is a four-step reaction, written as eq 7:67
Figure 4. The dependence of the vapor pressure of alkylaminium sulfate salts in their molecular weight.
In general, addition of a substituted alkyl group increases the alkylamine basicity. Since the alkyl groups stabilize the charge distribution of the acidic form, the enthalpy of the reaction will increase within each series of primary, secondary, and tertiary amines.63 However, the calculated enthalpy of TMAS and TEAS were lower than those of DMAS and DEAS, respectively (see Table 2 and Figure 5).
− − NH+4 Cl(crystal) ↔ NH+4 Cl(s)
↔ NH3 − HCl(g) ↔ NH3(g) + HCl(g)
(7)
Similarly, previous studies showed that the vapor phase over ammonium sulfate solid consists of ammonia and sulfuric acid.62,68 In the third mechanism gaseous ions are formed. This mechanism requires substantial energy to overcome the salt’s lattice energy. Since proton transfer reactions are favorable, as was shown for example in decomposition of NH4NO364 and NH4ClO4,69 it is unlikely that this mechanism will be dominant in atmospheric conditions. We suggest that the higher vapor pressure and lower enthalpy of vaporization of TMAS and TEAS are the result of their solid structure. The bulky structure of the trialkyl cation significantly increase its ionic radius63 and reduce the lattice energy of the compound70 (or prevent it from crystallizing). As a result, an increase in the vapor pressure and a decrease in the enthalpy required for phase transition and for vaporization are expected. This substantial effect of the solid state on the volatility is also known for C2−C5 dicarboxylic acids showing a decrease of 1−3 orders of magnitude between supercooled melt and solid state particle.71
Figure 5. The dependence of the enthalpy of vaporization of alkylaminium sulfate salts on molecular weight.
This irregularity may be explained by the mechanism of the evaporation, similar to the one proposed for ammonium sulfate. The evaporation and thermal decomposition of alkylaminium sulfate salts can possibly proceed by three mechanisms: 1. Evaporation of the salt molecule as a molecular entity. 2. Evaporation of neutral reactants (alkylamine and sulfuric acid). 22416
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Table 3. Parameters Used for Sensitivity Analysis parameter
starting value
temperature range
418−446 K
Qa
0.28 L min−1
γb
0.0441 J m−2
ε/kBc
966.05 K
σd
5.19 Å
ρe
1.7739 g cm−3
p0 (298 K)
deviated value
2.01 × 10
+1 K −1 K +0.05 L min−1 −0.05 L min−1 +0.04 J m−2 −0.03 J m−2 +10 K −10 K +0.2 Å −0.2 Å +0.07 g cm−3 −0.07 g cm−3
change (10−9 Pa)
% change
+0.3 −0.3 +0.4 −0.6 +0.4 −0.6 +0.4 −0.6 +0.4 −0.6 +0.4 −0.6
−15% +15% +2% −2%
