The Interaction of Propionic and Butyric Acids with Ice and HNO₃‑Doped Ice Surfaces at 195–212 K

The interaction of propionic and butyric acids on ice and HNO3-doped ice were studied between 195 and 212 K and low concentrations, using a Knudsen flow reactor coupled with a quadrupole mass spectrometer. The initial uptake coefficients (γ0) of propionic and butyric acids on ice as a function of temperature are given by the expressions: γ0(T) = (7.30 ± 1.0) × 10–10 exp[(3216 ± 478)/T] and γ0(T) = (6.36 ± 0.76) × 10–11 exp[(3810 ± 434)/T], respectively; the quoted error limits are at 95% level of confidence. Similarly, γ0 of propionic acid on 1.96 wt % (A) and 7.69 wt % (B) HNO3-doped ice with temperature are given as γ0,A(T) = (2.89 ± 0.26) × 10–8 exp[(2517 ± 266)/T] and γ0,B(T) = (2.77 ± 0.29) × 10–7 exp[(2126 ± 206)/T], respectively. The results show that γ0 of C1 to C4 n-carboxylic acids on ice increase with the alkyl-group length, due to lateral interactions between alkyl-groups that favor a more perpendicular orientation and well packing of H-bonded monomers on ice. The high uptakes (>1015 molecules cm–2) and long recovery signals indicate efficient growth of random multilayers above the first monolayer driven by significant van der Waals interactions. The heterogeneous loss of both acids on ice and HNO3-doped ice particles in dense cirrus clouds is estimated to take a few minutes, signifying rapid local heterogeneous removal by dense cirrus clouds

Absolute Rate Coefficient Determination and Reaction Mechanism Investigation for the Reaction of Cl Atoms with CH₂I₂ and the Oxidation Mechanism of CH₂I Radicals

The gas-phase reaction of atomic chlorine with diiodomethane was studied over the temperature range 273−363 K with the very low-pressure reactor (VLPR) technique. The reaction takes place in a Knudsen reactor at pressures below 3 mTorr, where the steady-state concentration of both reactants and stable products is continuously measured by electron-impact mass spectrometry. The absolute rate coefficient as a function of temperature was given by k = (4.70 ± 0.65) × 10-11 exp[−(241 ± 33)/T] cm3molecule-1s-1, in the low-pressure regime. The quoted uncertainties are given at a 95% level of confidence (2σ) and include systematic errors. The reaction occurs via two pathways:  the abstraction of a hydrogen atom leading to HCl and the abstraction of an iodine atom leading to ICl. The HCl yield was measured to be ca. 55 ± 10%. The results suggest that the reaction proceeds via the intermediate CH2I2−Cl adduct formation, with a I−Cl bond strength of 51.9 ± 15 kJ mol-1, calculated at the B3P86/aug-cc-pVTZ-PP level of theory. Furthermore, the oxidation reactions of CHI2 and CH2I radicals were studied by introducing an excess of molecular oxygen in the Knudsen reactor. HCHO and HCOOH were the primary oxidation products indicating that the reactions with O2 proceed via the intermediate peroxy radical formation and the subsequent elimination of either IO radical or I atom. HCHO and HCOOH were also detected by FT-IR, as the reaction products of photolytically generated CH2I radicals with O2 in a static cell, which supports the proposed oxidation mechanism. Since the photolysis of CH2I2 is about 3 orders of magnitude faster than its reactive loss by Cl atoms, the title reaction does not constitute an important tropospheric sink for CH2I2

Water Interactions with Acetic Acid Layers on Ice and Graphite

Adsorbed organic compounds modify the properties of environmental interfaces with potential implications for many Earth system processes. Here, we describe experimental studies of water interactions with acetic acid (AcOH) layers on ice and graphite surfaces at temperatures from 186 to 200 K. Hyperthermal D₂O water molecules are efficiently trapped on all of the investigated surfaces, with only a minor fraction that scatters inelastically after an 80% loss of kinetic energy to surface modes. Trapped molecules desorb rapidly from both μm-thick solid AcOH and AcOH monolayers on graphite, indicating that water has limited opportunities to form hydrogen bonds with these surfaces. In contrast, trapped water molecules bind efficiently to AcOH-covered ice and remain on the surface on the observational time scale of the experiments (60 ms). Thus, adsorbed AcOH is observed to have a significant impact on water–ice surface properties and to enhance the water accommodation coefficient compared to bare ice surfaces. The mechanism for increased water uptake and the implications for atmospheric cloud processes are discussed

Uptake Measurements of Acetic Acid on Ice and Nitric Acid-Doped Thin Ice Films over Upper Troposphere/Lower Stratosphere Temperatures

The adsorption of gaseous acetic acid (CH₃C­(O)­OH) on thin ice films and on ice doped with nitric acid (1.96 and 7.69 wt %) was investigated over upper troposphere and lower stratosphere (UT/LS) temperatures (198–208 K), and at low gas concentrations. Experiments were performed in a Knudsen flow reactor coupled to a quadrupole mass spectrometer. The initial uptake coefficients, γ₀, on thin ice films or HNO₃-doped ice films were measured at low surface coverage. In all cases, γ₀ showed an inverse temperature dependence, and for pure thin ice films, it was given by the expression<i> </i>γ₀(<i>T</i>) = (4.73 ± 1.13) × 10^–17 exp[(6496 ± 1798)/<i>T</i>]; the quoted errors are the 2σ precision of the linear fit, and the estimated systematic uncertainties are included in the pre-exponential factor. The inverse temperature dependence suggests that the adsorption process occurs via the formation of an intermediate precursor state. Uptakes were well represented by the Langmuir adsorption model, and the saturation surface coverage, <i>N</i>_max, on pure thin ice films was (2.11 ± 0.16) × 10¹⁴ molecules cm^–2, independent of temperature in the range 198–206 K. Light nitration (1.96 and 7.69 wt %) of ice films resulted in more efficient CH₃C­(O)­OH uptakes and larger <i>N</i>_max values that may be attributed to in-bulk diffusion or change in nature of the gas–ice surface interaction. Finally, it was estimated that the rate of adsorption of acetic acid on high-density cirrus clouds in the UT/LS is fast, and this is reflected in the short atmospheric lifetimes (2–8 min) of acetic acid; however, the extent of this uptake is minor resulting in at most a 5% removal of acetic acid in UT/LS cirrus clouds

Water Accommodation on Ice and Organic Surfaces: Insights from Environmental Molecular Beam Experiments

Water uptake on aerosol and cloud particles in the atmosphere modifies their chemistry and microphysics with important implications for climate on Earth. Here, we apply an environmental molecular beam (EMB) method to characterize water accommodation on ice and organic surfaces. The adsorption of surface-active compounds including short-chain alcohols, nitric acid, and acetic acid significantly affects accommodation of D₂O on ice. <i>n</i>-Hexanol and <i>n</i>-butanol adlayers reduce water uptake by facilitating rapid desorption and function as inefficient barriers for accommodation as well as desorption of water, while the effect of adsorbed methanol is small. Water accommodation is close to unity on nitric-acid- and acetic-acid-covered ice, and accommodation is significantly more efficient than that on the bare ice surface. Water uptake is inefficient on solid alcohols and acetic acid but strongly enhanced on liquid phases including a quasi-liquid layer on solid <i>n</i>-butanol. The EMB method provides unique information on accommodation and rapid kinetics on volatile surfaces, and these studies suggest that adsorbed organic and acidic compounds need to be taken into account when describing water at environmental interfaces

Kinetic Study of the Reactions of Cl Atoms with CF₃CH₂CH₂OH, CF₃CF₂CH₂OH, CHF₂CF₂CH₂OH, and CF₃CHFCF₂CH₂OH

The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF3CH2CH2OH (1), CF3CF2CH2OH (2), CHF2CF2CH2OH (3), and CF3CHFCF2CH2OH (4), were studied in the gas phase over the temperature range of 273−363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm3 molecule-1 s-1):  k1 = (4.42 ± 0.48) × 10-11 exp(−255 ± 20/T); k1(303) = (1.90 ± 0.17) × 10-11, k2 = (2.23 ± 0.31) × 10-11 exp(−1065 ± 106/ T); k2(303) = (6.78 ± 0.63) × 10-13, k3 = (8.51 ± 0.62) × 10-12 exp(−681 ± 72/T); k3(303) = (9.00 ± 0.82) × 10-13 and k4 = (6.18 ± 0.84) × 10-12 exp(−736 ± 42/T); k4(303) = (5.36 ± 0.51) × 10-13. The quoted 2σ uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (RF−CHO; RF = −CH2CF3, −CF2CF3, −CF2CHF2, and −CF2CHFCF3), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the α-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2−C5 ethers and C2−C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm3 molecule-1 s-1) = (5.8 ± 1.4) − (1.56 ± 0.13) × (ionization potential (in eV))

Kinetic Study of the Reactions of Cl Atoms with CF₃CH₂CH₂OH, CF₃CF₂CH₂OH, CHF₂CF₂CH₂OH, and CF₃CHFCF₂CH₂OH

The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF3CH2CH2OH (1), CF3CF2CH2OH (2), CHF2CF2CH2OH (3), and CF3CHFCF2CH2OH (4), were studied in the gas phase over the temperature range of 273−363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm3 molecule-1 s-1):  k1 = (4.42 ± 0.48) × 10-11 exp(−255 ± 20/T); k1(303) = (1.90 ± 0.17) × 10-11, k2 = (2.23 ± 0.31) × 10-11 exp(−1065 ± 106/ T); k2(303) = (6.78 ± 0.63) × 10-13, k3 = (8.51 ± 0.62) × 10-12 exp(−681 ± 72/T); k3(303) = (9.00 ± 0.82) × 10-13 and k4 = (6.18 ± 0.84) × 10-12 exp(−736 ± 42/T); k4(303) = (5.36 ± 0.51) × 10-13. The quoted 2σ uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (RF−CHO; RF = −CH2CF3, −CF2CF3, −CF2CHF2, and −CF2CHFCF3), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the α-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2−C5 ethers and C2−C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm3 molecule-1 s-1) = (5.8 ± 1.4) − (1.56 ± 0.13) × (ionization potential (in eV))