Vassileios C. Papadimitriou

UV absorption spectrum of the ClO dimer (Cl2O2) between 200 and 420 nm.

The UV photolysis of Cl(2)O(2) (dichlorine peroxide) is a key step in the catalytic destruction of polar stratospheric ozone. In this study, the gas-phase UV absorption spectrum of Cl(2)O(2) was measured using diode array spectroscopy and absolute cross sections, sigma, are reported for the wavelength range 200-420 nm. Pulsed laser photolysis of Cl(2)O at 248 nm or Cl(2)/Cl(2)O mixtures at 351 nm at low temperature (200-228 K) and high pressure (approximately 700 Torr, He) was used to produce ClO radicals and subsequently Cl(2)O(2) via the termolecular ClO self-reaction. The Cl(2)O(2) spectrum was obtained from spectra recorded following the completion of the gas-phase ClO radical chemistry. The spectral analysis used observed isosbestic points at 271, 312.9, and 408.5 nm combined with reaction stoichiometry and chlorine mass balance to determine the Cl(2)O(2) spectrum. The Cl(2)O(2) UV absorption spectrum peaks at 244.5 nm with a cross section of 7.6(-0.5)(+0.8) x 10(-18) cm(2) molecule(-1) where the quoted error limits are 2sigma and include estimated systematic errors. The Cl(2)O(2) absorption cross sections obtained for wavelengths in the range 300-420 nm are in good agreement with the Cl(2)O(2) spectrum reported previously by Burkholder et al. (J. Phys. Chem. A 1990, 94, 687) and significantly higher than the values reported by Pope et al. (J. Phys. Chem. A 2007, 111, 4322). A possible explanation for the discrepancy in the Cl(2)O(2) cross section values with the Pope et al. study is discussed. Representative, atmospheric photolysis rate coefficients are calculated and a range of uncertainty estimated based on the determination of sigma(Cl(2)O(2))(lambda) in this work. Although improvements in our fundamental understanding of the photochemistry of Cl(2)O(2) are still desired, this work indicates that major revisions in current atmospheric chemical mechanisms are not required to simulate observed polar ozone depletion.

Rate coefficients for the gas-phase reaction of the hydroxyl radical with CH2=CHF and CH2=CF2.

Rate coefficients, k, for the gas-phase reaction of the OH radical with CH(2)=CHF (k(1)) and CH(2)=CF(2) (k(2)) were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH and laser-induced fluorescence (PLP-LIF) to detect it. Rate coefficients were measured over a range of temperature (220-373 K) and bath gas pressure (20-600 Torr; He, N(2)). The rate coefficients were found to be independent of pressure. The measured rate coefficient for reaction 1 at room temperature was k(1)(296 K) = (5.18 +/- 0.50) x 10(-12) cm(3) molecule(-1) s(-1), independent of pressure, and the temperature dependence is given by the Arrhenius expression k(1)(T) = (1.75 +/- 0.20) x 10(-12) exp[(316 +/- 25)/T] cm(3) molecule(-1) s(-1); the rate coefficients for reaction 2 were k(2)(296 K) = (2.79 +/- 0.25) x 10(-12) cm(3) molecule(-1) s(-1) and k(2)(T) = (1.75 +/- 0.20) x 10(-12) exp[(140 +/- 20)/T] cm(3) molecule(-1) s(-1). The quoted uncertainties are 2sigma (95% confidence level) and include estimated systematic errors. The fall-off parameters for reaction 2 of k(infinity) = 3 x 10(-12) cm(3) molecule(-1) s(-1) and k(0)(296 K) = 1.8 x 10(-28) cm(6) molecule(-2) s(-1) with F(c) = 0.6 reproduce the room temperature data obtained in this study combined with the low pressure rate coefficient data from Howard (J. Chem. Phys. 1976, 65, 4771). OH radical formation was observed for reactions 1 and 2 in the presence of O(2), and the mechanism was investigated using (18)OH and OD rate coefficient measurements with CH(2)=CHF and CH(2)=CF(2) over a range of temperature (260-373 K) and pressure (20-100 Torr, He). Quantum chemical calculations using density functional theory (DFT) were used to determine the geometries and energies of the reactants and adducts formed in reactions 1 and 2 and the peroxy radicals formed following the addition of O(2). The atmospheric lifetimes of CH(2)=CHF and CH(2)=CF(2) due to loss by reaction with OH are approximately 2 and 4 days, respectively. Infrared absorption spectra of CH(2)=CHF and CH(2)=CF(2) were measured, and global warming potentials (GWP) values of 0.7 for CH(2)=CHF and 0.9 for CH(2)=CF(2) were obtained for the 100 year time horizon.

Atmospheric chemistry of CF3CF═CH2 and (Z)-CF3CF═CHF: Cl and NO3 rate coefficients, Cl reaction product yields, and thermochemical calculations.

Rate coefficients, k, for the gas-phase reactions of Cl atoms and NO(3) radicals with 2,3,3,3-tetrafluoropropene, CF(3)CF═CH(2) (HFO-1234yf), and 1,2,3,3,3-pentafluoropropene, (Z)-CF(3)CF═CHF (HFO-1225ye), are reported. Cl-atom rate coefficients were measured in the fall-off region as a function of temperature (220-380 K) and pressure (50-630 Torr; N(2), O(2), and synthetic air) using a relative rate method. The measured rate coefficients are well represented by the fall-off parameters k(0)(T) = 6.5 × 10(-28) (T/300)(-6.9) cm(6) molecule(-2) s(-1) and k(∞)(T) = 7.7 × 10(-11) (T/300)(-0.65) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and k(0)(T) = 3 × 10(-27) (T/300)(-6.5) cm(6) molecule(-2) s(-1) and k(∞)(T) = 4.15 × 10(-11) (T/300)(-0.5) cm(3) molecule(-1) s(-1) for (Z)-CF(3)C═CHF with F(c) = 0.6. Reaction product yields were measured in the presence of O(2) to be (98 ± 7)% for CF(3)C(O)F and (61 ± 4)% for HC(O)Cl in the CF(3)CF═CH(2) reaction and (108 ± 8)% for CF(3)C(O)F and (112 ± 8)% for HC(O)F in the (Z)-CF(3)CF═CHF reaction, where the quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. NO(3) reaction rate coefficients were determined using absolute and relative rate methods. Absolute measurements yielded upper limits for both reactions between 233 and 353 K, while the relative rate measurements yielded k(3)(295 K) = (2.6 ± 0.25) × 10(-17) cm(3) molecule(-1) s(-1) and k(4)(295 K) = (4.2 ± 0.5) × 10(-18) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF, respectively. The Cl-atom reaction with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF leads to decreases in their atmospheric lifetimes and global warming potentials and formation of a chlorine-containing product, HC(O)Cl, for CF(3)CF═CH(2). The NO(3) reaction has been shown to have a negligible impact on the atmospheric lifetimes of CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF. The energetics for the reaction of Cl, NO(3), and OH with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF in the presence of O(2) were investigated using density functional theory (DFT).

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) x 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 (2sigma) 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.

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 (CH(3)C(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, γ(0), on thin ice films or HNO(3)-doped ice films were measured at low surface coverage. In all cases, γ(0) showed an inverse temperature dependence, and for pure thin ice films, it was given by the expression γ(0)(T) = (4.73 ± 1.13) × 10(-17) exp[(6496 ± 1798)/T]; 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, N(max), on pure thin ice films was (2.11 ± 0.16) × 10(14) 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 CH(3)C(O)OH uptakes and larger N(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.

CH3CO + O2 + M (M = He, N2) Reaction Rate Coefficient Measurements and Implications for the OH Radical Product Yield

The gas-phase CH3CO + O2 reaction is known to proceed via a chemical activation mechanism leading to the formation of OH and CH3C(O)OO radicals via bimolecular and termolecular reactive channels, respectively. In this work, rate coefficients, k, for the CH3CO + O2 reaction were measured over a range of temperature (241-373 K) and pressure (0.009-600 Torr) with He and N2 as the bath gas and used to characterize the bi- and ter-molecular reaction channels. Three independent experimental methods (pulsed laser photolysis-laser-induced fluorescence (PLP-LIF), pulsed laser photolysis-cavity ring-down spectroscopy (PLP-CRDS), and a very low-pressure reactor (VLPR)) were used to characterize k(T,M). PLP-LIF was the primary method used to measure k(T,M) in the high-pressure regime under pseudo-first-order conditions. CH3CO was produced by PLP, and LIF was used to monitor the OH radical bimolecular channel reaction product. CRDS, a complementary high-pressure method, measured k(295 K,M) over the pressure range 25-600 Torr (He) by monitoring the temporal CH3CO radical absorption following its production via PLP in the presence of excess O2. The VLPR technique was used in a relative rate mode to measure k(296 K,M) in the low-pressure regime (9-32 mTorr) with CH3CO + Cl2 used as the reference reaction. A kinetic mechanism analysis of the combined kinetic data set yielded a zero pressure limit rate coefficient, kint(T), of (6.4 ± 4) × 10(-14) exp((820 ± 150)/T) cm(3) molecule(-1) s(-1) (with kint(296 K) measured to be (9.94 ± 1.3) × 10(-13) cm(3) molecule(-1) s(-1)), k0(T) = (7.39 ± 0.3) × 10(-30) (T/300)(-2.2±0.3) cm(6) molecule(-2) s(-1), and k∞(T) = (4.88 ± 0.05) × 10(-12) (T/300)(-0.85±0.07) cm(3) molecule(-1) s(-1) with Fc = 0.8 and M = N2. A He/N2 collision efficiency ratio of 0.60 ± 0.05 was determined. The phenomenological kinetic results were used to define the pressure and temperature dependence of the OH radical yield in the CH3CO + O2 reaction. The present results are compared with results from previous studies and the discrepancies are discussed.

Investigation of the photochemical reactivity of soot particles derived from biofuels toward NO2. A kinetic and product study.

In the current study, the heterogeneous reaction of NO2 with soot and biosoot surfaces was investigated in the dark and under illumination relevant to atmospheric conditions (J(NO2) = 0.012 s(-1)). A flat-flame burner was used for preparation and collection of soot samples from premixed flames of liquid fuels. The biofuels were prepared by mixing 20% v/v of (i) 1-butanol (CH3(CH2)3OH), (ii) methyl octanoate (CH3(CH2)6COOCH3), (iii) anhydrous diethyl carbonate (C2H5O)2CO and (iv) 2,5 dimethyl furan (CH3)2C4H2O additive compounds in conventional kerosene fuel (JetA-1). Experiments were performed at 293 K using a low-pressure flow tube reactor (P = 9 Torr) coupled to a quadrupole mass spectrometer. The initial and steady-state uptake coefficients, γ0 and γ(ss), respectively, as well as the surface coverage, N(s), were measured under dry and humid conditions. Furthermore, the branching ratios of the gas-phase products NO (∼80-100%) and HONO (<20%) were determined. Soot from JetA-1/2,5-dimethyl furan was the most reactive [γ0 = (29.1 ± 5.8) × 10(-6), γ(ss)(dry) = (9.09 ± 1.82) × 10(-7) and γ(ss)(5.5%RH) = (14.0 ± 2.8)(-7)] while soot from JetA-1/1-butanol [γ0 = (2.72 ± 0.544) × 10(-6), γ(ss)(dry) = (4.57 ± 0.914) × 10(-7), and γ(ss)(5.5%RH) = (3.64 ± 0.728) × 10(-7)] and JetA-1/diethyl carbonate [γ0 = (2.99 ± 0.598) × 10(-6), γ(ss)(dry) = (3.99 ± 0.798) × 10(-7), and γ(ss)(5.5%RH) = (4.80 ± 0.960) × 10(-7)] were less reactive. To correlate the chemical reactivity with the physicochemical properties of the soot samples, their chemical composition was analyzed employing Raman spectroscopy, NMR, and high-performance liquid chromatography. In addition, the Brunauer-Emmett-Teller adsorption isotherms and the particle size distributions were determined employing a Quantachrome Nova 2200e gas sorption analyzer. The analysis of the results showed that factors such as (i) soot mass collection rate, (ii) porosity of the particles formed, (iii) aromatic fraction, and (iv) pre-existence of nitro-containing species in soot samples (formed during the combustion process) can be used as indicators of soot reactivity with NO2.

1,2-Dichlorohexafluoro-Cyclobutane (1,2-c-C4F6Cl2, R-316c) a Potent Ozone Depleting Substance and Greenhouse Gas: Atmospheric Loss Processes, Lifetimes, and Ozone Depletion and Global Warming Potentials for the (E) and (Z) stereoisomers

The atmospheric processing of (E)- and (Z)-1,2-dichlorohexafluoro-cyclobutane (1,2-c-C4F6Cl2, R-316c) was examined in this work as the ozone depleting (ODP) and global warming (GWP) potentials of this proposed replacement compound are presently unknown. The predominant atmospheric loss processes and infrared absorption spectra of the R-316c isomers were measured to provide a basis to evaluate their atmospheric lifetimes and, thus, ODPs and GWPs. UV absorption spectra were measured between 184.95 to 230 nm at temperatures between 214 and 296 K and a parametrization for use in atmospheric modeling is presented. The Cl atom quantum yield in the 193 nm photolysis of R-316c was measured to be 1.90 ± 0.27. Hexafluorocyclobutene (c-C4F6) was determined to be a photolysis co-product with molar yields of 0.7 and 1.0 (±10%) for (E)- and (Z)-R-316c, respectively. The 296 K total rate coefficient for the O((1)D) + R-316c reaction, i.e., O((1)D) loss, was measured to be (1.56 ± 0.11) × 10(-10) cm(3) molecule(-1) s(-1) and the reactive rate coefficient, i.e., R-316c loss, was measured to be (1.36 ± 0.20) × 10(-10) cm(3) molecule(-1) s(-1) corresponding to a ~88% reactive yield. Rate coefficient upper-limits for the OH and O3 reaction with R-316c were determined to be <2.3 × 10(-17) and <2.0 × 10(-22) cm(3) molecule(-1) s(-1), respectively, at 296 K. The quoted uncertainty limits are 2σ and include estimated systematic errors. Local and global annually averaged lifetimes for the (E)- and (Z)-R-316c isomers were calculated using a 2-D atmospheric model to be 74.6 ± 3 and 114.1 ± 10 years, respectively, where the estimated uncertainties are due solely to the uncertainty in the UV absorption spectra. Stratospheric photolysis is the predominant atmospheric loss process for both isomers with the O((1)D) reaction making a minor, ~2% for the (E) isomer and 7% for the (Z) isomer, contribution to the total atmospheric loss. Ozone depletion potentials for (E)- and (Z)-R-316c were calculated using the 2-D model to be 0.46 and 0.54, respectively. Infrared absorption spectra for (E)- and (Z)-R-316c were measured at 296 K and used to estimate their radiative efficiencies (REs) and GWPs; 100-year time-horizon GWPs of 4160 and 5400 were obtained for (E)- and (Z)-R-316c, respectively. Both isomers of R-316c are shown in this work to be long-lived ozone depleting substances and potent greenhouse gases.

OH Radical Reaction Rate Coefficients, Infrared Spectrum, and Global Warming Potential of (CF3)2CFCH═CHF (HFO-1438ezy(E))

Rate coefficients, k(T), for the OH radical + (E)-(CF3)2CFCH═CHF ((E)-1,3,4,4,4-pentafluoro-3-(trifluoromethyl)-1-butene, HFO-1438ezy(E)) gas-phase reaction were measured using pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) between 214 and 380 K and 50 and 450 Torr (He or N2 bath gas) and with a relative rate method at 296 K between 100 and 400 Torr (synthetic air). Over the range of pressures included in this study, no pressure dependence in k(T) was observed. k(296 K) obtained using the two techniques agreed to within ∼3% with (3.26 ± 0.26) × 10(-13) cm(3) molecule(-1) s(-1) (2σ absolute uncertainty) obtained using the PLP-LIF technique. k(T) displayed non-Arrhenius behavior that is reproduced by (7.34 ± 0.30) × 10(-19)T(2) exp[(481 ± 10)/T) cm(3) molecule(-1) s(-1). With respect to OH reactive loss, the atmospheric lifetime of HFO-1438ezy(E) is estimated to be ∼36 days and HFO-1438ezy(E) is considered a very short-lived substance (VSLS) (the actual lifetime will depend on the time and location of the HFO-1438ezy(E) emission). On the basis of the HFO-1438ezy(E) infrared absorption spectrum measured in this work and its estimated lifetime, a radiative efficiency of 0.306 W m(-2) ppb(-1) (well-mixed gas) was calculated and its 100-year time-horizon global warming potential, GWP100, was estimated to be 8.6. CF3CFO, HC(O)F, and CF2O were identified using infrared spectroscopy as stable end products in the oxidation of HFO-1438ezy(E) in the presence of O2. Two additional fluorinated products were observed and theoretical calculations of the infrared spectra of likely degradation products are presented. The photochemical ozone creation potential of HFO-1438ezy(E) was estimated to be ∼2.15.

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 γ(0,A)(T) = (2.89 ± 0.26) × 10(-8) exp[(2517 ± 266)/T] and γ(0,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 (>10(15) 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.