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Ozonolysis of isoprene, the most abundant alkene, produces three distinct Criegee intermediates (CIs): CH2OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO2 by CIs is a potential source of H2SO4, an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti-MACRO with SO2 has been found, kSO2 = (1.5 ± 0.4) × 10−10 cm3 s−1 (±1σ, σ is the standard deviation of the data) at 298 K (150 − 500 Torr), which is ca. 4 times the value for syn-MVKO. However, the reaction of anti-MACRO with water vapor has been observed to be quite slow with an effective rate coefficient of (9 ± 5) × 10−17 cm3 s−1 (±1σ) at 298 K (300 to 500 Torr), which is smaller than current literature values by 1 or 2 orders of magnitude. Our results indicate that anti-MACRO has an atmospheric lifetime (best estimate ca. 18 ms at 298 K and RH = 70%) much longer than previously thought (ca. 0.3 or 3 ms), resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti-MACRO on the oxidation of atmospheric SO2 would be substantial, even more than that of syn-MVKO.

Isoprene is the most abundant unsaturated hydrocarbon in the atmosphere. Ozonolysis of isoprene produces methyl vinyl ketone oxide (MVKO), which may react with atmospheric SO2, formic acid, and other important species at substantial levels. In this study, we utilized ultraviolet absorption to monitor the unimolecular decay kinetics of syn-MVKO in real time at 278-319 K and 100-503 Torr. After removing the contributions of radical reactions and wall loss, the unimolecular decay rate coefficient of syn-MVKO was measured to be kuni = 70 ± 15 s-1 (1σ uncertainty) at 298 K with negligible pressure dependence. In addition, kuni increases from ca. 30 s-1 at 278 K to ca. 175 s-1 at 319 K with an effective Arrhenius activation energy of 8.3 ± 2.5 kcal mol-1, kuni(T) = (9.3 × 107)exp(-4200/T) s-1. Our results indicate that unimolecular decay is the major sink of MVKO in the troposphere. The data would improve the estimation for the steady-state concentrations of MVKO and thus its oxidizing ability.

We report a type of highly efficient double hydrogen atom transfer (DHAT) reaction. The reactivities of 3-aminopropanol and 2-aminoethanol towards Criegee intermediates (syn- and anti-CH3 CHOO) were found to be much higher than those of n-propanol and propylamine. Quantum chemistry calculation has confirmed that the main mechanism of these very rapid reactions is DHAT, in which the nucleophilic attack of the NH2 group is catalyzed by the OH group which acts as a bridge of HAT. Typical gas-phase DHAT reactions are termolecular reactions involving two hydrogen bonding molecules; these reactions are typically slow due to the substantial entropy reduction of bringing three molecules together. Putting the reactive and catalytic groups in one molecule circumvents the problem of entropy reduction and allows us to observe the DHAT reactions even at low reactant concentrations. This idea can be applied to improve theoretical predictions for atmospherically relevant DHAT reactions.

Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CH3CHOO, has two conformers, syn- and anti-conformers. Syn-CH3CHOO would undergo fast unimolecular decomposition to form OH radical via 1,4 H-atom transfer. In this work, unimolecular decomposition of syn-CH3CHOO was probed in real time with UV absorption spectroscopy at 278–318 K and 100–700 torr. We used water vapor as the scavenger of anti-CH3CHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH3CHOO (at 300 torr), which increases from (67 ± 15) s−1 at 278 K, (146 ± 31) s−1 at 298 K, to (288 ± 81) s−1 at 318 K with an Arrhenius activation energy of ca. 6.4 kcal mol−1 and a weak pressure dependence for 100–700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of syn-CH3CHOO at higher pressures, which are more relevant to atmospheric conditions.

Isoprene is the most abundant alkene in the atmosphere. Ozonolysis of isoprene produces three very reactive carbonyl oxides (Criegee intermediates), including formaldehyde oxide, methyl vinyl ketone oxide (MVKO, CH3(C2H3)COO), and methacrolein oxide. The latter two Criegee intermediates are resonance-stabilized due to the vinyl group. Recently, the electronic spectrum of thermalized MVKO has been reported [Caravan, et al., Proc. Natl. Acad. Sci. U. S. A., 2020, 117, 9733]. In this work, we utilized this strong UV/visible absorption to investigate the reaction kinetics of MVKO with SO2 under a wide pressure range of 4 to 700 Torr. We followed a new method [Barber, et al., J. Am. Chem. Soc., 2018, 140, 10866], in which MVKO is produced through the reaction of a resonance-stabilized iodoalkene radical with O2. The experimental data are consistent with a kinetic model that the reaction goes through an adduct of CH3(C2H3)CIOO, similar to the cases of H/alkyl substituted Criegee intermediates. However, different from the H/alkyl adducts, which are stable over the time scales of typical kinetic experiments, this vinyl adduct CH3(C2H3)CIOO is less stable and decomposes to MVKO + I at a time scale of 10-3 s (faster at higher temperature), consistent with the results of quantum chemistry calculations and the fact that the resonance stabilization is disrupted at the adduct structure. The adduct decomposition is the major pathway that forms MVKO for pressures higher than 50 Torr. In addition, temperature dependence has been investigated for 278-319 K. The experimental activation energy of the adduct decomposition was measured to be 12.7 ± 0.3 kcal mol-1, consistent with the calculated dissociation energy of the adduct to MVKO + I (14 kcal mol-1). Furthermore, the temperature dependent rate coefficient of MVKO + SO2 reaction has been measured to be kSO2 = (4.0 ± 0.6) × 10-11 cm3 s-1 at 4-700 Torr and 298 K with a negative activation energy of -3.7 ± 0.4 kcal mol-1.

The kinetics of the reaction of the simplest Criegee intermediate CH2OO with CH3SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17) × 10–12 cm3 s–1. The results exhibit a weak negative temperature dependence: the activation energy Ea (k = Ae–Ea/RT) is −1.83 ± 0.05 kcal mol–1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6 × 10–12 cm3 s–1 at 298 K; Ea = – 2.80 kcal mol–1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH2OO with CH3SH are compared to the reactions of CH2OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicat...

We report a very significant cooperative effect of water–ammonia hydrogen bonding in their reactions with a Criegee intermediate, syn-CH3CHOO. Under near ambient conditions, we found that the react...

The reaction of the simplest Criegee intermediate, CH2OO, with ammonia and water vapor has been investigated at 278–308 K and under 100–760 Torr by monitoring the strong UV absorption of CH2OO. We found that the observed decay rate of CH2OO becomes much larger when ammonia and water vapor are both present; the combinational effect of ammonia and water vapor is significantly greater than the sum of their individual contributions, revealing a strong synergic effect. The kinetic data are consistent with a termolecular process of CH2OO + NH3 + H2O reaction, of which the reaction rate coefficient was determined to be kNH3+H2O = (8.2 ± 1.2) × 10−31 cm6 s−1 at 298 K with a negative activation energy, Ea = −8.0 ± 0.8 kcal mol−1 [kNH3+H2O(T) = 1.04 × 10−36 exp(4047/T)]. Quantum chemistry calculation (at the QCISD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2d,2p) level) found a low-energy reaction pathway, on which water accepts a hydrogen atom (or proton) from ammonia and releases another hydrogen atom to the terminal oxygen of CH2OO. The predicted products are H2NCH2OOH and a new H2O molecule, indicating water catalysis. This reaction is very fast and probably barrierless, which poses a theoretical challenge to modeling the related kinetics.

Carbonyl oxides, also known as Criegee intermediates, are generated from ozonolysis of unsaturated hydrocarbons in the atmosphere. Alcohols are often used as a scavenger of the Criegee intermediates in laboratory studies. In this work, the reaction kinetics of CH3CHOO with methanol vapor was investigated at various temperatures, pressures, and isotopic substitutions using time-resolved UV absorption spectroscopy. The observed rate coefficients of the reaction of anti-CH3CHOO with methanol show a linear dependence on [CH3OH]. The bimolecular rate coefficient was determined to be k1Ha = (4.8 ± 0.5) × 10−12 cm3 s−1 at 298 K and 250 Torr with a negative activation energy Ea = −2.8 ± 0.3 kcal mol−1 for T = 288–315 K [k(T) = A exp(−Ea/RT)]. For the reaction of syn-CH3CHOO with methanol vapor, the observed rate coefficients show a quadratic dependence on [CH3OH], indicating that two methanol molecules participate in the reaction. The termolecular rate coefficient was determined to be k2Hs = (8.0 ± 1.0) × 10−32 cm6 s−1 at 298 K and 250 Torr with a strong negative temperature dependence (Ea = −13.2 ± 0.3 kcal mol−1) at 273–323 K. No significant pressure effect was observed at 250–760 Torr. A kinetic isotope effect, k2Hs/k2Ds = 2.5, was observed by changing CH3OH to CH3OD. Quantum chemistry and transition state theory calculations suggest that the observed isotope effect is mainly attributed to the changes of the vibrational zero-point energies and partition functions while tunneling plays a very minor role. The reaction of syn-CH3CHOO with one CH3OH molecule was not observed in the studied concentration range.

Criegee intermediates (CIs) are formed in the ozonolysis of unsaturated hydrocarbons and play a role in atmospheric chemistry as a non-photolytic OH source or a strong oxidant. Using a relative rate method in an ozonolysis experiment, Newland et al. (2015) reported high reactivity of isoprene-derived Criegee intermediates towards dimethyl sulfide (DMS) relative to that towards SO2 with the ratio of the rate coefficients kDMS+CI/kSO2+CI = 3.5 ± 1.8. Here we reinvestigated the kinetics of DMS reactions with two major Criegee intermediates formed in isoprene ozonolysis, CH2OO, and methyl vinyl ketone oxide (MVKO). The individual CI was prepared following the reported photolytic method with suitable (diiodo) precursors in the presence of O2. The concentration of CH2OO or MVKO was monitored directly in real time through their intense UV–visible absorption. Our results indicate the reactions of DMS with CH2OO and MVKO are both very slow; the upper limits of the rate coefficients are 4 orders of magnitude smaller than the rate coefficient reported by Newland et al. (2015) These results suggest that the ozonolysis experiment could be complicated such that interpretation should be careful and these CIs would not oxidize atmospheric DMS at any substantial level.

Isoprene has the highest emission into Earth’s atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO(2) and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO(3) and identifying organic hydroperoxide formation from reaction with SO(2) and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

The ν4 fundamental of the simplest Criegee intermediate, CH2OO, has been monitored with high-resolution infrared (IR) transient absorption spectroscopy under total pressures of 4–94 Torr. This IR spectrum provides an unambiguous identification of CH2OO and is potentially useful to determine the number density of CH2OO in various laboratory studies. Here we utilized an ultraviolet (UV) and IR coupled spectrometer to measure the UV and IR absorption spectra of CH2OO simultaneously; the absolute IR cross section can then be determined by using a known UV cross section. Due to significant pressure broadening in the studied pressure range, we integrated the IR absorption spectra between 1285.2 and 1286.4 cm–1 (covering the Q branch), and then we converted this integrated absorbance to the absolute integral IR cross section of CH2OO (for the Q branch); its absolute value is (3.7 ± 0.6) × 10–19 cm·molecule–1 or 2.2 ± 0.4 km·mol–1. The whole rotational band (P, Q, and R branches) can be adequately simulated by us...

The kinetics of the reaction of the simplest Criegee intermediate (CH2OO) with ammonia has been measured under pseudo-first-order conditions with two different experimental methods. We investigated the rate coefficients at 283, 298, 308, and 318 K at a pressure of 50 Torr using an OH laser-induced fluorescence (LIF) method. Weak temperature dependence of the rate coefficient was observed, which is consistent with the theoretical activation energy of -0.53 kcal mol-1 predicted by quantum chemistry calculation at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. At 298 K, the rate coefficient at 50 Torr from the OH LIF experiment was (5.64 ± 0.56) × 10-14 cm3 molecule-1 s-1 while at 100 Torr we obtained a slightly larger value of (8.1 ± 1.0) × 10-14 cm3 molecule-1 s-1 using the UV transient absorption method. These experimental values are within the theoretical error bars of the present as well as previous theoretical results. Our experimental results confirmed the previous conclusion that ammonia is negligible in the consumption of CH2OO in the atmosphere. We also note that CH2OO may compete with OH in the oxidation of ammonia under certain circumstances, such as at night-time, high altitude and winter time.

The kinetics of CH$_2$OO reaction with ozone has been studied by monitoring CH$_2$OO with time-resolved infrared (IR) absorption spectroscopy, which utilized fast chirped IR pulse train from a quantum cascade laser [J. Chem. Phys. 146, 244302 (2017)]. CH$_2$OO was prepared by photolyzing a gas mixture of CH$_2$I$_2$/O$_2$/O$_3$ at 352 nm; the photolysis wavelength was chosen to minimize the photodissociation of O$_3$. The measured rate coefficient at 298 K and 30 Torr is (6.7$\pm$0.5)x10$^{-14}$ cm$^3$sec$^{-1}$, independent of pressure from 30 to 100 Torr. The result indicates that previous ab initio calculations either underestimated or overestimated this reaction rate by one order of magnitude or more. The result also implies that in laboratory studies of ozonolysis of alkenes, the reaction of Criegee intermediate with ozone may play a role. However, this reaction would not compete with other CH$_2$OO sinks in the atmosphere.

Recent theoretical work by Kumar and Francisco suggested that the high reactivity of Criegee intermediates (CIs) could be utilized for designing efficient carbon capture technologies. Because the anti-CH3CHOO + CO2 reaction has the lowest barrier in their study, we chose to investigate it experimentally. We probed anti-CH3CHOO with its strong UV absorption at 365 nm and measured the rate coefficient to be ≤2 × 10–17 cm3 molecule–1 s–1 at 298 K, which is consistent with our theoretical value of 2.1 × 10–17 cm3 molecule–1 s–1 at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level but inconsistent with their results obtained at the M06-2X/aug-cc-pVTZ level, which tends to underestimate the barrier heights. The experimental result indicates that the reaction of a Criegee intermediate with atmospheric CO2 (400 ppmv) would be inefficient (keff < 0.2 s–1) and cannot compete with other decay processes of Criegee intermediates like reactions with water vapor (∼103 s–1) or thermal decomposition (∼102 s–1).

The ultraviolet (UV) absorption spectrum of the simplest deuterated Criegee intermediate CD2OO has been measured and compared with that of CH2OO. While the UV spectra of CH2OO and CD2OO are similar in the overall shape, distinctive oscillatory structures at the long wavelength side of the absorption band show clear effect of isotopic substitution. The average spacing between the vibrational peaks decreases from 606 cm−1 for CH2OO to 528 cm−1 for CD2OO. This large isotope effect cannot be explained by one-dimensional model along the dissociative O−O bond. Instead, vibrational modes involving motions of the H-atoms are expected to be responsible for the observed oscillatory structure. This isotope effect offers a stringent test for theoretical investigations on the absorption spectrum and excited-state dynamics of the simplest Criegee intermediate.

The reaction of the simplest Criegee intermediate CH2OO with hydrogen sulfide was measured with transient UV absorption spectroscopy in a temperature-controlled flow reactor, and bimolecular rate coefficients were obtained from 278 to 318 K and from 100 to 500 Torr. The average rate coefficient at 298 K and 100 Torr was (1.7 ± 0.2) × 10–13 cm3 s–1. The reaction was found to be independent of pressure and exhibited a weak negative temperature dependence. Ab initio quantum chemistry calculations of the temperature-dependent reaction rate coefficient at the QCISD(T)/CBS level are in reasonable agreement with the experiment. The reaction of CH2OO with H2S is 2–3 orders of magnitude faster than the reaction with H2O monomer. Though rates of CH2OO scavenging by water vapor under atmospheric conditions are primarily controlled by the reaction with water dimer, the H2S loss pathway will be dominated by the reaction with monomer. The agreement between experiment and theory for the CH2OO + H2S reaction lends creden...

International audience; Recent works [Jara-Toro et al., Angew. Chem. Int. Ed. 2017, 56, 2166 and PCCP 2018, 20, 27885] suggest that the rate coefficient of OH reactions with alcohols would increase by up to 2 times from dry to high humidity. This finding would have an impact on the budget of alcohols in the atmosphere and that it may explain differences in measured and modeled methanol concentrations. The results were based on a relative technique carried out in a small Teflon bag, which might suffer from wall reactions. We have re-investigated this effect using a direct fluorescence probe of OH radicals, and no catalytic effect of H2O could be found. Experiments in a Teflon bag were also carried out, but we were not able to reproduce the results of Jara-Toro et al. Further theoretical calculations show that the water-mediated reactions have negligible rates compared to the bare reaction and that even though water molecules can lower the barriers of reactions, it cannot make up for the entropy cost. Methanol (CH3OH) is one of the most abundant oxygenated volatile organic compounds (OVOC) in the atmosphere [1, [2]. Direct emissions are the main source, but some oxidation pathways of methane also contribute to its abundance, especially in the remote troposphere [3, [4, [5]. Concentrations range from 1-15 ppbv in the continental boundary layer and up to 1 ppbv in the remote troposphere [6, [7]. Despite numerous efforts, global atmospheric chemical models are presently unable to reconcile the modeled and measured methanol concentrations [7, [8, [9]. The atmospheric degradation of CH3OH is governed by its reaction with OH radicals, which proceeds by abstraction of H-atoms from either the methyl or the hydroxyl site [10]. The rate coefficient shows a non-Arrhenius behavior and increases at temperatures below 200 K, due to enhanced stabilization of a pre-reactive H-bonded complex that can undergo tunneling [10]. The rate coefficient at room temperature (298 K) has been measured many times [11, [12] and is recommended [13] to be k1 = 9.0 × 10-13 cm 3 s-1. Very recently, Jara-Toro et al. [14] reported that the reaction of OH with CH3OH is significantly catalyzed by water (enhanced by a factor of 2 between 20 to 95% of relative humidity) even at 294 K. The same group also reported the water catalysis effect on reactions of OH with ethanol and n-propanol [15]. Their results are based on a well-known relative method, where the consumption of the alcohol is measured relative to the consumption of a reference compound (C5H12) following their simultaneous reactions with OH radicals. The reactions took place in a 80 liter Teflon bag, and OH radicals were generated continuously from the 254 nm photolysis of H2O2. The ratio of the consumption of CH3OH versus the consumption of C5H12 gives the ratio of the rate coefficients, kCH 3 OH+OH / kRef+OH. This type of experiments was carried out at different relative humidity (RH) up to 95%, and it was found that the ratio increased with increasing RH. Under the assumption that the rate of the OH reaction with unpolar C5H12 is independent on RH, this increased ratio was assigned to an increased rate of OH reaction with CH3OH due to water catalysis. More surprisingly, they found a quadratic dependence of the rate enhancement with RH, and speculated that this effect was dismissed in earlier works, because earlier experiments were commonly carried out under low RH, where the water effect would be too weak to observe. Accompanying theoretical work proposed a mechanism of reaction barrier lowering by adding H2O. However, no rate coefficient has been calculated to estimate the impact of this lowering in the reaction barrier heights. Using the rate coefficients taking into account the water catalytic effect could decrease the atmospheric lifetime of CH3OH by a factor of 2 in tropical region with high RH, which would have a non-negligible effect on the global CH3OH budget. It is therefore important to verify this new finding by using different methods. Indeed, the experiments of Jara-Toro et al. have been carried out in a relatively small Teflon bag, where heterogeneous consumption of CH3OH on the walls may take place. This effect may increase with RH and may become complicated when photochemistry occurs (which produces radicals and radical reactions may change the wall property). They did not vary the surface to volume ratio to quantify this effect. In this work, we have reinvestigated the influence of H2O on the rate coefficient of OH reaction with CH3OH using a direct method: laser photolysis coupled to a time-resolved detection of OH radicals by laser induced fluorescence (LIF) after gas expansion (FAGE-Fluorescence Assay by Gas Expansion) [12, [16]. We have also repeated experiments using the same relative method as Jara-Toro et al., and in addition to varying RH, we [a]

The role of water in gas-phase reactions has gained considerable interest. Here we report a direct kinetic measurement of the reaction of syn-CH3CHOO (a Criegee intermediate or carbonyl oxide) with methanol at various relative humidity (RH = 0–80%) under near-ambient conditions (298 K, 250–755 Torr). The data indicate that a single water molecule expedites the reaction by up to a factor of three. The rate coefficient of the corresponding reaction, syn-CH3CHOO + CH3OH + H2O → products, has been determined to be (1.95 ± 0.11) × 10–32 cm6 s–1 at 298 K, with no observable pressure dependence for 250–755 Torr. Quantum chemistry calculation shows that the dominating pathway involves a hydrogen-bonded ring structure, in which methanol is donating a hydrogen atom to water, water is donating a hydrogen atom to the terminal oxygen atom of the Criegee intermediate, and, on the product side, H2O is reformed and acts as a catalyst.

The absolute absorption cross sections of (CH 3 ) 2 COO under a jet-cooled condition were measured via laser depletion to be (1.32 ± 0.10) × 10 −17 cm 2 molecule −1 at 308 nm and (9.6 ± 0.8) × 10 −18 cm 2 molecule −1 at 352 nm. The peak UV cross section is estimated to be (1.75 ± 0.14) × 10 −17 cm 2 molecule −1 at 330 nm, according to the UV spectrum of (CH 3 ) 2 COO (Huang et al., 2015) scaled to the absolute cross section at 308 nm.

The unimolecular decomposition of (CH3)2COO and (CD3)2COO was measured by direct detection of the Criegee intermediate at temperatures from 283 to 323 K using time-resolved UV absorption spectroscopy. The unimolecular rate coefficient kd for (CH3)2COO shows a strong temperature dependence, increasing from 269 ± 82 s(-1) at 283 K to 916 ± 56 s(-1) at 323 K with an Arrhenius activation energy of ∼6 kcal mol(-1). The bimolecular rate coefficient for the reaction of (CH3)2COO with SO2, kSO2, was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for kd and kSO2 are consistent with previously reported relative rate coefficients kd/kSO2 of (CH3)2COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of kd for (CH3)2COO are consistent with the experiment, and the combination of experiment and theory for (CD3)2COO indicates that tunneling plays a significant role in (CH3)2COO unimolecular decomposition. The fast rates of unimolecular decomposition for (CH3)2COO measured here, in light of the relatively slow rate for the reaction of (CH3)2COO with water previously reported, suggest that thermal decomposition may compete with the reactions with water and with SO2 for atmospheric removal of the dimethyl-substituted Criegee intermediate.

Organically modified layered silicates (OLS) were prepared via ionic exchanging intercalation of natural clays, such as sodium montmorillonite (MMT), by fatty alkyl amines, such as n-octadecylamine (ODA), and used for hydrocarbon recovery. Comparisons of OLS modified by different stoichiometries of ODA and by poly(oxypropylene)amines were conducted. OLS were characterized by X-ray diffraction (XRD) to study the correlation between their basal interlayer spacings and the efficiency of their oil absorption. With different ratios of ODA to the cationic exchanging capacity (CEC) of clay, the basal spacings were expanded from the pristine 12 A to 28 A (0.5 CEC), 30 A (1.0 CEC), 30 A (1.5 CEC) and 34 A (2.0 CEC). After crude oil absorption, the OLS were further expanded in interlayer spacings to 45–48 A or even evolved into a featureless via XRD. The observed oil absorption efficiencies were up to 9 times by mass that of oil-on-clay; moreover, OLS/oil aggregation into lumps from water was observed. The oil absorption–desorption cycle was repeated three times, demonstrating the feasibility of clay reuse and oil recovery. Our findings revealed the stability of the clay/oil structure and the process suitability of fatty amines as the intercalating organics. The high oil-absorbing efficiency and recyclability indicate suitability of OLS for practical applications, including in the remedy of oil pollution in water, enhancing oil recovery and hydraulic fracture and the shale-oil fracking process.

In this study, we performed ab initio calculations and obtained the bimolecular rate coefficients for the CH2OO/CH3CHOO reactions with H2O/(H2O)2. The energies were calculated with QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) and the partition functions were estimated with anharmonic vibrational corrections by using the second order perturbation theory. Furthermore, we directly measured the rate of the CH2OO reaction with water vapor at high temperatures (348 and 358 K) to reveal the contribution of the water monomer in the CH2OO decay kinetics. We found that the theoretical rate coefficients reproduce the experimental results of CH2OO for a wide range of temperatures. For anti- (syn-) CH3CHOO, we obtained theoretical rate coefficients of 1.60 × 10(-11) (2.56 × 10(-14)) and 3.40 × 10(-14) (1.98 × 10(-19)) cm(3) s(-1) for water dimer and monomer reactions at room temperature. From the detailed analysis of the quantum chemistry and approximations for the thermochemistry calculations, we conclude that our calculated values would be within a factor of 3 of the correct values. Furthermore, at [H2O] = 1 × 10(17) to 5 × 10(17) cm(-3), we estimate that the effective first-order rate coefficients for CH2OO, anti- and syn-CH3CHOO reactions with water vapor will be ∼10(3), ∼10(4), and ∼10(1) s(-1), respectively. Thereby, for Criegee intermediates with a hydrogen atom on the same side as the terminal oxygen atom, the reaction with water vapor will likely dominate the removal processes of these CIs in the atmosphere.

The kinetics of the reaction of CH2OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH2OO decay rate is second order with respect to the H2O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH2OO reaction with water dimer can be described by an Arrhenius expression k(T) = A exp(-Ea/RT) with an activation energy of -8.1 ± 0.6 kcal mol(-1) and k(298 K) = (7.4 ± 0.6) × 10(-12) cm(3) s(-1). Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH2OO by a factor of ~2.5 at 278 K and decreases by a factor of ~2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere.

The reactions of Criegee intermediates with NO2 have been proposed as a potentially significant source of the important nighttime oxidant NO3, particularly in urban environments where concentrations of ozone, alkenes and NOx are high. However, previous efforts to characterize the yield of NO3 from these reactions have been inconclusive, with many studies failing to detect NO3. In the present work, the reactions of formaldehyde oxide (CH2OO) and acetaldehyde oxide (CH3CHOO) with NO2 are revisited to further explore the product formation over a pressure range of 4–40 Torr. NO3 is not observed; however, temporally resolved and [NO2]-dependent signal is observed at the mass of the Criegee–NO2 adduct for both formaldehyde- and acetaldehyde-oxide systems, and the structure of this adduct is explored through ab initio calculations. The atmospheric implications of the title reaction are investigated through global modelling.

We investigate the photolytic production of two radical intermediates in the reaction of OH with propene, one from addition of the hydroxyl radical to the terminal carbon and the other from addition to the center carbon. In a collision-free environment, we photodissociate a mixture of 1-bromo-2-propanol and 2-bromo-1-propanol at 193 nm to produce these radical intermediates. The data show two primary photolytic processes occur: C-Br photofission and HBr photoelimination. Using a velocity map imaging apparatus, we measured the speed distribution of the recoiling bromine atoms, yielding the distribution of kinetic energies of the nascent C3H6OH radicals + Br. Resolving the velocity distributions of Br((2)P(1/2)) and Br((2)P(3/2)) separately with 2 + 1 REMPI allows us to determine the total (vibrational + rotational) internal energy distribution in the nascent radicals. Using an impulsive model to estimate the rotational energy imparted to the nascent C3H6OH radicals, we predict the percentage of radicals having vibrational energy above and below the lowest dissociation barrier, that to OH + propene; it accurately predicts the measured velocity distribution of the stable C3H6OH radicals. In addition, we use photofragment translational spectroscopy to detect several dissociation products of the unstable C3H6OH radicals: OH + propene, methyl + acetaldehyde, and ethyl + formaldehyde. We also use the angular momenta of the unstable radicals and the tensor of inertia of each to predict the recoil kinetic energy and angular distributions when they dissociate to OH + propene; the prediction gives an excellent fit to the data.

SO2 scavenging and self-reaction of CH2OO were utilized for the decay of CH2OO to extract the absorption spectrum of CH2OO under bulk conditions. Absolute absorption cross sections of CH2OO at 308.4 and 351.8 nm were obtained from laser-depletion measurements in a jet-cooled molecular beam. The peak cross section is (1.23 ± 0.18) × 10(-17) cm(2) at 340 nm.

Clusters are important in bridging the gap between individual molecules in the gas phase and closely interacting molecules in condensed phases. The size of a cluster is a basic parameter that a researcher would like to control or measure. Whereas the size determination of ionic clusters is rather straightforward, determination of the size of a neutral cluster is much more difficult. Only a few methods are available for the size determination of a neutral cluster beam, including: 1) momentum transfer in crossed-beam scattering, 2) diffraction from a transmission grating, and 3) high-resolution spectroscopy. Ozone is not only a crucial molecule in our atmosphere but also a benchmark molecule in photochemistry and photophysics. Theoretical studies on its photodissociation were recently reviewed by Grebenshchikov et al. Although quite a few excited states and complicated non-adiabatic couplings are involved, high-level theoretical calculations can describe the relevant experimental observables of ozone such as its absorption spectrum, product distributions, etc. The detailed information about the ozone molecule may offer a good starting point for investigations of ozone clusters. Probst et al. synthesized the ozone dimer in a molecular beam and probed it with high-resolution electron-impact ionization near threshold. They also performed ab initio calculations on the ozone dimer and its possible ionic structures, mostly by means of density functional calculations with the B3LYP functional, and checked the results with a variety of other methods, such as CASSCF and QCISD or CCSD(T). For the neutral ozone dimer, their calculations indicate that two ozone molecules are only very weakly bound to each other. This result also agrees with the small MP2 dimerization energy calculated by Slanina and Adamowicz. Bahou et al. investigated the infrared spectroscopy and photochemistry at 266 nm of the ozone dimer trapped in an argon matrix. The observed frequency shift of the ozone antisymmetric stretching mode upon dimerization is small, about 1 to 3 cm , which is similar in magnitude to those induced by different trapping sites in the argon matrix, thus indicating weak interconstituent interactions in the dimer. In their photodissociation experiment, the photolysis crosssection of the matrix-isolated ozone dimer was estimated to be 1.5 10 18 cm at 266 nm, substantially smaller than the absorption cross-section of the ozone monomer in the gas phase (9.1 10 18 cm). It should be noted that the photolysis cross-section is the product of the absorption cross-section and the dissociation quantum yield; in the matrix environment the dissociation quantum yield is hard to establish. Herein, we report the synthesis of ozone clusters in a molecular beam by supersonic expansion. The photolysis crosssections of the ozone clusters were measured in a mass-resolved manner (with an electron-impact-ionization mass spectrometer) at selected excitation wavelengths (l). By tuning the temperature and backing pressure before the expansion, we were able to generate ozone clusters of different size distributions. Figure 1 shows typical electron-impact mass spectra of the ozone molecular beam under two expansion conditions. Evidence of cluster formation can be clearly seen: peaks at masses (m/z) 80 (O5 ), 96 (O6 ), 128 (O8 ), and 144 (O9 ) indicate that not only the dimer but also larger clusters could be formed. Owing to dissociative ionization in the electron-impact ionization process, the mass 80 peak is the most intense peak other than the monomer mass peaks (O3 + , masses 48 and 50). The relative intensities of [a] I.-C. Chen, A. F. Chen, W.-T. Huang, Dr. K. Takahashi, Prof. Dr. J. J. Lin Institute of Atomic and Molecular Sciences Academia Sinica PO Box 23-166, Taipei 10617 (Taiwan) Fax: (+886)2-23620200 E-mail : jimlin@gate.sinica.edu.tw [b] W.-T. Huang, Prof. Dr. J. J. Lin Department of Chemistry National Taiwan University Taipei 10617 (Taiwan) [c] Prof. Dr. J. J. Lin Department of Applied Chemistry National Chiao Tung University Hsinchu 30010 (Taiwan)

Using a crossed laser-molecular beam scattering apparatus and tunable photoionization detection, these experiments determine the branching to the product channels accessible from the 2-hydroxyethyl radical, the first radical intermediate in the addition reaction of OH with ethene. Photodissociation of 2-bromoethanol at 193 nm forms 2-hydroxyethyl radicals with a range of vibrational energies, which was characterized in our first study of this system ( J. Phys. Chem. A 2010 , 114 , 4934 ). In this second study, we measure the relative signal intensities of ethene (at m/e = 28), vinyl (at m/e = 27), ethenol (at m/e = 44), formaldehyde (at m/e = 30), and acetaldehyde (at m/e = 44) products and correct for the photoionization cross sections and kinematic factors to determine a 0.765:0.145:0.026:0.063:0.01 branching to the OH + C(2)H(4), H(2)O + C(2)H(3), CH(2)CHOH + H, H(2)CO + CH(3), and CH(3)CHO + H product asymptotes. The detection of the H(2)O + vinyl product channel is surprising when starting from the CH(2)CH(2)OH radical adduct; prior studies had assumed that the H(2)O + vinyl products were solely from the direct abstraction channel in the bimolecular collision of OH and ethene. We suggest that these products may result from a frustrated dissociation of the CH(2)CH(2)OH radical to OH + ethene in which the C-O bond begins to stretch, but the leaving OH moiety abstracts an H atom to form H(2)O + vinyl. We compare our experimental branching ratio to that predicted from statistical microcanonical rate constants averaged over the vibrational energy distribution of our CH(2)CH(2)OH radicals. The comparison suggests that a statistical prediction using 1-D Eckart tunneling underestimates the rate constants for the branching to the product channels of OH + ethene, and that the mechanism for the branching to the H(2)O + vinyl channel is not adequately treated in such theories.

The reactions of F 2 with isomers of butene were investigated with crossed-beam reactive scattering and ab initio computations. Only one product channel F + C 4 H 8 F was observed in the experiments; the HF + C 4 H 7 F channel was searched but not found. The reaction path of F 2 + C 4 H 8 → F + C 4 H 8 F was studied theoretically. Very low barriers of less than 1 kcal/mol were found for the F 2 reactions with cis -2-butene, trans -2-butene, and isobutene, which all contain two methyl substitution groups. This theoretical prediction, yet to be confirmed experimentally, suggests that double methyl substituted C C double bonds can react with F 2 almost without a barrier.

The photochemistry of the ClO dimer (ClOOCl) plays a central role in the catalytic destruction of polar stratospheric ozone. In spite of decades of intense investigations, some of its laboratory photochemical data had not reached the desired accuracy to allow a reliable simulation of the stratospheric ozone loss until recently. Inevitable impurities in ClOOCl samples have obstructed conventional measurements. In particular, an absorption measurement of ClOOCl in 2007, which gave much lower cross sections than previous studies, implied that the formation of the ozone hole cannot be explained with current chemical models. Scientists have wondered whether the model is insufficient or the data is erroneous. Efforts aiming to resolve this controversy are reviewed in this paper, which emphasizes newly developed experiments to determine two critical photochemical properties of ClOOCl--its absorption cross section and product branching ratio--including the first reported product branching ratio at 351.8 nm photolysis.

Following up a previous publication (J. Chem. Phys. 121, 813 (2004)) on the observation of a reactive resonance in the integral cross-section of the title reaction, this report elucidates the resonance fingerprints in the pair-correlated differential cross section (CDCS). By using a time-sliced, velocity-mapped (ion) imaging technique, we measured the DCS in a product pair-correlated manner under the crossed-beam conditions. Here, the focus is on the major vibration states of the two isotopic product channels: the CD3(v = 0) + HF(v) and CHD2(v = 0) + DF(v). A distinct ridge structure near reaction threshold that followed by oscillatory forward-backward peaking features is clearly revealed in the E c (collision energy)-evolution of the angular distribution of the paired products. The observed patterns are attributed to the characteristic imprints of a resonant reaction pathway. More intriguing is the observation that these resonance signatures occur at about the same collision energies in both HF + CD3 and...

We report mass resolved photoionization yield spectra of a molecular-beam cooled sample of HN(3) using photoionization mass spectrometry based on high-resolution monochromatized synchrotron radiation. Spectra are reported at mass-to-charge ratios of 14(N(+)), 15(NH(+)), 29(N(2)H(+)), 42(N(3)(+)), and 43(HN(3)(+)) in the region of each ionization threshold. The thresholds observed here are all lower than the previously reported ones obtained with electron impact ionization, which were the only ones available in the literature prior to this work. The appearance energies can be used to evaluate four key thermochemical quantities of relevance to HN(3): D(0)(H-N(3)), D(0)(N-N(2)), D(0)(N-H), and IE(NH). We observe the appearance energy of the parent ion (HN(3)(+)) to be 10.56 +/- 0.02 eV, somewhat below the reported ionization energies derived from photoelectron spectroscopy. Great care was taken to evaluate the importance of vibrational hot bands to the photoionization yield spectra. This experiment also provides a lower limit to the proton affinity of N(2) allowing us to bracket this quantity with improved certainty: 119.3 kcal/mol

High-resolution synchrotron-radiation-based photoionization mass spectrometry was applied to study the dissociative photoionization of ClN 3 under collision-free molecular beam conditions at ionization energies between 10 and 17 eV. No parent ion (ClN 3 + ) could be detected under our experimental conditions. This suggests that the ground and excited states of ClN 3 + are weakly bound or repulsive, a conclusion supported by electronic structure calculations also reported here. We recorded photoionization yield spectra at m / z = 49, 42, 35 and 14 from which we extracted the appearance potentials for NCl + , N 3 + , Cl + , and N + . The appearance potential of NCl + (10.17 ± 0.02 eV) observed here is close to the previously reported ionization potential of ClN 3 obtained form photoelectron spectroscopy. Using the theoretically calculated binding energy of ClN 3 + (0.2 eV), we derive an estimate of the adiabatic ionization potential of ClN 3 = 9.97 ± 0.02 eV. The measured appearance potentials for N 3 + , Cl + , and N + provide three independent determinations of the Cl–N bond energy in ClN 3 , which agree within their respective error limits. The observations of this work are consistent with a new value of the N–Cl bond energy in ClN 3 , D 0 (Cl–N 3 ) = 1.86 ± 0.05 eV, 0.3 eV lower than previously reported values, which are however experimentally derived upper limits. The bond energy reported here is consistent with high level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit, which yield a value: D 0 (Cl–N 3 ) = 1.87 eV.

Using a crossed-molecular-beam apparatus and a synchrotron as a tunable vacuum-ultraviolet light source, we investigated oxysilanes formed in the reaction of atomic oxygen with silane. We measured the time-of-flight spectra, photoionization yields, and branching ratios of dissociative ionization of reaction products H 3 SiO/H 2 SiOH, H 2 SiO/HSiOH, HSiO/SiOH and SiO as a function of photoionization energy. The observed ionization thresholds correspond to isomeric products H 2 SiOH, HSiOH, and SiOH. The H 2 -loss channel has a branching ratio greater than the H-loss channel for the dissociative ionization of H 3 SiO/H 2 SiOH, whereas this situation is reversed for H 2 SiO/HSiOH, which we interpret according to the enthalpies of reactions.

The recombination of oxygen atoms with oxygen molecules to form ozone exhibits several strange chemical characteristics, including unusually large differences in formation rate coefficients when different isotopes of oxygen participate. Purely statistical chemical reaction rate theories cannot describe these isotope effects, suggesting that reaction dynamics must play an important role. We investigated the dynamics of the 18O + 32O2 --O3(*) --16O + 34O2 isotope exchange reaction (which proceeds on the same potential energy surface as ozone formation) using crossed atomic and molecular beams at a collision energy of 7.3 kcal mol(-1), providing the first direct experimental evidence that the dissociation of excited ozone exhibits significant nonstatistical behavior. These results are compared with quantum statistical and quasi-classical trajectory calculations in order to gain insight into the potential energy surface and the dynamics of ozone formation.

Criegee intermediates are thought to play a role in atmospheric chemistry, in particular, the oxidation of SO2, which produces SO3 and subsequently H2SO4, an important constituent of aerosols and acid rain. However, the impact of such oxidation reactions is affected by the reactions of Criegee intermediates with water vapor, because of high water concentrations in the troposphere. In this work, the kinetics of the reactions of dimethyl substituted Criegee intermediate (CH3)2COO with water vapor and with SO2 were directly measured via UV absorption of (CH3)2COO under near-atmospheric conditions. The results indicate that (i) the water reaction with (CH3)2COO is not fast enough (kH2O < 1.5 × 10(-16) cm(3) s(-1)) to consume atmospheric (CH3)2COO significantly and (ii) (CH3)2COO reacts with SO2 at a near-gas-kinetic-limit rate (kSO2 = 1.3 × 10(-10) cm(3) s(-1)). These observations imply a significant fraction of atmospheric (CH3)2COO may survive under humid conditions and react with SO2, very different from the case of the simplest Criegee intermediate CH2OO, in which the reaction with water dimer predominates in the CH2OO decay under typical tropospheric conditions. In addition, a significant pressure dependence was observed for the reaction of (CH3)2COO with SO2, suggesting the use of low pressure rate may underestimate the impact of this reaction. This work demonstrates that the reactivity of a Criegee intermediate toward water vapor strongly depends on its structure, which will influence the main decay pathways and steady-state concentrations for various Criegee intermediates in the atmosphere.

This work investigates the unimolecular dissociation of the methoxycarbonyl, CH(3)OCO, radical. Photolysis of methyl chloroformate at 193 nm produces nascent CH(3)OCO radicals with a distribution of internal energies, determined by the velocities of the momentum-matched Cl atoms, that spans the theoretically predicted barriers to the CH(3)O + CO and CH(3) + CO(2) product channels. Both electronic ground- and excited-state radicals undergo competitive dissociation to both product channels. The experimental product branching to CH(3) + CO(2) from the ground-state radical, about 70%, is orders of magnitude larger than Rice-Ramsperger-Kassel-Marcus (RRKM)-predicted branching, suggesting that previously calculated barriers to the CH(3)OCO --> CH(3) + CO(2) reaction are dramatically in error. Our electronic structure calculations reveal that the cis conformer of the transition state leading to the CH(3) + CO(2) product channel has a much lower barrier than the trans transition state. RRKM calculations using this cis transition state give product branching in agreement with the experimental branching. The data also suggest that our experiments produce a low-lying excited state of the CH(3)OCO radical and give an upper limit to its adiabatic excitation energy of 55 kcal/mol.

The ground state reactions of Cl + CH4 and CD4 were studied in a crossed-beam experiment, in which the ground state methyl products were probed using a time-sliced velocity imaging technique. By taking the images over the energy range of chemical significance - from reaction threshold to about 10 kcal/mol, the reactive excitation functions as well as the dependences of product angular distributions and of the energy disposals on initial collision energies were obtained. Comparing the dynamical attributes of the two isotopic reactions revealed an interesting isotope effect in product angular distributions. The previous reduced-dimensional quantum scattering calculations of Yu and Nyman appear to reproduce this isotope effect rather well. Yet, a physical understanding of its origin awaits further theoretical investigations.

We report time-of-flight spectra of fragments from photodissociation of various molecules using tunable radiation from a synchrotron for ionization. Ionization energy, population of electronic states, and distribution of internal energy of products result in varied quantum yields for ionization. We successfully used soft photoionization to detect, among other species, atomic D, O (1D), HCOH and CH2 ( a ˜ A 1 1 ) without interference from H2, O (3P), CH2O and CH2 ( X ˜ B 1 3 ), respectively.

Concentrations and distributions of three major water-soluble ion species (sulfate, nitrate, and ammonium) contained in ambient particles were measured at three sampling sites in the Kao-ping ambient air quality basin, Taiwan. Ambient particulate matter (PM) samples were collected in a Micro-orifice Uniform Deposit Impactor from February to July 2003 and were analyzed for water-soluble ion species with an ion chromatograph. The PM1/ PM2.5 and PM1/PM10 concentration ratios at the emission source site were 0.73 and 0.53 and were higher than those (0.68 and 0.48) at the background site because there are more combustion sources (i.e., industrial boilers and traffic) around the emission source site. Mass-size distributions of PM NO3- were found in both the fine and coarse modes. SO4(2-)and NH4+ were found in the fine particle mode (PM2.5), with significant fractions of submicron particles (PM1). The source site had higher PM1/PM10(79, 42, and 90%) and PM1/PM2.5 concentration ratios (90, 58, and 93%) for the three major inorganic secondary aerosol components (SO4(2-), NO3-, and NH4+) than the receptor site (65, 27, and 65% for PM1/PM10, 69, 51, and 70% for PM1/PM2.5. Results obtained in this study indicate that the PM1 (submicron aerosol particles) fraction plays an important role in the ambient atmosphere at both emission source and receptor sites. Further studies regarding the origin and formation of ambient secondary aerosols are planned.

The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF+CD3 product channel is surmized, which echoes the reactive resonances found previously for the F+HD--HF+D reaction and more recently for the F+CH4 reaction.

The dynamics of the 18O(1D) + 44CO2 oxygen isotope exchange reaction has been studied using a crossed molecular beam apparatus at collision energies of 4.2 and 7.7 kcal/mol. At both collision energies, two reaction channels are observed: isotope exchange in which quenching to O(3P) occurs and isotope exchange in which the product oxygen atom remains on the singlet surface. Electronic quenching of O(1D) is the major channel at both collision energies, accounting for 84% of isotope exchange at 4.2 kcal/mol and 67% at 7.7 kcal/mol. Both channels proceed via a CO3* complex that is long-lived with respect to its rotational period. Combined with recent ab initio and statistical calculations by Mebel et al., the long complex lifetimes suggest that statistical isotope exchange occurs in the CO3* complex (apart from zero-point energy isotope effects), although the existence of a small, dynamically driven unconventional isotope effect in this reaction cannot yet be ruled out. These new molecular-level details may ...

Ambient particulate matter mass concentrations were measured in terms of PM1 and PM1−10 for 8 months in 2000–2001, at a sampling site in the urban city of Kaohsiung, Taiwan. On average for the 8-month samples, 52±20% of the PM10 was made up of PM1. The concentration of particulate n-alkanes was in the range of 75–265 ng m−3 in PM1 and 89–346 ng m−3 in PM10, considerably higher than the concentration levels measured at other urban areas. The mass content of n-alkanes in ambient submicron aerosol particles (PM1)(0.38%) was statistically higher than that in PM1−10 (0.13) and suggests that the n-alkanes exist mainly in PM1 in Kaohsiung city. The concentration ratio of the homologues (C19–C33) of alkanes indicated that homologues were associated with submicron particles in general and that there was a high level of fractions (0.87±0.09 for C19) in PM1 for low carbon n-alkanes than for high carbon n-alkanes (0.64±0.09 for C33). For all the collected samples in this study, the average and standard deviation of CPI (carbon preference index) was 1.26±0.17 for PM1 and 1.39±0.28 for PM1−10. The plant wax contributions to the total alkanes for PM1 and PM1−10 were estimated to be 13.4% (5.9–21.7%) and 17.1% (4.8–32.0%), respectively. These observed results suggest that the organic matter in the submicron aerosol particles (PM1) in Kaohsiung city contained compounds that were predominantly contributed by anthropogenic sources with only a minor contribution from biogenic sources.

The reaction of atomic chlorine with methane was studied in crossed-beam experiments at a collisional energy of Ec ≈ 4.7 kcal/mol. Using a time-sliced product imaging technique, several outstanding...

Contains fulltext : 60528.pdf (Publisher’s version ) (Open Access) The velocity map ion imaging technique was applied to measure the reaction excitation function for the first time. It was found that the "raw" excitation function was significantly distorted by the density-to-flux transformation of the title reaction. Through a systematic investigation, possible reasons for such a dramatic effect are outlined. In addition, the state-resolved, pair-correlated excitation functions and branching ratios are presented. Effects of imperfect time slicing in the time-sliced velocity imaging technique in general are also discussed. (C) 2004 American Institute of Physics.