Peter Neeb

Formation of hydroxymethyl hydroperoxide and formic acid in alkene ozonolysis in the presence of water vapour

Abstract Ozonolysis experiments of a series of terminal alkenes were performed to study the products formed in the presence of water vapour. Alkenes investigated were ethene, propene, isobutene and isoprene. Concentrations of the reactants were 4–6 ppmv alkene and 2 ppm ozone. The concentration of water vapour was varied from 0.5 ppmv to 17,000 ppmv. Hydroxymethyl hydroperoxide (HMHP) was found to be the sole product in the reactions of the stabilized Criegee biradical CH2OO with water vapour. The yield of HMHP relative to ozone consumption was measured to be 42% ethene, 14% propene, 13% isobutene and 30% isoprene in the presence of 9,000–18,000 ppmv H2O. HMHP was not stable under the experimental conditions and decomposed to HCOOH and water, presumably via a heterogeneous process. The atmospheric fate of HMHP and the relative rate constants of the CH2OO Criegee biradical with H2O, HCOOH and NOx are discussed.

Products and Mechanism of the Gas Phase Reaction of Ozone with β-Pinene

Gas phase ozonolysis of β-pinene was performedin a 570 l static reactor at 730 Torr and 296 K insynthetic air and the products were analysed by acombination of gas phase FTIR spectroscopy, HPLC andIC analyses of gas phase and aerosol samples,respectively. The reaction mechanism was investigatedby adding HCHO, HCOOH and H2O as Criegeeintermediate scavenger and cyclohexane as OH radicalscavenger. Main identified products (yields inparentheses) in the presence of cyclohexane as OHradical scavenger were HCHO (0.65 ± 0.04),nopinone (0.16 ± 0.04), 3-hydroxy-nopinone (0.15± 0.05), CO2 (0.20 ± 0.04), CO (0.030± 0.002), HCOOH (0.020 ± 0.002), the secondaryozonide of β-pinene (0.16 ± 0.05), andcis-pinic acid (0.02 ± 0.01). The decompositionof the primary ozonide was found to yieldpredominantly the excited C9-Criegee intermediateand HCHO (0.84 ± 0.04) and to a minor extent theexcited CH2OO intermediate and nopinone (0.16± 0.04). Roughly 40% of the excitedC9-Criegee intermediate becomes stabilised andcould be shown to react with HCHO, HCOOH and H2O. The atmospherically important reaction of thestabilised C9-Criegee intermediate with H2Owas found to result in a nopinone increase of (0.35± 0.05) and in the formation of H2O2(0.24 ± 0.03). Based on the observed products,the unimolecular decomposition/isomerisationchannels of the C9-Criegee intermediate arediscussed in terms of the hydroperoxide and esterchannels. Subsequent reactions of the nopinonylradical, formed in the hydroperoxide channel, lead tomajor products like 3-hydroxy-nopinone but also tominor products like cis-pinic acid. A mechanismfor the formation of this dicarboxylic acid isproposed and its possible role in aerosol formationprocesses discussed.

Formation of formic acid and organic peroxides in the ozonolysis of ethene with added water vapour

Ozonolysis of C2H4 was carried out in a 580 l glass reaction vessel at 1–5 ppm reactant concentrations, with added water vapour. Under dry conditions ([H2O]0 = 0.5 ppm), HCHO, CO, CO2, (CHO)2O (formic acid anhydride), H2O2, and CH3OOH were identified as the reaction products. Under wet conditions ([H2O]0 = 2 × 104 ppm), HCOOH yields approaching ca. 20% of the converted C2H4, were observed, while no (CHO)2O was formed. Hydroxymethyl hydroperoxide, HOCH2OOH, was observed as the major peroxide, and found to be formed only in the presence of water vapour. Direct reactions of H2O vapour with the excited CH2OO* radicals and with stabilized CH2OO radicals are postulated to explain the formation of HCOOH and HOCH2OOH in the presence of water vapour, respectively.

Gas‐phase ozonolysis of ethene in the presence of hydroxylic compounds

Ozonolysis of C2H4 was carried out at 295 K in 730 torr synthetic air in the concentration ranges of [O3]0 = 1.9–8.2 ppm and [C2H4]0 = 4.0–15.0 ppm, in the absence and presence of the added HCOOH (1 ppm), CH3COOH (1–10 ppm), and CH3OH (36–100 ppm). In the absence of the added compounds, a nearly complete analysis of the reaction products was achieved, with the yields expressed relative to the converted C2H4: HCHO 0.98, CO 0.26, CO2 0.18, HCOOH 0.05, and the sum of formic acid anhydride (FAN) and hydroperoxymethyl formate (HPMF), CHO(SINGLE BOND)O(SINGLE BOND)CH2OOH, 0.19. In the presence of the added HCOOH, the yield of [FAN + HPMF] increased. The addition of CH3COOH suppressed the formation of FAN and HPMF completely. The addition of large excesses of CH3OH also decreased the yield of [FAN + HPMF] significantly. In both cases, new products with the formula R(SINGLE BOND)O(SINGLE BOND)CH2OOH where R = CH3CO and CH3 for CH3COOH and CH3OH, respectively, were formed. The present results, together with the formation of hydroxymethyl hydroperoxide, HO(SINGLE BOND)CH2OOH, with added water vapor (Horie et al., Geophys. Res. Lett., 21, 1523, (1994)) were explained by the reaction of the Criegee biradical CH2OO with the added hydroxy compounds ROH. Formation of the products with the general formula R(SINGLE BOND)O(SINGLE BOND)CH2OOH indicates that the RO(SINGLE BOND)H bond fission has taken place.

The nature of the transitory product in the gas-phase ozonolysis of ethene

Abstract One of the reactants for the formation of previously identified transitory product in the gas-phase ozonolysis of C 2 H 4 was shown to be HCOOH. The most probable structure of this compound is HOOCH 2 OCHO. Its concentration increased with the addition of HCOOH but decreased with the addition of HCHO which had previously been assumed as one of the reactants. This compound slowly decomposed to formic acid anhydride and water.

Formation of secondary ozonides in the gas-phase ozonolysis of simple alkenes

Secondary propene ozonide and isobutene ozonide were formed in the gas-phase ozonolysis of ethene with added acetaldehyde and acetone, respectively. Combined with the formation of hydroperoxymethyl formate and methoxymethyl hydroperoxide in the ethene-ozone reaction system in the presence of HCOOH and CH3OH, respectively, formation of the secondary ozonides reveals a close similarity between the gas-phase and the liquid-phase ozonolysis of alkenes.

The reactions of the Criegee intermediate CH3 CHOO in the gas-phase ozonolysis of 2-butene isomers

Ozonolysis of cis- and trans-2-butene isomers were carried out in a 570 l spherical glass vessel in 730 torr synthetic air at 295 ± 3 K. The initial concentrations were 5 to 10 ppmv for the isomers and 2 to 5 ppmv for ozone. Quantitative yields were determined by FTIR spectroscopy for CH3CHO, HCHO, CH4, CH3OH, CO, and CO2. By means of computational subtraction of the spectral contribution of the identified products from the product spectra, residual spectra have been obtained. Formation of 2-butene ozonide, propene ozonide, and l-hydroperoxyethyl formate CH3CH(OOH)(SINGLE BOND)O(SINGLE BOND)CH(O) have been identified in the residual spectra. These products have been shown to be formed in the reactions of the Criegee intermediate CH3CHOO with CH3CHO, HCHO, and HCOOH, respectively. Mechanistic implications and atmospheric relevance of these observations are discussed.

Structure-Reactivity Based Estimation of the Rate Constants for Hydroxyl Radical Reactions with Hydrocarbons

The reaction with the OH radical constitutes the singlemost important removal process for most organiccompounds found in the atmosphere. Efforts to measurethe OH radical rate constants of all troposphericconstituents remain incomplete due to the largevariety of primary emitted compounds and theirtropospheric degradation products.Based on the measured rate constants of ≈250molecules with the OH radical, a structure-activityrelationship (SAR) for OH reactions has beendeveloped. The molecules used in the dataset includemost classes of tropospheric compounds (includingalkanes, alkenes, and oxygenated hydrocarbons), withthe exception of aromatic and halogen-containingcompounds. Using a new parameterization of themolecular structure, the overall agreement betweenmeasured values and those estimated using the SARdeveloped in this study is usually very good, with10% of the molecules showing deviations larger than50%. In particular, the estimated rate constants ofethers and ketones are in better agreement withexperimental data than with previous SARs (Kwok andAtkinson, Atmos. Environ.29, 1685–1695,1995). Rate constants of organic nitrates werenot well described by the SAR used in thisstudy. The basic assumption that the additive rateconstant for a chemical group is only influenced byneighbouring functional groups did not allow a goodparameterization for the rate constants of organicnitrates. The use of a second parameter to alter thereactivity of C-H bonds in β-position to thefunctional group resulted in markedly better agreementbetween calculated and measured rate constants, butwas not extended due to the limited set of data. This indicates that strong electron withdrawing groups(e.g., nitrate groups) might influence the reactivityof C-H bonds that are not directly adjacent.

Ozonolysis of nonmethane hydrocarbons as a source of the observed mass independent oxygen isotope enrichment in tropospheric CO

Combined 17 O and 18 O measurements of tropospheric CO from two northern hemisphere (NH) sites reveal systematically enhanced 17 O levels, confirming the existence of significant mass independent oxygen isotope enrichment in this important trace gas. When CO levels increase in the NH winter, the mass independent enrichment decreases proportionally. A possible source of this rare isotope effect in CO is transfer of the mass independent enrichment from O 3 to the CO pool via ozonolysis of unsaturated hydrocarbons such as isoprene and terpenes. Laboratory ozonolysis experiments indeed confirm that this process is a potentially important source of mass independently enriched CO. The extra 17 O enrichment found in ozonolysis-derived CO is similar to the mass independent fractionation measured on ozone. If ozonolysis is the only source of mass independent enrichment in CO, it is estimated that about 10% of all CO in middle to high latitude winter, at ground level, originates from the ozonolysis of unsaturated hydrocarbons. While the laboratory experiments prove that ozonolysis is a source of mass independent enrichment in CO, a further problem unfolds. Because tropospheric O 3 is strongly enriched in 18 O as well, this enrichment is also transferred to the CO inventory. When 10 % of the CO inventory has the strong 18 O enrichment, the measured low 18 O abundance of atmospheric CO requires for compensation the existence of a strongly 18 O depleted CO source, or a selective isotopic modification due to isotope fractionation. No such source has been identified to date, and thus ozonolysis may not be the only source of mass independent fractionation in atmospheric CO. Because the mass independent isotopic enrichment is by its nature indestructible by common (i.e., mass dependent) fractionation processes, it is an ideal tracer.

Rate constants for the reactions of methylvinyl ketone, methacrolein, methacrylic acid, and acrylic acid with ozone

Rate constants for the reaction of ozone with methylvinyl ketone (H2C(DOUBLEBOND)CHC(O)CH3), methacrolein (H2C(DOUBLEBOND)C(CH3)CHO), methacrylic acid (H2C(DOUBLEBOND)C(CH3)C(O)OH), and acrylic acid (H2C(DOUBLEBOND)CHC(O)OH) were measured at room temperature (296±2 K) in the presence of a sufficient amount of cyclohexane to scavenge OH-radicals. Results from pseudo-first-order experiments in the presence of excess ozone were found not to be consistent with relative rate measurements. It appeared that the formation of the so-called Criegee-intermediates leads to an enhanced decrease in the concentration of the two organic acids investigated. It is shown that the presence of formic acid, which is known to react efficiently with Criegee-intermediates, diminishes the observed removal rate of the organic acids. The rate constant for the reaction of ozone with the unsaturated carbonyl compounds methylvinyl ketone and methacrolein was found not to be influenced by the addition of formic acid. Rate constants for the reaction of ozone determined in the presence of excess formic acid are (in cm3 molecule−1 s−1): methylvinyl ketone (5.4±0.6)×10−18; methacrolein (1.3±0.14)×10−18; methacrylic acid (4.1±0.4)×10−18; and acrylic acid (0.65±0.13)×10−18. Results are found to be consistent with the Criegee mechanism of the gas-phase ozonolysis.