Gas-phase Oxidation Products Research Articles

Investigation of organic nitrate product formation during hydroxyl radical initiated photo-oxidation of β-pinene

Abstract A series of experiments have been carried out in the York University smog chamber designed to study the products and pathways of the HO radical oxidation of β-pinene. Experiments on the oxidation of β-pinene and its most prominent oxidation product, pinaketone, by HO radicals with initial NO ranging from 0 to 2.5 ppm have been undertaken. An atmospheric pressure chemical ionization (API) mass spectrometer was operated for online, real-time identification and time profiling of the gas phase oxidation products. The formation of six organic nitrate products has been distinguished of which two have been assigned identifications, C10 dihydroxy nitrate (C10H17NO5) and nitrooxy-pinaketone (C9H13NO4). The real-time profiling and sensitivity to initial NO mixing ratio of each product has provided new insight into the β-pinene + HO radical oxidation mechanism. The distinguished products exhibited either no sensitivity or enhanced formations upon increasing the initial NO mixing ratio. The results also provide evidence supporting the formation of higher order organic nitrate products with formation pathways independent of pinaketone oxidation.

Response of an aerosol mass spectrometer to organonitrates and organosulfates and implications for atmospheric chemistry.

Organonitrates (ON) are important products of gas-phase oxidation of volatile organic compounds in the troposphere; some models predict, and laboratory studies show, the formation of large, multifunctional ON with vapor pressures low enough to partition to the particle phase. Organosulfates (OS) have also been recently detected in secondary organic aerosol. Despite their potential importance, ON and OS remain a nearly unexplored aspect of atmospheric chemistry because few studies have quantified particulate ON or OS in ambient air. We report the response of a high-resolution time-of-flight aerosol mass spectrometer (AMS) to aerosol ON and OS standards and mixtures. We quantify the potentially substantial underestimation of organic aerosol O/C, commonly used as a metric for aging, and N/C. Most of the ON-nitrogen appears as NO(x)+ ions in the AMS, which are typically dominated by inorganic nitrate. Minor organonitrogen ions are observed although their identity and intensity vary between standards. We evaluate the potential for using NO(x)+ fragment ratios, organonitrogen ions, HNO(3)+ ions, the ammonium balance of the nominally inorganic ions, and comparison to ion-chromatography instruments to constrain the concentrations of ON for ambient datasets, and apply these techniques to a field study in Riverside, CA. OS manifests as separate organic and sulfate components in the AMS with minimal organosulfur fragments and little difference in fragmentation from inorganic sulfate. The low thermal stability of ON and OS likely causes similar detection difficulties for other aerosol mass spectrometers using vaporization and/or ionization techniques with similar or larger energy, which has likely led to an underappreciation of these species.

Aqueous-phase ozonolysis of methacrolein and methyl vinyl ketone: a potentially important source of atmospheric aqueous oxidants

Abstract. Recent studies indicate that isoprene and its gas-phase oxidation products could contribute a considerable amount of aerosol through aqueous-phase acid-catalyzed oxidation with hydrogen peroxide (H2O2), although the source of H2O2 is unclear. The present study revealed a potentially important route to the formation of aqueous oxidants, including H2O2, from the aqueous-phase ozonolysis of methacrolein (MAC) and methyl vinyl ketone (MVK). Laboratory simulation was used to perform the atmospheric aqueous-phase ozonolysis at different pHs and temperatures. Unexpectedly high molar yields of the products, including hydroxylmethyl hydroperoxide (HMHP), formaldehyde (HCHO) and methylglyoxal (MG), of both of these reaction systems have been seen. Moreover, these yields are almost independent of pH and temperature and are as follows: (i) for MAC–O3, 70.3±6.3% HMHP, 32.3±5.8% HCHO and 98.6±5.4% MG; and (ii) for MVK–O3, 68.9±9.7% HMHP, 13.3±5.8% HCHO and 75.4±7.9% MG. A yield of 24.2±3.6% pyruvic acid has been detected for MVK–O3. HMHP is unstable in the aqueous phase and can transform into H2O2 and HCHO with a yield of 100%. We suggest that the aqueous-phase ozonolysis of MAC and MVK can contribute a considerable amount of oxidants in a direct and indirect mode to the aqueous phase and that these compounds might be the main source of aqueous-phase oxidants. The formation of oxidants in the aqueous-phase ozonolysis of MAC and MVK can lead to substantial aerosol formation from the aqueous-phase acid-catalyzed reaction of H2O2 with MAC, even if there are no other sources of oxidants.

On-line analysis of secondary ozonides from cyclohexene and d-limonene ozonolysis using atmospheric sampling townsend discharge ionization mass spectrometry

An on-line technique has been developed for analysis of gas-phase oxidation products formed in a reaction flow tube using different reaction times, concentrations and humidities. Products of ozonolysis, including thermally labile secondary ozonides (SOZ), were directly introduced into an atmospheric sampling townsend discharge ionization (ASTDI) source coupled to a triple quadrupole mass spectrometer (MS). Instant changes in the product composition were monitored in the total-ion chromatogram, or by fragment ions in the collision activated dissociation mass spectra by use of MS/MS scan techniques. Assignment of the individual ions was accomplished by inspection of the products’ mass spectra obtained by pre-concentration of the gas phase on a dedicated short column followed by chromatographic analysis. The observed reaction products correspond to those identified with other techniques. Of relevance for future mechanistic modelling, is the point that conditions of excess d-limonene favoured the formation of the d-limonene SOZ (major product), which was observed to be quite stable in dry and humid air, without oxidants. The d-limonene/ozone ratio was observed to be crucial for the stability of the SOZ, because it is prone to ozonolysis, and no SOZ could be detected in completely reacted 1:1 mixtures.

Comparative evolved gas analyses on thermal degradation of thiourea by coupled TG-FTIR and TG/DTA-MS instruments

Identification and monitoring of gaseous species released during thermal decomposition of pure thiourea, (NH2)2C=S in argon, helium and air atmosphere have been carried out by both online coupled TG-FTIR and simultaneous TG/DTA-MS apparatuses manufactured by TA Instruments (USA). In both inert atmospheres and air between 182 and 240°C the main gaseous products of thiourea are ammonia (NH3) and carbon disulfide (CS2), whilst in flowing air sulphur dioxide (SO2) and carbonyl sulphide (COS) as gas phase oxidation products of CS2, and in addition hydrogen cyanide (HCN) also occur, which are detected by both FTIR spectroscopic and mass spectrometric EGA methods. Some evolution of isothiocyanic acid (HNCS) and cyanamide (NH2CN) vapours have also observed mainly by EGA-FTIR, and largely depending on the experimental conditions. HNCS is hardly identified by mass spectrometry. Any evolution of H2S has not been detected at any stage of thiourea degradation by either of the two methods. The exothermic heat effect of gas phase oxidation process of CS2 partially compensates the endothermicity of the corresponding degradation step producing CS2.

A one‐dimensional sectional aerosol model integrated with mesoscale meteorological data to study marine boundary layer aerosol dynamics

The dynamics of aerosols in the marine boundary layer are simulated with a one‐dimensional, multicomponent, sectional aerosol model using vertical profiles of turbulence, relative humidity, temperature, vertical velocity, cloud cover, and precipitation provided by 3‐D mesoscale meteorological model output. The Naval Research Laboratory's (NRL) sectional aerosol model MARBLES (Fitzgerald et al., 1998a) was adapted to use hourly meteorological input taken from NRL's Coupled Ocean‐Atmosphere Prediction System (COAMPS). COAMPS‐generated turbulent mixing coefficients and large‐scale vertical velocities determine vertical exchange within the marine boundary layer and exchange with the free troposphere. Air mass back trajectories were used to define the air column history along which the meteorology was retrieved for use with the aerosol model. Details on the integration of these models are described here, as well as a description of improvements made to the aerosol model, including transport by large‐scale vertical motions (such as subsidence and lifting), a revised sea‐salt aerosol source function, and separate tracking of sulfate mass from each of the five sources (free tropospheric, nucleated, condensed from gas phase oxidation products, cloud‐processed, and produced from heterogeneous oxidation of S(IV) on sea‐salt aerosol). Results from modeling air masses arriving at Oahu, Hawaii, are presented, and the relative contribution of free‐tropospheric sulfate particles versus sea‐salt aerosol from the surface to CCN concentrations is discussed. Limitations and benefits of the method are presented, as are sensitivity analyses of the effect of large‐scale vertical motions versus turbulent mixing.

Indoor Secondary Pollutants from Household Product Emissions in the Presence of Ozone: A Bench-Scale Chamber Study

Ozone-driven chemistry is a source of indoor secondary pollutants of potential health concern. This study investigates secondary air pollutants formed from reactions between constituents of household products and ozone. Gas-phase product emissions were introduced along with ozone at constant rates into a 198-L Teflon-lined reaction chamber. Gas-phase concentrations of reactive terpenoids and oxidation products were measured. Formaldehyde was a predominant oxidation byproduct for the three studied products, with yields for most conditions of 20-30% with respect to ozone consumed. Acetaldehyde, acetone, glycolaldehyde, formic acid, and acetic acid were each also detected for two or three of the products. Immediately upon mixing of reactants, a scanning mobility particle sizer detected particle nucleation events that were followed by a significant degree of secondary particle growth. The production of secondary gaseous pollutants and particles depended primarily on the ozone level and was influenced by other parameters such as the air-exchange rate. Hydroxyl radical concentrations in the range 0.04-200 x 10(5) molecules cm(-3) were determined by an indirect method. OH concentrations were observed to vary strongly with residual ozone level in the chamber, which was in the range 1-25 ppb, as is consistent with expectations from a simplified kinetic model. In a separate chamber study, we exposed the dry residue of two products to ozone and observed the formation of gas-phase and particle-phase secondary oxidation products.

Formation of secondary organic particle phase compounds from isoprene gas-phase oxidation products: An aerosol chamber and field study

Acid-catalyzed multiphase reactions of isoprene with hydrogen peroxide have been suggested to produce hydroxylated oxidation products, namely the diastereoisomeric 2-methyltetrols (2-methylthreitol and 2-methylerythritol), compounds which were found quite recently in aerosols above forests. In this study aerosol chamber experiments have been performed reacting isoprene or known isoprene oxidation products with hydrogen peroxide in the presence of acidic particles. Starting from isoprene, 2-methyl-3-butene-1,2-diol or 2-methyl-2-vinyloxirane the formation of the 2-methyltetrols could be observed. The reaction was most effective for 2-methyl-2-vinyloxirane. The last process can lead to an annual 2-methyltetrol formation in atmospheric particles of about 1 Tg C. Additionally, results from size segregated 2-methyltetrol measurements in Melpitz site (Germany) are presented. The concentrations are between 0.5 and 1.7 ng m −3. The 2-methyltetrols are enriched in the fine size fraction as expected for secondary organic aerosol compounds.

Development and initial evaluation of a dynamic species‐resolved model for gas phase chemistry and size‐resolved gas/particle partitioning associated with secondary organic aerosol formation

A module for predicting the dynamic evolution of the gas phase species and the aerosol size and composition distribution during formation of secondary organic aerosol (SOA) is presented. The module is based on the inorganic gas‐aerosol equilibrium model Simulating the Composition of Atmospheric Particles at Equilibrium 2 (SCAPE2) and updated versions of the lumped Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics (MPMPO). The aerosol phase generally consists of an organic phase and an aqueous phase containing dissolved inorganic and organic components. Simulations are presented in which a single salt (either dry or aqueous), a volatile organic compound, and oxides of nitrogen undergo photo‐oxidation to form SOA. Predicted SOA mass yields for classes of aromatic and biogenic hydrocarbons exhibit the proper qualitative behavior when compared to laboratory chamber data. Inasmuch as it is currently not possible to represent explicitly aerosol phase chemistry involving condensed products of gas phase oxidation, the present model can be viewed as the most detailed SOA formation model available yet will undergo continued improvement in the future.

Formation of secondary organic aerosols from isoprene and its gas-phase oxidation products through reaction with hydrogen peroxide

Aerosols produced over forests impair visibility and may affect climate by scattering and absorbing solar radiation and by serving as cloud condensation nuclei. Here, we introduce, to our knowledge, a new route to secondary organic aerosol formation from isoprene and its gas-phase oxidation products, methacrolein and methacrylic acid, namely, multiphase acid-catalysed oxidation with hydrogen peroxide, a perfect analogue to atmospheric sulphate formation. We demonstrate that the formation of major secondary organic aerosol components that are present in natural forest aerosols collected at K-puszta, Hungary, during the summer of 2003, namely, 2-methyltetrols and 2,3-dihydroxymethacrylic acid, can be explained by this mechanism.

Formation of methane sulfinic acid in the gas-phase OH-radical initiated oxidation of dimethyl sulfoxide.

Dimethyl sulfoxide (CH3S(O)CH3: DMSO) is an important product of dimethyl sulfide (CH3SCH3: DMS) photooxidation. The mechanism of the OH-radical initiated oxidation of DMSO is still highly uncertain and a major aim of recent studies has been to establish if methane sulfinic acid (CH3S(O)OH: MSIA) is a major reaction product In the present work the products of the OH-radical gas-phase oxidation of dimethyl sulfoxide have been investigated in the absence and presence of NOx All experiments were performed in a 1,080 L reaction chamber in 1,000 mbar synthetic air at 284 +/- 2 K using long-path FT-IR spectroscopy and ion chromatography to monitor and quantify reactants and reaction products. Formation of methane sulfinic acid in high yield (80-99%) was observed in both in the absence and presence of NOx, and the results support that it is the major primary reaction product Other products observed included dimethyl sulfone (CH3S(O)2CH3: DMSO2), sulfur dioxide (SO2), methane sulfonic acid (CH3S(O)2OH: MSA), and methane sulfonyl peroxynitrate (CH3S(O)2OONO2: MSPN). The formation behavior of these products is in line with their source being mainly secondary production via oxidation of a primary product, i.e. MSIA.

Gas-Phase Oxidation Products of Biphenyl and Polychlorinated Biphenyls

Our laboratory recently measured the gas-phase reaction rate constants of polychlorinated biphenyls (PCBs) with the hydroxyl radical (OH) and concluded that OH reactions are the primary removal pathway of PCBs from the atmosphere. With the reaction system previously employed for kinetics, we have now investigated the products of these PCB−OH reactions. Experiments were carried out in either air or He as the diluent gas at approximately 1 atm in a 160-mL quartz chamber. Temperatures ranged between 318 and 363 K in order to enhance the vapor pressures of these less volatile compounds. OH was produced in situ by the photolysis of ozone in the presence of H2O. Reaction products from biphenyl, the three monochlorobiphenyls, and five dichlorobiphenyls were extracted from the chamber, derivatized by diazomethane, and analyzed by gas chromatographic mass spectrometry (GC/MS). Experiments gave benzoic acid and the appropriate chlorinated benzoic acids in significant product yields (8−17%).

Product and kinetic study of the oh-initiated gas-phase oxidation of Furan, 2-methylfuran and furanaldehydes at ≈ 300 K

The products of the OH-initiated gas-phase oxidation of furan and 2-methylfuran were studied at 298 ± 2 K in the absence of NO, at 1000 mbar pressure of air using in situ long-path FTIR absorption spectroscopy to monitor reactants and products; the 254 nm photolysis of H 20 2 was used as the OH-radical source under NO. -free conditions. The major products were cis/trans-butenedial and cis/trans-4-oxo-2-pentenal for the oxidation of furan and 2-methylfuran with estimated carbon balances of ≈ 100 and ≈ 87% C, respectively. This study represents the first product determination for both furan and 2-methylfuran; a mechanism is proposed to explain the results. Rate coefficients have also been determined for the gas-phase reaction of OH radicals with the aromatic heterocyclics furan-2-aldehyde, furan-3-aldehyde, and 5-methylfurfural at 300 ± 2 K in 1000 mbar of synthetic air, using a relative kinetic technique. The rate coefficients obtained using trans-2-butene as reference compound are (units of 10 −11 cm 3 molecule −1 s −1): furan-2-aldehyde, 3.51 ± 0.11; furan-3-aldehyde, 4.85 ± 0.16; 5-methylfurfural, 5.10 ± 0.21. This study represents the first determination of the rate coefficients for these compounds.

Laboratory investigations of the deposition of oxidation products of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) to aqueous solutions

Laboratory experiments were conducted to investigate the deposition to aqueous media of the gas phase oxidation products of the following hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs): HCFC-22 (CClF2H), HFC-41 (CH3F), HCFC-123 (CCl2HCF3), HCFC-124 (CClFHCF3), HFC-125 (CF3CF2H), HFC-134a (CF3CFH2), HCFC-141b (CCl2FCH3), HCFC-142b (CClF2CH3), and HFC-152a (CF2HCH3). Single component experiments were conducted were the oxidation products CF3CFO, COF2, CF3C(O)Cl were exposed under laminar flow conditions to alkaline, acidic, and neutral solutions in a aluminum exposure trough. The anionic composition of the exposure solutions were used to determine the effective deposition velocity. Exposures to neutral solutions were also conducted for irradiated HFC and HCFC/Cl2/air mixtures. The combined single component and irradiated mixture experiments were used to measure the effective deposition velocities of C(O)F2, C(O)FCl, HFC(O), CF(O)OOCF(O), CF3CCl(O), CF3CF(O). The deposition velocities differed by as much as a factor of two with the largest velocities found for C(O)F2 and CF3CF(O). The data were insufficient to determine the extent to which gas and liquid phase resistance controlled the overall deposition. However, the data were used to estimate lower limits for the laboratory aqueous resistance and the results were found to be consistent with the recent conclusions of Wine and Chameides who reported the deposition of the oxidation products to oceans and cloudwater was sufficiently fast that there was little likelihood that the products would be transported to the stratosphere.

Soot oxidation in high temperature N 2O/Ar and NO/Ar mixtures

The oxidation rate of soot particles suspended in N 2 O/Ar and NO/Ar mixtures was studied in the temperature range 1940 K≤ T ≤3500 K. The total pressure behind the incident shock waves varied from 0.7 to 1.5 bar. In the first series of experiments the oxidation of soot particles by O atoms generated by the thermal dissociation of N 2 O was studied. In the second series of experiments soot was dispersed in different NO/Ar mixtures. The aim was to investigate the surface oxidation of soot by NO or its decomposition products. The progress in surface oxidation was determined from the gas phase oxidation products and educts. The time dependent concentrations of the reaction product CO and/or the reaction educt NO were measured by a pulsed IR-diode laser rapidly scanned over individual absorption lines. In addition, the particle properties were determined by laser light extinction measurements at three different wavelengths. The measured concentration profiles were interpreted by computer simulations including a well established mechanism for homogeneous reactions combined with global heterogeneous reactions. It was possible to deduce the reaction probabilities α O =0.23 α NO =1.82·exp(−15000 K/ T ) for the surface oxidation of soot by O and NO.

Room temperature oxidation of soot by oxygen atoms

Soot from a paraffin candle diffusion flame was collected on a quartz plate and exposed to oxygen atoms in a flow tube apparatus under carefully controlled conditions. The gaseous combustion products, CO 2 and CO, were measured as a function of time during the combustion using a mass spectrometer, while the weight change of the soot was monitored by a microbalance. The striking feature of this oxidation chemistry is the substantial net adsorption of oxygen atoms by the soot. Under our experimental conditions, the soot adsorbed up to 25 % by weight oxygen, compared to an initial soot composition of less than 5 % by weight oxygen. This is a chemical adsorption, in which the oxygen atoms are strongly bonded to carbon atoms in the soot. Statistically this 25 % oxygen corresponds to one oxygen atom for every four carbon atoms in the soot. At 23 °C, oxygen atom adsorption on fresh soot is much faster than the production of gas-phase oxidation products. Furthermore, the oxygen laden soot does not produce gaseous CO 2 and CO at room temperature in the absence of O atoms; additional energy and/or chemistry is required to produce these gaseous oxidation products.

Gas-phase oxidation of alkenes: decomposition of hydroxy-substituted peroxyl radicals

Carbonyl compounds are the main products of the low-temperature gas-phase oxidation of alkenes. It is suggested that they are formed by consecutive addition of a hydroxyl radical and an oxygen molecule to the alkene, the hydroxy-substituted peroxyl radical subsequently decomposing to yield two molecules of carbonyl compounds and the chain carrier. A model system has been investigated, the fuel chosen being 2,3-dimethylbutan-2-ol which gives acetone as the major product at 578 K. In reactions with [18O]-2,3-dimethylbutan-2-ol and oxygen, the ketone is enriched by ca. 50%, and this result is consistent with the decomposition of the radical (II). [graphic omitted]