Reactions Of Nitrate Radical Research Articles

Rate Coefficients for the Gas-Phase Reactions of Nitrate Radicals with a Series of Furan Compounds

The atmospheric reaction of a series of furan compounds (furan (F), 2-methylfuran (2-MF), 3-methylfuran (3-MF), 2,5-dimethylfuran (2,5-DMF), and 2,3,5-trimethylfuran (2,3,5-TMF)) with nitrate radical (NO3) has been investigated using the relative rate kinetic method in the CHamber for the Atmospheric Reactivity and the Metrology of the Environment (CHARME) simulation chamber at the laboratoire de Physico-Chimie de l'Atmosphere (LPCA) laboratory (Dunkerque, France). The experiments were performed at (294 ± 2) K atmospheric pressure and under dry conditions (relative humidity, RH < 2%) with proton transfer mass reaction-time of flight-mass spectrometer (PTR-ToF-MS) for the chemical analysis. The following rate coefficients (in units cm3 molecule-1 s-1) were determined: furan, k(F) = (1.51 ± 0.38) × 10-12, 2-methylfuran, k(2-MF) = (1.91 ± 0.32) × 10-11, 3-methylfuran, k(3-MF) = (1.49 ± 0.33) × 10-11, 2,5-dimethylfuran, k(2,5-DMF) = (5.82 ± 1.21) × 10-11, and 2,3,5-trimethylfuran, k(2,3,5-TMF) = (1.66 ± 0.69) × 10-10. The uncertainty on the measured rate coefficient (ΔkFC) includes both the uncertainty on the measurement and that on the rate coefficient of the reference molecule. To our knowledge, this work represents the first determination for the rate coefficient of the 2,3,5-TMF reaction with NO3. This work shows that the reaction between furan and methylated furan compounds with nitrate radical (NO3) is the dominant removal pathway during the night with lifetimes between 0.5 and 55 min for the studied molecules.

Seasonal variation in nitryl chloride and its relation to gas-phase precursors during the JULIAC campaign in Germany

Abstract. Ambient measurements of nitryl chloride (ClNO2) were performed at a rural site in Germany, covering three periods in winter, summer, and autumn 2019, as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed to understand the photochemical processes in air masses typical of midwestern Europe. Measurements were conducted at 50 m aboveground, which was mainly located in the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO2 is produced at night by the heterogeneous reaction of dinitrogen pentoxide (N2O5) on chloride (Cl−) that contains aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO2 mixing ratio was 1.6 ppbv (parts per billion by volume; 15 min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO2 mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N2O5 necessary for ClNO2 formation comes from the reaction of nitrate radicals (NO3) with nitrogen dioxide (NO2), where NO3 itself is formed by a reaction of NO2 with ozone (O3). Measured concentrations of ClNO2, NO2, and O3 were used to quantify ClNO2 production efficiencies, i.e., the yield of ClNO2 formation per NO3 radical formed, and a box model was used to examine the idealized dependence of ClNO2 on the observed nocturnal O3 and NO2 concentrations. Results indicate that ClNO2 production efficiency was most sensitive to the availability of NO2 rather than that of O3 and increased with decreasing temperature. The average ClNO2 production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO2 production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO2 chemistry in rural environments in midwestern Europe.

Mechanism and Kinetics of the Reaction of Nitrate Radicals with Carboxylic Acids.

Rate constants for the reaction of nitrate radical (NO3 ⋅) with several carboxylic acids (RCO2 H) were measured in acetonitrile using laser flash photolysis, and found to be on the order of 105 -106 M-1 s-1 . No observable H/D kinetic isotope effect was observed at the carboxyl O-H group, α-C-H bond and (possibly) in the case of formic acid, the formylic C-H bond. This suggests that NO3 ⋅ does not abstract hydrogen from any of these positions despite the fact that all these processes are thermodynamically favorable. Reactivity increases with increased length and/or branching of the alkyl side chain (R), and approaches, but does not quite reach, that of an alkane towards NO3 ⋅. The relative inertness of carboxylic acids towards NO3 ⋅ can be explained by the polar effect.

Theoretical study of the NO3 radical reaction with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2.

The potential reaction of the nitrate radical (NO3), the main nighttime atmospheric oxidant, with five alkyl halides, halons (CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2) has been studied theoretically. The most favorable reaction corresponds to a hydrogen atom transfer. The stationary points on the potential energy surfaces of these reactions have been characterized. The reactions can be classified into two groups based on the number of hydrogen atoms in the halon molecules (1 or 2). The reactions with halons with only one hydrogen atom show more exothermic profiles than those with two hydrogen atoms. In addition, the kinetics of the reaction of NO3 + CH2BrI was studied in much higher detail using a multi-well Master Equation solver as a representative example of the nitrate radical reactivity against these halocarbons. These results indicate that the chemical lifetime of the alkyl halides would not be substantially affected by nitrate radical reactions, even in the case of NO3-polluted atmospheric conditions.

Insight into Hydrogen Abstractions by Nitrate Radical: Structural, Solvent Effects, and Evidence for a Polar Transition State.

The role of a polarized transition state and solvent effects on nitrate radical reactions was examined with a previously under-reported class of substrates, ethers, for their atmospheric implications. Absolute rate constants for hydrogen abstraction from a series of alcohols, ethers, and alkanes by nitrate radical have been measured in acetonitrile, water, and mixtures of these two solvents. Across all of these classes of compounds, using a modified form of the Evans-Polanyi relationship, it is demonstrated that the observed structure/reactivity trends can be reconciled by considering the number of abstractable hydrogens, strength of the C-H bond, and ionization potential (IP) of the substrate. Hydrogen abstractions by nitrate radical occur with low selectivity and are characterized by an early transition state (α ≈ 0.3). The dependence of the rate constant on IP suggests a polar transition state with some degree (<10%) of charge transfer. These conclusions stand for reactions conducted in solution (CH3CN and H2O) as well as gas-phase values. Because of this polar transition state, the rate constants increase going from the gas phase to a polar solvent, and the magnitude of the increase is consistent with Kirkwood theory.

Gas-Particle Partitioning and SOA Yields of Organonitrate Products from NO3-Initiated Oxidation of Isoprene under Varied Chemical Regimes.

Alkyl nitrate (AN) and secondary organic aerosol (SOA) from the reaction of nitrate radicals (NO3) with isoprene were observed in the Simulation of Atmospheric PHotochemistry In a large Reaction (SAPHIR) chamber during the NO3Isop campaign in August 2018. Based on 15 day-long experiments under various reaction conditions, we conclude that the reaction has a nominally unity molar AN yield (observed range 90 ± 40%) and an SOA mass yield of OA + organic nitrate aerosol of 13–15% (with ∼50 μg m–3 inorganic seed aerosol and 2–5 μg m–3 total organic aerosol). Isoprene (5–25 ppb) and oxidant (typically ∼100 ppb O3 and 5–25 ppb NO2) concentrations and aerosol composition (inorganic and organic coating) were varied while remaining close to ambient conditions, producing similar AN and SOA yields under all regimes. We observe the formation of dinitrates upon oxidation of the second double bond only once the isoprene precursor is fully consumed. We determine the bulk partitioning coefficient for ANs (Kp ∼ 10–3 m3 μg–1), indicating an average volatility corresponding to a C5 hydroxy hydroperoxy nitrate.

Secondary organic aerosol formation from the gas-phase reaction of guaiacol (2-methoxyphenol) with NO3 radicals

Abstract Methoxyphenols are oxygenated aromatic compounds emitted by wood combustion (consequently to the pyrolysis of lignin). The atmospheric reaction of nitrate radical (NO3) with guaiacol (2-methoxyphenol), one of the principal representatives of this class of compounds has been investigated in the dark at (294 ± 2) K, atmospheric pressure and low relative humidity (RH

Investigation of dark condition nitrate radical- and ozone-initiated aging of toluene secondary organic aerosol: Importance of nitrate radical reactions with phenolic products

To investigate the reaction mechanisms during the NO3- and O3-initiated aging of aromatic secondary organic aerosol (SOA), the gases and particles formed from the OH-initiated oxidation of toluene were exposed to NO3 radicals and O3 in a laboratory chamber. Hydroxyl radicals for the oxidation of toluene were produced under light conditions through H2O2 photolysis and dark conditions through tetramethylethylene ozonolysis. Aerodyne aerosol mass spectrometer results demonstrated an increase in SOA and nitrate aerosol concentrations only after N2O5 injection, but no clear change after O3 injection. The results of proton transfer reaction-quadrupole interface-time of flight mass spectrometer showed that the signal of m/z 109.07 decreased and the signals of m/z 123.05 and m/z 154.05 increased after N2O5 injection, suggesting that gaseous cresols formed from toluene oxidation are rapidly degraded and gaseous nitrocresols and methylbenzoquinones are newly formed. The results of liquid chromatography-mass spectrometry analysis showed that nitrocresols and nitrosalicylic alcohols were the major products formed after N2O5 injection in the particle phase. To confirm the contribution of cresols on aging, NO3 oxidation of o-cresol was carried out, which resulted in the formation of similar products, i.e. gaseous nitrocresols and particulate nitrosalicylic alcohols. Yield curve analysis using two product model estimated that cresol accounted for ~62% of the increase in SOA during NO3-initiated aging of toluene SOA.

Development of a protocol for the auto-generation of explicit aqueous-phase oxidation schemes of organic compounds

Abstract. This paper presents a new CAPRAM–GECKO-A protocol for mechanism auto-generation of aqueous-phase organic processes. For the development, kinetic data in the literature were reviewed and a database with 464 aqueous-phase reactions of the hydroxyl radical with organic compounds and 130 nitrate radical reactions with organic compounds has been compiled and evaluated. Five different methods to predict aqueous-phase rate constants have been evaluated with the help of the kinetics database: gas–aqueous phase correlations, homologous series of various compound classes, radical reactivity comparisons, Evans–Polanyi-type correlations, and structure–activity relationships (SARs). The quality of these prediction methods was tested as well as their suitability for automated mechanism construction. Based on this evaluation, SARs form the basis of the new CAPRAM–GECKO-A protocol. Evans–Polanyi-type correlations have been advanced to consider all available H atoms in a molecule besides the H atoms with only the weakest bond dissociation enthalpies (BDEs). The improved Evans–Polanyi-type correlations are used to predict rate constants for aqueous-phase NO3 and organic compounds reactions. Extensive tests have been performed on essential parameters and on highly uncertain parameters with limited experimental data. These sensitivity studies led to further improvements in the new CAPRAM–GECKO-A protocol but also showed current limitations. Biggest uncertainties were observed in uptake processes and the estimation of Henry's law coefficients as well as radical chemistry, in particular the degradation of alkoxy radicals. Previous estimation methods showed several deficits, which impacted particle growth. For further evaluation, a 1,3,5-trimethylbenzene oxidation experiment has been performed in the aerosol chamber “Leipziger Aerosolkammer” (LEAK) at high relative humidity conditions and compared to a multiphase mechanism using the Master Chemical Mechanism (MCMv3.2) in the gas phase and using a methylglyoxal oxidation scheme of about 600 reactions generated with the new CAPRAM–GECKO-A protocol in the aqueous phase. While it was difficult to evaluate single particle constituents due to concentrations close to the detection limits of the instruments applied, the model studies showed the importance of aqueous-phase chemistry in respect to secondary organic aerosol (SOA) formation and particle growth. The new protocol forms the basis for further CAPRAM mechanism development towards a new version 4.0. Moreover, it can be used as a supplementary tool for aerosol chambers to design and analyse experiments of chemical complexity and help to understand them on a molecular level.

Characterization and possible sources of nitrated mono- and di-aromatic hydrocarbons containing hydroxyl and/or carboxyl functional groups in ambient particles in Nagoya, Japan

By understanding the sources and temporal variations of nitrated mono- and di-aromatic hydrocarbons (nitroaromatic hydrocarbons, NAHCs) in ambient particles, we can evaluate their impact on the climate, environment, and human health. This work quantifies (for the first time) a series of NAHCs (3,5-dinitrosalicylic acid, 2,4-dinitro-1-naphthol, 4-nitrophthalic acid, and 2-hydroxy-5-nitrobenzyl alcohol) in ambient particles collected from an urban site in Nagoya, Japan. The data were collected throughout the day/night cycles during the summer and fall of 2013. To evaluate the possible sources of the NAHCs, the observed NAHC concentrations were compared with organic tracers of anthropogenic secondary organic aerosols (ASOAs) and biomass burning. Nitrocatechols were mainly influenced by emissions from biomass burning during the fall, and by secondary formation from anthropogenic volatile organic compounds (AVOCs) during the summer. In both seasons, AVOC oxidation mainly produced nitrosalicylic acids, 4-nitro-1-naphthol, and 4-nitrophthalic acid, whereas biomass burning in the fall produced a significant amount of 4-nitro-1-naphthol. The concentrations of 3,5-dinitrosalicylic acid and 4-nitrophthalic acid were highly correlated with those of other ASOA tracers, and were strongly related to wind direction, oxidant concentration, and solar radiation. Therefore, these NAHCs are suggested as new tracers of ASOA. Meanwhile, although 2-hydroxy-5-nitrobenzyl alcohol was generated by AVOCs photo-oxidation during the day, its concentration remained high at night, indicating additional nighttime sources such as nitrate radical reactions. Our results suggest that different NAHCs could have different sources, large seasonal and day-to-night variations in their major ones.

Theoretical study on gas-phase reactions of nitrate radicals with methoxyphenols: Mechanism, kinetic and toxicity assessment

Creosol and 4-ethylguaiacol are two important methoxyphenols, lignin pyrolysis products, which are discharge into the atmosphere in large quantities. In this work, theoretical calculations of the reaction mechanism towards the two compounds with NO3 radicals was performed using DFT method. The rate constants and toxicity assessment were also investigated. The atmospheric lifetime for creosol and 4-ethylguaiacol were 0.82 and 0.19 h, respectively. A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl, which has not yet been clarified in previous studies. The toxicity of methoxyphenols and their degradation products is closely related to their hydrophobicity. Although most degradation products are less toxic, they also should be pay more attention, especially for nitro-substituents.A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl. The toxicity is closely related to their hydrophobicity.

Secondary organic aerosol (SOA) yields from NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; radical + isoprene based on nighttime aircraft power plant plume transects

Abstract. Nighttime reaction of nitrate radicals (NO3) with biogenic volatile organic compounds (BVOC) has been proposed as a potentially important but also highly uncertain source of secondary organic aerosol (SOA). The southeastern United States has both high BVOC and nitrogen oxide (NOx) emissions, resulting in a large model-predicted NO3-BVOC source of SOA. Coal-fired power plants in this region constitute substantial NOx emissions point sources into a nighttime atmosphere characterized by high regionally widespread concentrations of isoprene. In this paper, we exploit nighttime aircraft observations of these power plant plumes, in which NO3 radicals rapidly remove isoprene, to obtain field-based estimates of the secondary organic aerosol yield from NO3 + isoprene. Observed in-plume increases in nitrate aerosol are consistent with organic nitrate aerosol production from NO3 + isoprene, and these are used to determine molar SOA yields, for which the average over nine plumes is 9 % (±5 %). Corresponding mass yields depend on the assumed molecular formula for isoprene-NO3-SOA, but the average over nine plumes is 27 % (±14 %), on average larger than those previously measured in chamber studies (12 %–14 % mass yield as ΔOA ∕ ΔVOC after oxidation of both double bonds). Yields are larger for longer plume ages. This suggests that ambient aging processes lead more effectively to condensable material than typical chamber conditions allow. We discuss potential mechanistic explanations for this difference, including longer ambient peroxy radical lifetimes and heterogeneous reactions of NO3-isoprene gas phase products. More in-depth studies are needed to better understand the aerosol yield and oxidation mechanism of NO3 radical + isoprene, a coupled anthropogenic–biogenic source of SOA that may be regionally significant.

The Reactivity of the Nitrate Radical (• NO3 ) in Aqueous and Organic Solutions

Rate constants for the nitrate (•NO3) radical reaction with alcohols, alkanes, alkenes, and several aromatic compounds were measured in aqueous and tert-butanol solution for comparison to aqueous and acetonitrile values from the literature. The measured trends provide insight into the reactions of the •NO3 radical in various media. The reaction with alcohols primarily consists of hydrogen-atom abstraction from the alpha-hydroxy position and is faster in solvents of lower polarity where the diffusivity of the radical is greater. Alkenes react faster than alkanes, and their rate constants are also faster in nonpolar solution. The situation is reversed for the nitrate radical reaction with the aromatic compounds, where the rate constants in tert-butanol are slower. This is attributed to the need to solvate the NO3− anion and corresponding tropylium cation produced by the •NO3 radical electron transfer reaction. A linear correlation was found between measured rate constants in water and acetonitrile, which can be used to estimate aqueous nitrate radical rate constants for compounds having low water solubility.

Room-temperature and temperature-dependent QSRR modelling for predicting the nitrate radical reaction rate constants of organic chemicals using ensemble learning methods

Experimental determinations of the rate constants of the reaction of NO3 with a large number of organic chemicals are tedious, and time and resource intensive; and the development of computational methods has widely been advocated. In this study, we have developed room-temperature (298 K) and temperature-dependent quantitative structure–reactivity relationship (QSRR) models based on the ensemble learning approaches (decision tree forest (DTF) and decision treeboost (DTB)) for predicting the rate constant of the reaction of NO3 radicals with diverse organic chemicals, under OECD guidelines. Predictive powers of the developed models were established in terms of statistical coefficients. In the test phase, the QSRR models yielded a correlation (r2) of >0.94 between experimental and predicted rate constants. The applicability domains of the constructed models were determined. An attempt has been made to provide the mechanistic interpretation of the selected features for QSRR development. The proposed QSRR models outperformed the previous reports, and the temperature-dependent models offered a much wider applicability domain. This is the first report presenting a temperature-dependent QSRR model for predicting the nitrate radical reaction rate constant at different temperatures. The proposed models can be useful tools in predicting the reactivities of chemicals towards NO3 radicals in the atmosphere, hence, their persistence and exposure risk assessment.

Kinetic Study of the Gas-Phase Reactions of Nitrate Radicals with Methoxyphenol Compounds: Experimental and Theoretical Approaches.

The gas-phase reactions of five methoxyphenols (three disubstituted and two trisubstituted) with nitrate radicals were studied in an 8000 L atmospheric simulation chamber at atmospheric pressure and 294 ± 2 K. The NO3 rate constants were investigated with the relative kinetic method using PTR-ToF-MS and GC-FID to measure the concentrations of the organic compounds. The rate constants (in units of cm(3) molecule(-1) s(-1)) determined were: 2-methoxyphenol (guaiacol; 2-MP), k(2-MP) = (2.69 ± 0.57 × 10(-11); 3-methoxyphenol (3-MP), k(3-MP) = (1.15 ± 0.21) × 10(-11); 4-methoxyphenol (4-MP), k(4-MP) = (13.75 ± 7.97) × 10(-11); 2-methoxy-4-methylphenol, k(2-M-4-MeP) = (8.41 ± 5.58) × 10(-11) and 2,6-dimethoxyphenol (syringol; 2,6-DMP), k(2,6-DMP) = (15.84 ± 8.10) × 10(-11). The NO3 rate constants of the studied methoxyphenols are compared with those of other substituted aromatics, and the differences in the reactivity are construed regarding the substituents (type, number and position) on the aromatic ring. This study was also supplemented by a theoretical approach of the methoxyphenol reactions with nitrate radicals. The upper limits of the NO3 overall rate constants calculated were in the same order of magnitude than those experimentally determined. Theoretical calculations of the minimum energies of the adducts formed from the reaction of NO3 radicals with the methoxyphenols were also performed using a DFT approach (M06-2X/6-31G(d,p)). The results indicate that the NO3 addition reactions on the aromatic ring of the methoxyphenols are exothermic, with energy values ranging between -13 and -21 kcal mol(-1), depending on the environment of the carbon on which the oxygen atom of NO3 is attached. These energy values allowed identifying the most suitable carbon sites for the NO3 addition on the aromatic ring of the methoxyphenols: at the exception of the 3-MP, the NO3 ipso-addition to the hydroxyl group is one of the favored sites for all the studies compounds.

Kinetics and mechanism of the gas-phase reaction of nitrate radical with phenol

To study the kinetics and mechanism of the gas-phase reaction of nitrate radical (NO3) with phenol (C6H5OH), its rate constant was measured by the relative rate method using a Fourier transform infrared spectrometer with a long path length gas cell. Using the observed rate constant ratio, k(NO3 + C6H5OH)/k(NO3 + CH3SCH3) = (4.09 ± 0.14) in 100 Torr of synthetic air diluent at (300 ± 3) K, the rate constant was determined to be (4.5 ± 2.1) × 10−12 cm3 molecule−1 s−1. The pressure dependence of the rate constant of NO3 + C6H5OH was investigated for the first time and the results showed that the reaction has no pressure dependence in 50, 100, 200, and 300 Torr of synthetic air diluent or even in 15 Torr of He diluent. Using the same experimental apparatus, an investigation of the reaction product yields was also performed. The results showed that 2-nitrophenol was predominantly formed in the reaction of NO3 with C6H5OH and the formation yield of 2-nitrophenol was (54 ± 2) %.

Kinetic study of the reaction of nitrate radicals with ethylperoxy radicals between 277 and 358 K

The rate constants of the reaction of nitrate and ethylperoxy radicals are measured using laser photolysis coupled with time resolved UV⿿vis spectrometry and laser diode spectroscopy over the temperature range 277⿿358K, for pressures varying between 380 and 550Torr. This work provides the first kinetic data for the reaction of NO3+C2H5O2 as a function of temperature. The Arrhenius expression (in cm3molecule⿿1s⿿1) is as follows: k1 (T)=(8.70±2.40)ÿ10⿿12 exp(⿿390±85/T). At room temperature, the rate constant value obtained is in good agreement with the three values previously reported for the NO3+C2H5O2 reaction.

DFT and theoretical kinetics studies on the reaction of nitrate radical with α-pinene and β-pinene

The potential energy surfaces for the reactions of α-pinene and β-pinene with NO3 radicals are explored by using M06-2X hybrid meta density functional theory (HMDFT) method. On the basis of the optimized geometries and energies of the intermediates and transition states, the mechanisms of the title reactions are explained and the importance of different product channels is discussed. It is found that pinonaldehyde and NO radical is the most important products of the α-pinene+NO3 reaction. The major products of the β-pinene+NO3 reaction are predicted to be 10-pinenyl radical, nitric acid, 6,6-dimethylbicyclo[3.1.1]heptan-2-one, NO radical and formaldehyde. Variable Reaction Coordinate-Transition State Theory (VRC-TST) is used to compute the overall rate coefficients as a function of temperature. The computed rate constants are compared with the available experimental data.

Gas-surface reactions of nitrate radicals with vinyl-terminated self-assembled monolayers.

Interfacial reactions between gas-phase nitrate radicals, a key nighttime atmospheric oxidant, and a model unsaturated organic surface have been investigated to determine the reaction kinetics and probable reaction mechanism. The experimental approach employs in situ reflection-absorption infrared spectroscopy (RAIRS) to monitor bond rupture and formation while a well-characterized effusive flux of NO3 impinges on the organic surface. Model surfaces are created by the spontaneous adsorption of vinyl-terminated alkanethiols (HS(CH2)16CHCH2) onto a polycrystalline gold substrate. The H2C=CH-terminated self-assembled monolayers provide a well-defined surface with the double bond positioned precisely at the gas-surface interface. The surface reaction kinetics obtained from RAIRS revealed that the consumption rate of the terminal vinyl groups is nearly identical to the formation rate of a surface-bound nitrate species and implies that the mechanism is one of direct addition to the vinyl group rather than hydrogen abstraction. Upon nitrate radical collisions with the surface, the initial reaction probability for consumption of carbon-carbon double bonds was determined to be (2.3 ± 0.5) × 10(-3). This reaction probability is approximately two orders of magnitude greater than the probability of ozone reactions on the same surface, which suggests that oxidation of surface-bound vinyl groups by nighttime nitrate radicals may play an important role in atmospheric chemistry despite their relatively low concentration.

Atmospheric organic matter in clouds: exact masses and molecular formula identification using ultrahigh-resolution FT-ICR mass spectrometry

Abstract. Clouds alter the composition of atmospheric aerosol by acting as a medium for interactions between gas- and particulate-phase substances. To determine the cloud water atmospheric organic matter (AOM) composition and study the cloud processing of aerosols, two samples of supercooled clouds were collected at the Storm Peak Laboratory near Steamboat Springs, Colorado (3220 m a.s.l.). Approximately 3000 molecular formulas were assigned to ultrahigh-resolution mass spectra of the samples after using a reversed-phase extraction procedure to isolate the AOM components from the cloud water. Nitrogen-containing compounds (CHNO compounds), sulfur-containing compounds (CHOS and CHNOS compounds) and other oxygen-containing compounds (CHO compounds) with molecular weights up to 700 Da were observed. Average oxygen-to-carbon ratios of ∼0.6 indicate a slightly more oxidized composition than most water-soluble organic carbon identified in aerosol studies, which may result from aqueous oxidation in the clouds. The AOM composition indicates significant influences from biogenic secondary organic aerosol (SOA) and residential wood combustion. We observed 60% of the cloud water CHO molecular formulas to be identical to SOA samples of α-pinene, β-pinene, d-limonene, and β-caryophyllene ozonolysis. CHNO compounds had the highest number frequency and relative abundances and are associated with residential wood combustion and NOx oxidation. Multiple nitrogen atoms in the assigned molecular formulas for the nighttime cloud sample composite were observed, indicating the significance of nitrate radical reactions on the AOM composition. Several CHOS and CHNOS compounds with reduced sulfur (in addition to the commonly observed oxidized sulfur-containing compounds) were also observed; however further investigation is needed to determine the origin of the reduced sulfur-containing compounds. Overall, the molecular composition determined using ultrahigh-resolution Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry provides an unambiguous identification of the cloud water organic anion composition in the Rocky Mountain area that could help to improve the understanding of aqueous-phase processes.