Relative-rate study of the gas-phase reaction of hydroxy radicals with difunctional organic nitrates at 298 K and atmospheric pressure

Rate coefficients for the reactions of difunctional nitrates with atmospherically important OH radicals are not currently available in the literature. This study represents the first determination of rate coefficients for a number of C(3) and C(4) carbonyl nitrates and dinitrates with OH radicals in a 38 l glass reaction chamber at 1000 mbar total pressure of synthetic air by 298±2 K using a relative kinetic technique.The following rate coefficients (in units of 10^-12 cm³ molecule^-1 s^-1) were obtained: 1,2-propandiol dinitrate, <0.31; 1,2-butandiol dinitrate, 1.70±0.32; 2,3-butandiol dinitrate, 1.07±0.26; -nitrooxyacetone, <0.43; 1-nitrooxy-2-butanone, 0.91±0.16; 3-nitrooxy-2-butanone, 1.27±0.14; 1,4-dinitrooxy-2-butene, 15.10±1.45; 3,4-dinitrooxy-1-butene, 10.10±0.50.The possible importance of reaction of OH as an atmospheric sink for the compounds compared to other loss processes is considered.     

Near UV absorption spectra and photolysis products of difunctional organic nitrates: Possible importance as NO x reservoirs

Difunctional organic nitrates are important products of the atmospheric reaction of NO₃ radicals with unsaturated hydrocarbons about which relatively little is known. In a continuation of the investigation of the atmospheric chemistry of such compounds, the UV absorption spectra of the following organic dinitrates and keto nitrates have been quantitively measured in the gas phase at 298±2 K and atmospheric pressure: 1,2-propandiol dinitrate, CH₃CH(ONO₂)CH₂(ONO₂); 1,2-butandiol dinitrate, CH₃CH₂CH(ONO₂)CH₂(ONO₂); 2,3-butandiol dinitrate, CH₃CH(ONO₂)CH(ONO₂)CH₃;cis 1,4-dinitrooxy-2-butene, CH₂(ONO₂)CH=CHCH₂(ONO₂); 3,4-dinitrooxy-1-butene, CH₂(ONO₂CH(ONO₂)CH=CH₂; -nitrooxy acetone, CH₃COCH₂(ONO₂); 1-nitrooxy-2-butanone, CH₃CH₂COCH₂(ONO₂); 3-nitrooxy-2-butanone, CH₃CH(ONO₂)COCH₃.Although the UV spectra of the nitrates are all very similar in shape those of the keto nitrates are red-shifted compared to the dinitrates and in the spectral range of atmospheric interest (>290 nm) their absorption cross-sections are approximately a factor of 5 higher. The cross-sections of the dinitrates are a factor of 2 higher than those reported in the literature for the corresponding alkyl mononitrates.The UV absorption cross-sections of the difunctional nitrates were used in combination with solar actinic flux data to estimate photolysis frequencies and consequently atmospheric lifetimes for these compounds. The results indicate that for the saturated difunctional nitrates studied in this work photolysis will generally be somewhat some important than reaction with OH radicals as an atmospheric removal process. However, for unsaturated nitrates loss due to reaction with OH will dominate over photolysis as an atmospheric sink.Preliminary FT-IR analyses of the photolysis products of -nitrooxy acetone, 3-nitrooxy-2-butanone and 2,3-butandiol dinitrate using both mercury and fluorescent lamps indicate that NO₂ is released in the primary step. The further reactions of the radicals thus produced result in the formation of CO, aldehydes and PAN. The possible significance of the results for difunctional organic nitrate as reservoirs for reactive odd nitrogen NO _y in the atmosphere, especially during the night, is briefly discussed.     

Preparation of organic nitrates from alcohols and N2O5 for species identification in atmospheric samples

Several mono- and bifunctional organic nitrates of atmospheric interest were prepared in a one-step synthesis, based on the reaction of alcohols with N₂O₅ (dinitrogen pentoxide) at low temperatures. The method is well suitable for micro-scale preparations. The compounds were characterized by GC/ECD, GC/NO _y -specific detection, and GC/CI (chemical ionization) mass spectrometry.     

Gas phase reactions of alkyl nitrates with hydroxyl radicals under tropospheric conditions in comparison with photolysis

Rate constants for the reaction of OH radicals with some branched alkyl nitrates have been measured applying a competitive technique. Methyl nitrite photolysis in synthetic air was used as OH radical source at 295±2 K and 1000 mbar total pressure. Using a rate constant of 2.53×10^-12 cm³ s^-1 for the reaction of OH radicals with n-butane as reference, the following rate constants were obtained (units: 10^-12 cm³ s^-1): isopropyl nitrate, 0.59±0.22; isobutyl nitrate, 1.63±0.20; 3-methyl-2-butyl nitrate, 1.95±0.15; 2-methyl-1-butyl nitrate, 2.50±0.15; 3-methyl-1-butyl nitrate, 2.55±0.35. These values have been combined with the literature data to recalculate the substituent factors F(X) for the different nitrate groups which can be used to predict OH rate constants for organic nitrates for which experimental data are not available.Preliminary measurements of the photolysis frequency of isopropyl nitrate have shown that for this nitrate as a model substance, OH reactions and direct photolysis are of equal importance under tropospheric conditions.     

Alkyl nitrate formation from the atmospheric photoxidation of alkanes; a revised estimation method

The available experimental data concerning the yields of alkyl nitrates in the reactions of alkyl peroxy radicals with NO have been used to derive a revised expression for the estimation of alkyl nitrate yields in the atmospheric photooxidation of alkanes as a function of temperature and pressure. This revised expression gives more reasonable predictions of alkyl nitrate yields under high altitude tropospheric conditions than that which has been previously published.     

Henry's law and hydrolysis-rate constants for peroxyacyl nitrates (PANs) using a homogeneous gas-phase source

The solubilities and hydrolysis rates of PAN (peroxyacetyl nitrate) and its homologues PPN (peroxypropionyl nitrate), PnBN (peroxy-n-butyl nitrate), PiBN (peroxy-isobutyl nitrate) and MPAN (peroxymethacryloyl nitrate) in liquid water have been studied at 20 °C. Temperature dependencies were measured for PAN and PPN. The solubilities of peroxyacyl nitrates decrease smoothly with increasing carbon-chain length fromH (293 K)=4.1 M atm^–1 (PAN) toH (293 K)=1.0 M atm^–1 (PiBN). Hydrolysis-rate constants, which cover the range fromk _h (293 K)=(2.4–7.4)×10^–4 s^–1, do not show a systematic chain-length dependency. Solubilities of PAN and PPN in solutions which mimic the composition and ionic strength of sea water are 15% and 20% lower than in pure water. The hydrolysis rate constants are not affected.     

Equilibria of the marine multiphase ammonia system

A lack of empirical data has made it difficult to ascertain whether ammonia is in equilibrium between the oceanic, atmospheric gas and atmospheric particle phases in the remote marine environment. Reported here are simultaneous measurements of the saturation concentration of ammonia relative to ammonia concentrations in ocean surface waters; total seawater ammonia; atmospheric gas phase ammonia; and atmospheric particulate-phase ammonium, non-seasalt sulfate, methanesulfonate, and nitrate. Sampling was performed in May of 1987 in the northeast Pacific Ocean environment and in April and May of 1988 in the central Pacific Ocean environment.These measurements were used to determine the degree to which ammonia approached equilibrium between the oceanic and atmospheric gas and aerosol particle phases. The experimental atmospheric gas phase ammonia concentrations were compared with calculated equilibrium concentrations assuming a Henry's law type of partitioning between the gas and condensed phases. Characteristic times of the processes controlling the fate of ammonia in the marine environment also were compared.The measured atmospheric gas phase and oceanic concentrations of ammonia indicate that ammonia is not in a Henry's law equilibrium across the air/sea interface. This disequilibrium is a result of the long air/sea exchange equilibration time relative to the lifetime of ammonia in the atmosphere. Comparison of the calculated equilibrium gas phase ammonia concentrations with the measured gas phase ammonia concentrations shows that attainment of equilibrium between the atmospheric gas and particle phases is a strong function of the chemical composition of the aerosol particles. The data suggest that fully neutralized aerosol particles are not in Henry's law equilibrium with the gas phase while equilibrium is observed for particles with an average ammonium to non-seasalt sulfate molar ratio less than 1.8.     

Measurements of the Henry's Law Coefficients of 2-Methyl-3-buten-2-ol, Methacrolein, and Methylvinyl Ketone

Using an equilibrium headspace technique, Henry's law coefficients were measured for methacrolein (H = 6.5 ± 0.7 M atm-1) and methylvinyl ketone (41 ± 7.0 M atm-1) in water at 25 °C. In addition, 2-methyl-3-buten-2-ol was studied at 30 °C in water and in an aqueous ionic solution representative of plant tissue. Similar values were found in deionized water (65 ± 3.5 M atm-1) and in a 0.05 mol kg-1 Ca2+, K+, NO3-, SO42- solution (62 ± 0.8 M atm-1). These Henry's Law coefficients are too small to allow for significant partitioning of methacrolein, methylvinyl ketone or methylbutenol into cloud water under equilibrium conditions.     

Modelling of the nighttime nitrogen and sulfur chemistry in size resolved droplets of an orographic cloud

A chemistry module has been incorporated into a Lagrangian type model that computes the dynamics and microphysics of an orographical cloud formed in moist air flowing over the summit of Great Dun Fell (GDF) in England. The cloud droplets grow on a maritime aerosol which is assumed to be an external mixture of sea-salt particles and ammonium-sulfate particles. The dry particle radii are in the range 10 nm<r<1 µm. The gas-phase chemical reaction scheme considers reactions of nitrogen compounds that are important at night. The treatment of scavenging of gases into the aqueous phase in the model takes into account the different solubilities and accommodation coefficients. The chemistry in the aqueous phase focusses on the oxidation of S(IV) via different pathways.Sensitivity analyses have been performed to investigate deviations from gas-liquid equilibria according to Henry's law and also to study the influence of iron and of nitrogen compounds on the aqueous-phase oxidation of dissolved SO₂. When addressing these questions, special attention has been given to the dependence on the droplet size distribution and on the chemical composition of the cloud condensation nuclei on which the droplets have formed. It was found that the oxidation of S(IV) via a chain reaction of sulfur radicals can be important under conditions where H₂O₂ is low. However, major uncertainties remain with respect to the interaction of iron with the radical chain. It was shown that mixing of individual cloud droplets, which are not in equilibrium according to Henry's law, can result in a bulk sample in equilibrium with the ambient air. The dependence of the aqueous-phase concentrations on the size of the cloud droplets is discussed for iron, chloride and NO₃.     

Mechanisms and products of the reactions of NO3 with cycloalkenes

The NO₃ radical initiated oxidation of cyclopentene, cyclohexene and 1-methyl-cyclohexene has been studied. The products formed in an N₂O₅-NO₂-N₂-O₂-cycloalkene-static reactor system, at 0.1 MPa and 296 K, were investigated using long path FTIR. The principal products were aldehydes formed via a ring opening process. The reactions also resulted in significant yields of three types of ring retaining nitrooxy-substituted compounds. The average yields of alkyl nitrates from, e.g., reactions with cycloalkene were 25.1% 2-oxo-cyclohexyl nitrate, 22.8% 2-hydroxy-cyclohexyl nitrate and 4.0% 1,2-cyclohexyl dinitrate. The mechanisms involved resembles those proposed for acyclic alkenes. In absence of NO, -oxo and -hydroxy-cycloalkyl nitrates are formed via self reactions of -nitrooxy substituted cycloalkyl peroxy radicals. Estimated branching ratios for the reactants leading to ring retaining products in the presence and in the absence of NO are given and the possible relevance of these reactions for cycloalkenes under tropospheric conditions is discussed.     

Gas and Particle Partitioning Behavior of Aldehyde in the Presence of Diesel Soot and Wood Smoke Aerosols

Outdoor smog chamber experiments were performed to investigate gas/particle (G/P) partitioning behavior of aldehyde compounds in atmospheric acidic aerosols. Diesel soot and wood smoke aerosols were selected as acidic aerosols and octanal, decanal, undecanal, and cis-pinonaldehyde for aldehydes compounds. Aerosol acidity was measured with the equivalent sulfuric acid amounts in aerosol mass: 0.2–0.6 wt% in diesel soot and 0.04–0.1 wt% in wood smoke aerosols. Experimentally determined partitioning coefficients of aldehyde along with other classes of semivolatile organic compounds (SOCs) were compared with the estimation. All experimental G/P partitioning coefficients of aldehyde compounds were 10–200 times higher than estimated partitioning coefficients. Aldehyde partitioning coefficients in wood soot were similar or less than diesel soot aerosols.     

Henry's law coefficients for aqueous solutions of acetone,acetaldehyde and acetonitrile,and equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite

Vapor phase concentrations of acetone, acetaldehyde and acetonitrile over their aqueous solutions were measured to determine Henry's law partition coefficients for these compounds in the temperature range 5–40 °C. The results are for acetone: ln(H ₁/atm)=–(5286±100)T+(18.4±0.3); acetaldehyde: ln(H ₁/atm)=–(5671±22)/T+(20.4±0.1); and acetonitrile: ln(H ₁/atm)=–(4106±101)/T+(13.8±0.3). Artificial seawater of 3.5% salinity in place of deiionized water raisesH ₁ by about 15%. A similar technique has been used to measure the equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite in aqueous solution. The results are ln(K ₁/M ^–1)=(4972±318)/T–(11.2±1.1) and ln(K ₁/M ^–1)=(6240±427)/T–(8.1±1.3), respectively. The results are compared and partly combined with other data in the literature to provide an average representation.     

Kinetics of the reactions of hydroxyl radicals with alkyl nitrates and with some oxygen-containing organic compounds studied under simulated atmospheric conditions

Rate constants have been measured for the reactions of hydroxyl radicals with alkyl nitrates and with some oxygen-containing organic compounds by a competitive technique. Mixtures of synthetic air containing a few ppm of nitrous acid, ethylene and the organic substrate were photolysed in a Teflon bag smog chamber. Based on the value k _HO+C2H₄}=8.1×10^-12 cm³ molecule^-1 s^-1 the following rate constants were obtained for the hydroxyl radical reactions at 750 Torr and at 303 K in units of 10^-12 cm³ molecule^-1: CH₃ONO₂, 0.37±0.09; C₂H₅ONO₂, 0.48±0.20; n-C₃H₇ONO₂, 0.70±0.22; C₂H₅OH, 3.6±0.4; CH₃COCH₃, 0.26±0.08; CH₃CO₂ i-C₃H₇, 3.0±0.8; CH₃CO₂ n-C₃H₇, 2.4±0.2. The results are discussed in relation to the available literature data and the implications of the results are considered in terms of the smog reactivity of these molecules.     

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 -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.     

Atmospheric Loss Processes of Dimethyl and Diethyl Carbonate

A combined study of the OH gas phase reaction and uptake on aqueous surfacesof two carbonates, dimethyl and diethyl carbonate has been carried out todetermine the atmospheric lifetimes of these compounds. Rate coefficients havebeen measured for gas phase reactions of OH radicals with dimethyl and diethylcarbonate. The experiments were carried out using pulsed laser photolysis– laser induced fluorescence over the temperature range 263–372K and the kinetic data were used to derive the following Arrhenius expressions(in units of cm³ molecule^–1 s^–1):for dimethyl carbonate, k₁ = (0.83±0.27)×10^–12 exp [–(247± 98)/T] and fordiethyl carbonate, k₂ = (0.46±0.15)×10^–12 exp [(503± 203)/T]. At 298 K, therate coefficients obtained (in units of 10^–12 cm³molecule^–1 s^–1) are: k₁ =(0.35± 0.04) and k₂ = (2.31± 0.29). The results arediscussed in terms of structure-activity relationships.The uptake coefficients of both carbonates on aqueous surfaces were measuredas a function of temperature and composition of the liquid phase, using thedroplet train technique coupled to a mass spectrometric detection. Dimethyland diethyl carbonate show very similar results. For both carbonates, themeasured uptake kinetics were found to be independent of the aqueous phasecomposition (pure water, NaOH solutions) but dependent on gas-liquid contacttime which characterises a surface saturation effect. The uptake coefficientvalues show a slight negative temperature dependence for both carbonates.These values vary from 1.4×10^–2 to0.6×10^–2 in the temperature range of 265–279 Kfor dimethyl carbonate, from 2.4×10^–2 to0.9×10^–2 in the temperature range of 270–279 Kfor diethyl carbonate. From the kinetic data, the following Henry's lawconstants were derived between 279 and 265 K: dimethyl carbonate,H₁ = 20–106 M atm^–1; and diethyl carbonate,H₂ = 30–98 M atm^–1. The reported data showthat the OH reaction is the major atmospheric loss process of these twocarbonates with lifetimes of 33 and 5 days, respectively, while the wetdeposition is a negligible process.     

On the Measurement of PANs by Gas Chromatography and Electron Capture Detection

A fast, automated, gas chromatographic system for the airborne measurement of PAN and a series of its homologues is described and its performance is evaluated. Response factors for PAN, PPN, APAN, PiBN, and MPAN have been determined and are discussed with regard to ECD response and to potential losses in the analytical system. Calibration methods used for these tasks are described and compared. The results from this work should help investigators who are employing the widely used GC/ECD method for the measurement of peroxyacyl nitrates to evaluate peaks of PAN homologues that cannot be calibrated for by using the reported response factors.     

Solubility of HOCl in water and aqueous H2SO4 to stratospheric temperatures

Henry's law constants K_H (mol kg^–1 atm^–1) for the reaction HOCl_(g)=HOCl_(aq) near room temperature, literature data for the associated enthalpy change, and solubilities of HOCl in aqueous H₂SO₄ (46 to 60 wt%) at temperatures relevant to the stratosphere (200 KT230 K) are shown to be thermodynamically consistent. Effective Henry's law constants [H*=mHOCl/pHOCl, in mol kg^–1 atm^–1] of HOCl in aqueous H₂SO₄ are given by: ln(H*)=6.4946–mH₂SO₄(–0.04107+54.56/T)–5862 (1/T_o–1/T) where T(K) is temperature and T_o=298.15K. The activity coefficient of HOCl in aqueous H₂SO₄ has a simple Setchenow-type dependence upon H₂SO₄ molality.     

Calculation of updated coefficients for atmospheric temperature retrieval

In older to calculate updated coefficients for atmospheric temperature retrieval from satellite sounding data and radiosonde data, it is necessary to form statistical samples of real radiance and radiosonde data match-ups. A procedure is presented here for the data matchups. And a method of eigenvectors of statistical covariance matrices is used to produce updated coefficients for atmospheric temperature retrieval. The updated coefficients produced are tested using radiance observations from NOAA-7 satellite. Comparisons of these real-time retrieved data with radiosonde data show that the atmospheric temperature profiles retrieved have an accu-racy of RMS 2－3 degrees (oC). In addition, the error sources are also discussed.     

A procedure for the formulation of bulk transfer coefficients over water

A method suitable for predicting bulk transfer coefficients appropriate to any reasonable height of measurement in the atmospheric surface boundary layer and incorporating the effects of atmospheric stability is based on the assumption that eddy and molecular diffusivities are additive near a water surface. This assumption is supported in the case of sensible heat, by results obtained over Lake Michigan and over an industrial cooling pond at Dresden, Illinois, as well as by published measurements made over Lake Flevo, Holland. The verification appears to extend to wind speeds in the range 10–15 m s^–1. The results permit evaluation of transfer coefficients applicable in the demanding situations of inland lakes and artificial cooling ponds.Work performed under the auspices of the U.S. Energy Research and Development Administration.     

Carboxylic monoacids in the air of mayombe forest (Congo): Role of the forest as a source or sink

In the tropical rain forests of the Congo during the dry season, from June to September 1987, carboxylic acid partial pressures (P _gas) in the air above the canopy, at ground level, and at the boundary layer, were estimated from water samples such as fog and rainwater. The concentrations of these acids were also measured in the sap of tree leaves. Tree leaves act as a sink or as a source if the acid P _gas is greater of lower than the acid concentrations in molecular form in sap. For each of these soluble gases, there is a value of P _gas where the exchange is nul. This is called the compensation point. Values of the compensation point for some tree leaves were evaluated according to Henry's law. Henry's law coefficients at ppm levels were redetermined for formic (HCOOH), acetic (CH₃COOH), propionic (CH₃CH₂COOH), and isobutyric (CH₃CH(CH₃)COOH) acids.By comparison of P _gas and compensation points, it is concluded that the forest was a potential source for these acids. The soil-or the litter-acts as a significant source of a carboxylic acid of C₃ or C₄ atoms in the aliphatic chain. This carboxylic acid, not yet fully characterized, could play an important role in the rain acidity in forested zones of the equatorial areas.The direct emission of these carboxylic acids by vegetation was the main source in the boundary layer above the forest. The average emissions were 3.1×10⁹, 7.8×10⁹, and 8.4×10⁹ molecules cm^-2 s^-1 for HCOOH, CH₃COOH, and CH₃CH₂COOH, respectively. The savanna is an exogenous source of HCOOH and CH₃CH₂COOH during moderately rainy days for 30% of the time. The ozonolysis of isoprene seems to be a small source of HCOOH.