Photochemical activation of chlorine by iron-oxide aerosol

The photochemical activation of chlorine by dissolved iron in artificial sea-salt aerosol droplets and by highly dispersed iron oxide (Fe₂O₃) aerosol particles (mainly hematite, specific surface ~150 m² g^?1) exposed to gaseous HCl, was investigated in humidified air in a Teflon simulation chamber. Employing the radical-clock technique, we quantified the production of gaseous atomic chlorine (Cl) from the irradiated aerosol. When the salt aerosol contained Fe₂O₃ at pH 6, no significant Cl production was observed, even if the dissolution of iron was forced by “weathering” (repeatedly freezing and thawing for five times). Adjusting the pH in the stock suspension to 2.6, 2.2, and 1.9 and equilibrating for one week resulted in a quantifiable amount of dissolved iron (0.03, 0.2, and 0.6 mmol L^?1, respectively) and in gaseous Cl production rates of ~1.6, 6, and 8?×?10²¹ atoms cm^?2 h^?1, respectively. In a further series of experiments, the pure Fe₂O₃ aerosol was exposed to various levels of gaseous hydrogen chloride (HCl). The resulting Cl production rates ranged from 8?×?10²⁰ Cl atoms cm^?2 h^?1 (at ~4 ppb HCl) to 5?×?10²² Cl atoms cm^?2 h^?1 (at ~350 ppb HCl) and confirmed the uptake and conversion of HCl to atomic Cl (at HCl to Cl conversion yields of 2–5 %, depending on the relative humidity). The Fe₂O₃ experiments indicate that iron-induced Cl formation may be important for highly soluble combustion-aerosol particles in marine environments in the presence of gaseous HCl.     

Chlorine-hydrocarbon photochemistry in the marine troposphere and lower stratosphere

A one-dimensional photochemical model was used to explore the role of chlorine atoms in oxidizing methane and other nonmethane hydrocarbons (NMHCs) in the marine troposphere and lower stratosphere. Where appropriate, the model predictions were compared with available measurements. Cl atoms are predicted to be present in the marine troposphere at concentrations of approximately 10³ cm^-3, mostly as a consequence of the reaction of OH with HCl released from sea spray. Despite this low abundance, our results indicate that 20 to 40% of NMHC oxidation in the troposphere (0–10 km) and 40 to 90% of NMHC oxidation in the lower stratosphere (10–20 km) is caused by Cl atoms. At 15 km, NMHC-Cl reactions account for nearly 80% of the PAN produced.The model was also used to test the longstanding hypothesis that NOCl is an intermediate to HCl formation from sea salt aerosols. It was found that the NOCl concentration required (10 ppt) would be incompatible with field observations of reactive nitrogen and ozone abundance. Chlorine nitrate (ClONO₂) and methyl nitrate (CH₃ONO₂) were shown to be minor components of the total NO _y abundance. Heterogeneous reactions that might enhance photolysis of halocarbons or convert ClONO₂ to HOCl or Cl₂ were determined to be relatively unimportant sources of Cl atoms. Specific and reliable measurements of HCl and other reactive chlorine species are needed to better assess their role in tropospheric chemistry.     

Adsorption and reaction of trichlorofluoromethane on various particles

As a possible tropospheric sink of trichlorofluoromethane (CCl₃F), its adsorption and reaction on solid particles were studied with or without UV light longer than 310 nm. The adsorption and photodesorption occurred for most of the particles examined. The amounts depended not only on the BET surface area but also on the chemical property of the particles. The reaction rates were less than 0.1% h^-1. The surface induced degradation of CCl₃F in air was confirmed under photoirradiation at room temperature by the detection of Cl^- or F^- on some metal oxides. When the particles were pretreated with heat and evacuation, the reaction rate became larger and disproportionation of CCl₃F took place. The tropospheric lifetime of CCl₃F was reevaluated based on a reported model. It is suggested that the heterogeneous reaction of CCl₃F on particles may be a possible tropospheric sink.     

Regional Sources of Methyl Chloride, Chloroform and Dichloromethane Identified from AGAGE Observations at Cape Grim, Tasmania, 1998–2000

There are large uncertainties in identifying and quantifying the natural and anthropogenic sources of chloromethanes – methyl chloride (CH₃Cl), chloroform (CHCl₃) and dichloromethane (CH₂Cl₂), which are responsible for about 15% of the total chlorine in the stratosphere. We report two years of in situ observations of these species from the AGAGE (Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41° S, 145° E). The average background levels of CH₃Cl, CHCl₃ and CH₂Cl₂ during 1998–2000 were 551± 8, 6.3± 0.2 and 8.9± 0.2 ppt (dry air mole fractions expressed in parts per 10¹²) respectively, with a two-year average amplitude of the seasonal cycles in background air of 25, 1.1 and 1.5 ppt respectively. The CH₃Cl and CHCl₃ records at Cape Grim show clear episodes of elevated mixing ratios up to 1300 ppt and 55 ppt respectively, which are highly correlated, suggesting common source(s). Trajectory analyses show that the sources of CH₃Cl and CHCl₃ that are responsible for these elevated observations are located in coastal-terrestrial and/or coastal-seawater regions in Tasmania and the south-eastern Australian mainland. Elevated levels of CH₂Cl₂ (up to 70 ppt above background) are associated mainly with emissions from the Melbourne/Port Phillip region, a large urban/industrial complex (population 3.5 million) 300 km north of Cape Grim.Now at the Centre for Atmospheric ChemistryNow at School of Environmental Sciences     

Mechanism development and modelling of tropospheric multiphase halogen chemistry: The CAPRAM Halogen Module 2.0 (HM2)

A new detailed multiphase halogen mechanism, the CAPRAM Halogen Module 2.0 (HM2), has been developed and coupled to the multiphase chemistry mechanism RACM-MIM2ext/CAPRAM 3.0n. The overall mechanism comprises 1,705 reactions including 595 reactions of the HM2. Halogen chemistry box model studies have been, for the first time, performed with a non-permanent cloud scenario for pristine open ocean regions in mid-latitudes. Moreover, detailed time-resolved reaction flux analysis has been used to investigate the multiphase halogen reaction cycles in more detail. Clouds significantly change the multiphase halogen chemical system and new reaction cycles are proposed for in-cloud conditions. While most gas phase concentrations are decreased for chlorine and iodine species, they are increased for bromine. Flux analyses determined the relative contributions of the methylene dihalides CH₂IX (X = Cl, Br, I) as the main I atom source with a contribution of about 80 % to the total iodocarbon sources. Furthermore, HOI was confirmed to be important for chlorine activation. It is shown that 25 % of the ozone loss can be attributed to halogens. VOC oxidation by halogens is important as halogens account for about 20 % of the methane oxidation and up to 80 % of the oxidation of other VOCs. In other cases, enhanced VOC and VOC oxidation product concentration levels were found. For example, 15 % of the methyl peroxyl radicals are formed after the reaction of chlorine atoms with methane or methyl hydroperoxide. In the aqueous phase, changes in the oxidation of organics do only occur for highly oxidised organics without a C-H bond. For example, over 80 % of oxalic acid are oxidised by electron transfer with Cl₂ ^? in deliquescent particles during non-cloud periods.     

The Heterogeneous Reaction of NO<Subscript>2</Subscript> with NH<Subscript>4</Subscript>Cl: A Molecular Diffusion Tube Study

The heterogeneous interaction of nitrogen dioxide with ammonium chloride was investigated in a molecular diffusion tube experiment at 295–335 K and interpreted using Monte Carlo trajectory calculations. The surface residence time (τ_surf) of NO₂ on NH₄Cl is equal to 15 μs at 295 K, increases with temperature up to 323 K (τ_surf = 45 μs) and probably decreases beyond 323 K. The same experiment also yields uptake coefficients, γ, which are derived from the absolute number of surviving molecules effusing out of the diffusion tube. The rate of uptake of NO₂ on NH₄Cl followed a rate law first order in [NO₂] and the uptake coefficient γ is equal to 7 × 10⁻⁵ at 295 K, increases with temperature up to 323 K (γ = 2.1 × 10⁻⁴) and probably decreases beyond 323 K. Nitrous acid, water and nitrogen were detected as products. From these products, it is concluded that the reaction of NO₂ with NH₄Cl is a reverse disproportionation reaction where two moles of NO₂ result in ammonium nitrite, NH₄NO₂, as an intermediate, and nitryl chloride, NO₂Cl. NH₄NO₂ decomposes in two pathways, one to nitrous acid, HONO and NH₃, the other to nitrogen and water. The branching ratio for the production of HONO + NH₃ to that of N₂ + H₂O is approximately 20 at 298 K and increases with increasing temperature.     

Kinetics of the reactions of soot surface-bound polycyclic aromatic hydrocarbons with NO<Subscript>2</Subscript>

The kinetics of heterogeneous reactions of NO₂ with 17 polycyclic aromatic hydrocarbons (PAHs) adsorbed on laboratory generated kerosene soot surface was studied over the temperature range (255–330) K in a low pressure flow reactor combined with an electron-impact mass spectrometer. The kinetics of soot-bound PAH consumption due to their desorption and reaction with NO₂ were monitored using off-line HPLC measurements of their concentrations in soot samples as a function of reaction time, NO₂ concentrations in the gas phase being analyzed by mass spectrometer. No measurable decay of PAHs due to the reaction with NO₂ was observed under experimental conditions of the study (maximum NO₂ concentration of 5.5 × 10¹⁴ molecule cm⁻³ and reaction time of 45 min), which allowed to determine the upper limits of the first-order rate constants for the heterogeneous reactions of 17 soot-bound PAHs with NO₂: k < 5.0 × 10⁻⁵ s⁻¹ (for most PAHs studied). Comparison of these results to previous studies carried on different carbonaceous substrates, showed that heterogeneous reactivity of PAHs towards NO₂ is, probably, dependent on the substrate nature even for resembling, although different carbonaceous materials. Results show that particulate PAHs degradation by NO₂ alone is of minor importance in the atmosphere     

The Potential Influence of ClO·O2 on Stratospheric Ozone Depletion Chemistry

The inability to explain the observedoxygen suppression of chlorine photosensitized ozoneloss remains a gap in our understanding of thephotochemistry responsible for depletion of thestratospheric ozone layer. It has been suggested thatthe presence of a weakly bound ClO·O₂complexcould explain this effect. The existence of thiscomplex would alter the chlorine budget of thestratosphere, perhaps reducing the chlorine availablefor catalytic ozone destruction. On the other hand,the chemistry of ClO·O₂ provides two newpathwaysfor ClO dimer formation, which could increase the rateof catalytic ozone loss. In this paper, we constrainthe kinetic rate system of ClO·O₂ tomatch themeasured Cl_y budget. It is shown thatClO·O₂cannot be both fairly stable and rapidly form the ClOdimer, or the resulting partitioning of chlorinebecomes incompatible with observations of both ClO andtotal available chlorine. These constraints allow thateither: (1) the ClO·O₂ is fairly stable,but doesnot significantly enhance ClO dimer formation andtherefore has a negligible effect on ozone loss rates,or (2) the ClO·O₂ complex is only veryweaklystable, but does rapidly form the ClO dimer, andtherefore can influence stratospheric ozone depletion.Even at the ClO·O₂ mixing ratios allowedunderthe assumption of weak stability, 0.1 to 0.2 ppbv,significant ozone loss rate enhancements werecalculated. Of course, the chlorine budget constraintalso allows for a thirdpossibility; that ClO·O₂ is neither verystablenor forms Cl₂O₂ very rapidly. Measuredlimits on the reaction rates for ClO·O₂to form the ClO dimer would greatly aid the resolution of thisissue. Since the uncertainties aboutClO·O₂chemistry are so large, a potential role forClO·O₂ in stratospheric ozone loss cannotbe ruled out at this time.     

Chlorine initiated oxidation studies of hydrochlorofluorocarbons: Results for HCFC-123 (CF3CHCl2) and HCFC-141b (CFCl2CH3)

Oxidation reactions of the proposed CFC substitutes HCFC-123 (CF₃CHCl₂) and HCFC-141b (CFCl₂CH₃) have been studied in the laboratory using long-path Fourier transform infrared spectroscopy. The air oxidation of the HCFCs was initiated by the photolysis of Cl₂ forming Cl atoms that abstract H atoms from the HCFC. CF₃C(O)Cl was the only carbon containing compound observed in the infrared spectrum of the products of the HCFC-123/Cl₂ irradiations and its yield was approximately one. The product data are consistent with formation of CF₃C(O)Cl by Cl elimination of the intermediate halogenated alkoxy radical CF₃CCl₂O. The Cl-initiated oxidation of HCFC-141b led to the formation of CO and C(O)FCl. The product data are consistent with a 1 : 1 relationship between C(O)FCl formed and HCFC-141b reacted. Product data were compatible with both decomposition by cleavage of the C–C bond of the radical CFCl₂CH₂O leading to the prompt generation of C(O)FCl and reaction of the radical with O₂ forming the two carbon halogenated aldehyde CFCl₂CH(O), which in the presence of Cl was likely oxidized to C(O)FCl. An approximate method was developed in which the ratio was extracted from analysis of the time evolution of HCFC-141b, C(O)FCl, and CO. The data suggest that the contributions are comparable.     

An experimental technique for studying heterogeneous reactions of polyaromatic hydrocarbons on particle surfaces

An experimental technique for studying atmospheric heterogeneous reactions of polyaromatic hydrocarbons (PAH) on particle surfaces is reported. Particle bound organics were reacted in a 200 liter Teflon continuous stirred tank reactor (CSTR), with vapor phase oxidants. To provide a source of chemically stable particles for the CSTR, soot particles from a residential wood stove were first introduced during under darkness into a 25 m³ outdoor Teflon chamber. Air containing the particles was then added at a constant flow to the CSTR. The rates of heterogeneous reactions were obtained by comparing reacted particle samples with unreacted ones. The derivation of rate expressions for heterogeneous reactions in the CSTR is described. The use of the technique for a study of the nitration of selected soot particle bound PAH species by NO₂ and HNO₃ is demonstrated.     

The 1985 chlorine and fluorine inventories in the stratosphere based on ATMOS observations at 30° north latitude

The set of high-resolution infrared solar observations made with the Atmospheric Trace Molecule Spectroscopy (ATMOS)-Fourier transform spectrometer from onboard Spacelab 3 (30 April-1 May 1985) has been used to evaluate the total budgets of the odd chlorine and fluorine chemical families in the stratosphere. These budgets are based on volume mixing ratio profiles measured for HCl, HF, CH₃Cl, ClONO₂, CCl₄, CCl₂F₂, CCl₃F, CHClF₂, CF₄, COF₂, and SF₆ near 30° north latitude. When including realistic concentrations for species not measured by ATMOS, i.e., the source gases CH₃CCl₃ and C₂F₃Cl₃ below 25 km, and the reservoirs ClO, HOCl and COFCl between 15 and 40 km (five gases actually measured by other techniques), the 30° N zonal 1985 mean total mixing ratio of chlorine, Cl, was found to be equal to (2.58±0.10) ppbv (parts per billion by volume) throughout the stratosphere, with no significant decrease near the stratopause. The results for total fluorine indicate a slight, but steady, decrease of its volume mixing ratio with increasing altitude, around a mean stratospheric value of (1.15±0.12) ppbv. Both uncertainties correspond to one standard deviation. These mean springtime 1985 stratospheric budgets are commensurate with values reported for the tropospheric Cl and F concentrations in the early 1980s, when allowance is made for the growth rates of their source gases at the ground and the time required for tropospheric air to be transported into the stratosphere. The results are discussed with emphasis on conservation of fluorine and chlorine and the partitioning among source, sink, and reservoir gases throughout the stratosphere.     

Heterogeneous Chemistry of Nitryl Halides in Relation to Tropospheric Halogen Activation

The heterogeneous chemistry of nitryl chloride and nitryl bromide by salt containing solutions was studied as a function of temperature in the range from 275 to 293 K with the wetted-wall flowtube combined with FTIR and mass spectrometry detection. Uptake coefficients and values of the product Hk^1/2 on these saline solutions have been determined. For nitryl halides interacting with NaI and NaBr solutions, the values of the product Hk^1/2 are respectively 4384.7±326.7 and 103.1±18.7 M atm^-1 s^-1/2 for nitryl chloride at 275 K and 544.2±94.7 and 47.7±15.2 M atm^-1 s^-1/2 for nitryl bromide at 278 K. When reacting with NaI or NaBr solutions, these heterogeneous reactions release, as major products, the molecular forms of the halogen i.e., respectively I₂ and Br₂. A simplified reaction scheme explaining the formation of these products is presented and is inserted into a model simulating the chemistry in the marine boundary layer. The modelling effort showed Cl and BrO atoms concentrations up to 5×10⁴ and 1.8×10⁶ molecules cm^-3 respectively, which are comparable to values actually measured in field campaigns.     

Laboratory spectroscopic studies of atmospherically important radicals using fourier transform spectroscopy

This paper describes laboratory experiments designed to obtain the infrared spectra of some atmospherically important radical species and related compounds. A Fourier transform spectrometer was used that was capable of yielding resolutions as great as 0.0024 cm^-1, and optical paths of up to 512 m were employed. The objective of the experiments was to obtain the spectra for subsequent application to remote sounding measurements in the atmosphere.Radicals were generated by a variety of chemical reactions involving atoms or other highly reactive precursors. Spectra of the ₃ band of NO₃, at ca. 1500 cm^-1, were obtained with up to 0.005 cm^-1 resolution using the reaction between NO₂ and O₃ to produce the radical. The most satisfactory source of ClO was found to be the reaction between Cl and O₃, and the (1-0) vibration-rotation band in the region 829–880 cm^-1 was recorded at a resolution of 0.02 cm^-1. We were unable to observe infrared absorption of HO₂ with any of the radical sources that we tested. High-resolution survey spectra were obtained of compounds used as reactants, or formed as side-products in the radical-generating processes. These compounds included N₂O₅, HNO₃, ClONO₂, FNO₂, Cl₂O, HO₂NO₂, and probably FO₂.The ability to monitor concentrations of the NO₃ radical in the visible region of the spectrum as well as the concentrations of reactants and other products in the infrared region allowed us to undertake a study of the time-dependent interactions occurring when NO₂ reacts with O₃. The results indicate the importance of heterogeneous processes, especially when traces of water are present, and lend credence to suggestions that heterogeneous mechanisms in the NO₃–N₂O₅–H₂O system might be a viable source of HNO₃ in the atmosphere.     

The Influence of NaBr/NaCl Ratio on the Br--Catalysed Production of Halogenated Radicals

The activation of Br^- and Cl^- to atomic Br and Cl in sea-spray aerosol was investigated in smog-chamber experiments. In the presence of O₃, hydrocarbons and NaCl aerosol alone no activation was observed. By adding Br^- to the aerosol, the chain reaction: Br + O₃ BrO, BrO + HO₂ HOBr, HOBr HOBr(aq), HOBr(aq) + H⁺ + Br^- Br₂ (6), HOBr(aq) + H⁺ + Cl^- BrCl (7) was verified. The step from reaction (6) to (7) is accompanied by a decrease of the Br^-/Cl^- ratio from 1/600 to less than 1/2000. In the absence of sulphate, the chain is initiated by the reaction of OH(aq) with Br^-. The pH value decreases to less than 2 during the first minutes of the experiment and later on to almost 1 (in the absence of NO_x or SO₂). This is caused by the formation of oxalic acid from alkanes and toluene. In stopped flow experiments, the reduction of Br₂ by oxalic acid was observed to occur through a two-step mechanism: HC₂O₄ ^- + Br₂ Br^- + BrC₂O₄H (k₂₂, k_-22), BrC₂O₄H Br^- + H⁺ + 2 CO₂ (23) with the following rate constants and ratios of rate constants, k ± 2: k₂₂k_-23 / k_-22 = (2.9 ± 0.3) · 10^-4 s^-1, k_-22 / k_-23 = 7000 ± ₃₀₀₀ ¹³⁰⁰⁰ M^-1, k₂₂ = 2 ±_-1 ⁴ M^-1 s^-1, and k_-23 > 0.1 s^-1, k_-22 > 600 M^-1 s^-1. Oxalic acid may be responsible for the inhibition of the chain reaction observed at the end of the experiments.     

Rainwater Chemistry at Mt. Etna (Italy): Natural and Anthropogenic Sources of Major Ions

Major ion content of 37 wet-only rainwater samples collected on the southern flank of Mount Etna volcano was investigated. Measured pH values range from 3.80 to 7.22 and display a positive correlation with Ca²⁺ and an inverse correlation with NO₃ ^–, suggesting that anthropogenic NO_x are the most effective acidifying agents while Ca, likely as solid CaCO₃, is the prevailing proton acceptor. Na/Cl ratios indicate a dominant marine origin for both species, while K, mg and Ca contents point to additional sources (soil dust, fertilisers etc.). Nitrate and sulphate concentrations display a nearly constant ratio indicating a common anthropogenic origin, and only a few samples are characterised by sulphate excess. The analysis of time series reveals a good correlation between the excess sulphate in rainwater and SO₂ fluxes from the summit craters plume. Non sea salt chloride contents show also a significant correlation with volcanic activity indicating a magmatic sulphur and chloride contribution to rainwater. Meteoric flux estimations point to a prevailing magmatic origin for sulphur in the collected rainwaters while sea spray is the main source of chlorine.     

Transformations of Benzene Photoinduced by Nitrate Salts and Iron Oxide

This paper reports the results of a study on the transformation of benzene in the presence of solid nitrate salts (NaNO₃, NH₄NO₃) under irradiation in a gas-solid photoreactor. Sodium and ammonium nitrate have been chosen as representative of the composition of atmospheric particulate, benzene as a model aromatic molecule. The purpose is to simulate the transformations that aromatic compounds undergo on the surface of dispersed particles in the atmosphere. Irradiation of sodium nitrate causes hydroxylation and nitration of benzene, yielding phenol and nitrobenzene. This is most likely due to the generation of ^•OH and ^•NO₂ radicals upon nitrate photolysis, with ^•OH + O₂ leading to the formation of phenol and ^•OH + ^•NO₂ yielding nitrobenzene. The percentage of oxygen in the reaction environment influences the transformation pathways, with phenol formation being favoured and nitrobenzene formation depressed by high O₂ concentration. In the presence of hematite (α-Fe₂O₃, another component of atmospheric particulate) very relevant formation of nitrobenzene takes place even with 21% oxygen (simulated air), indicating that the interaction between hematite and nitrate can lead to the formation of aromatic nitroderivatives on the surface of atmospheric particulate. The effect of hematite is possibly due to protonation of peroxynitrite, formed upon nitrate photoisomerisation, to peroxynitrous acid, a powerful nitrating agent. A similar effect leads to relevant formation of nitrobenzene under atmospheric conditions upon irradiation of the acid salt ammonium nitrate.     

FTIR studies of reactions between the nitrate radical and haloethenes

Products and mechanisms for the gas-phase reactions of NO₃ radicals with CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂,cis-CHCl=CHCl andtrans-CHCl=CHCl in air have been studied. The experiments were carried out at 295±2 K and 740±5 Torr in a 480-L Teflon-coated reaction chamber and at 295±2 K and 760±5 Torr in a 250-L stainless steel reactor. NO₃ was generated by the thermal dissociation of N₂O₅. Experiments with¹⁵NO₃ and CD₂CDCl have also been performed. The initially formed nitrate peroxynitrates decay into carbonyl compounds, nitrates, HCl and ClNO₂. In adidtion, there are indications of nitrooxy acid chlorides being produced. The reactions with CH₂=CCl₂ and CHCl=CCl₂ are more complex due to release of chlorine atoms which eventually lead to formation of chloroacid chlorides.A general reaction mechanism is proposed and the observed concentration-time profiles of reactants and products are simulated for each compound. The rate constants for the initial step of NO₃ addition to the chloroethenes are determined as: (2.6±0.5, 9.4±0.9, 2.0±0.4 and 1.4±0.4) × 10^–16 cm³ molecule^–1 s^–1 for CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂ andcis-CHCl=CHCl, respectively.     

Modelling the multiphase near-surface chemistry related to ozone depletions in polar spring

Near-total depletions of ozone have been observed in the Arctic spring since the mid 1980s. The autocatalytic reaction cycles involving reactive halogens are now recognized to be of main importance for ozone depletion events in the polar boundary layer. We present sensitivity studies using the model MISTRA in the box-model mode on the influence of chemical species on these ozone depletion processes. In order to test the sensitivity of the chemistry under polar conditions, we compared base runs undergoing fluxes of either Br₂, BrCl, or Cl₂ to induce ozone depletions, with similar runs including a modification of the chemical conditions. The role of HCHO, H₂O₂, DMS, Cl₂, C₂H₆, HONO, NO₂, and RONO₂ was investigated. Cases with elevated mixing ratios of HCHO, H₂O₂, DMS, Cl₂, and HONO induced a shift in bromine speciation from Br / BrO to HOBr/HBr, while high mixing ratios of C₂H₆ induced a shift from HOBr/HBr to Br/BrO. The shifts from Br/BrO to HOBr/HBr accelerated the aerosol debromination, but also increased the total amount of deposited bromine at the surface (mainly via increased deposition of HOBr). For all NO_y species studied (HONO, NO₂, RONO₂) the chemistry is characterized by an increased bromine deposition on snow reducing the amount of reactive bromine in the air. Ozone is less depleted under conditions of high mixing ratios of NO_x. The production of HNO₃ led to the acid displacement of HCl, and the release of chlorine out of salt aerosol (Cl₂ or BrCl) increased.     

Laboratory measurements of the <Superscript>12</Superscript>C/<Superscript>13</Superscript>C kinetic isotope effects in the gas-phase reactions of unsaturated hydrocarbons with Cl atoms at 298 ± 3 K

The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k ₁₂/k ₁₃, are greater than unity or normal KIEs. The KIEs, reported in per mil according to ^Cl ɛ = (k ₁₂/k ₁₃−1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1); propene, 5.56 ± 0.18 (2); 1-butene, 5.93 ± 1.16 (1); 1-pentene, 4.86 ± 0.63 (1); cyclopentene, 3.75 ± 0.14 (1); toluene, 2.89 ± 0.31 (2); ethylbenzene, 2.17 ± 0.17 (2); o-xylene, 1.85 ± 0.54 (2). To our knowledge, these are the first reported KIE measurements for reactions of unsaturated NMHC with Cl atoms. Relative rate constants were determined concurrently to the KIE measurements. For the reactions of cyclopentene and ethylbenzene with Cl atoms, no rate constant has been reported in refereed literature. Our measured rate constants are: cyclopentene (7.32 ± 0.88) relative to propene (2.68 ± 0.32); ethylbenzene (1.15 ± 0.04) relative to o-xylene (1.35 ± 0.21), all × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The KIEs in reactions of aromatic hydrocarbons with Cl atoms are similar to previously reported KIEs in Cl-reactions of alkanes with the same numbers of carbon atoms. Unlike the KIEs for previously studied gas-phase hydrocarbon reactions, the KIEs for alkene–Cl reactions do not exhibit a simple inverse dependence on carbon number. This can be explained by competing contributions of normal and inverse isotope effects of individual steps in the reaction mechanism. Implications for the symmetries of the transition state structures in these reactions and the potential relevance of Cl-atom reactions on stable carbon isotope ratios of atmospheric NMHC are discussed.     

A numerical model for one‐dimensional simulation of stratospheric chemistry

Abstract

We describe a one‐dimensional (1‐D) numerical model developed to simulate the chemistry of minor constituents in the stratosphere. The model incorporates most of the chemical species presently found in the upper atmosphere and has been used to investigate the effect of increasing chlorofluorocarbon (CFC) emissions on ozone (O₃).

Our calculations confirm previous results that O₃ depletions in the 20–25 km region, the region of the O₃ maximum, are very sensitive to the relative abundances of Cl_x and NO_y in the lower stratosphere for high Cl_x amounts. The individual abundances of lower stratospheric Cl_x and NO_y amounts are very sensitive to upper tropospheric mixing ratios, which, in turn, are determined largely by surface input fluxes and heterogeneous loss processes. Thus the behaviour of column O₃ depletions at high Cl_x levels is greatly affected, albeit indirectly, by tropospheric processes. For high Cl_x levels the O_x flux from the stratosphere to the troposphere is dramatically reduced, leading to a large reduction in tropospheric O₃. Some of the variation between different published 1‐D model results is most likely due to this critical dependence of O₃ depletion on NO_y‐Cl_x ratios.

Model simulations of time‐dependent CFC effects on ozone indicate that if CFCs were to remain at constant 1980 emission rates while N₂O increased at 0.25% a^?1 and CH₄ increased at 1% a^?1, we could expect a 2.2% decrease in total column O₃ (relative to the 1980 atmosphere) by the year 2000. However, if CFC emission rates were to increase by 3% a^?1 (current estimates are 5–6% a^?1), we would predict a depletion of 2.7% by the year 2000. The calculations for times beyond the year 2000 suggest that the effects on total O₃ will begin to accelerate. If methyl chloroform emissions are added at 7% a^?1 (current estimates are 7–9% a^?1) to the above CFC‐N₂O‐CH₄ scenario we calculate total O₃ depletions by the year 2000 that are 41% larger than those calculated without. This suggests that if the emissions of methyl chloroform continue to increase at their present rate then methyl chloroform could have a significant effect upon total O₃.