Volume 224, Issue 7-8 -

The hydroxyl-peroxy radical, HO 3 , has been the subject of numerous theoretical and experimental studies, not least because of its potential importance in atmospheric chemistry. Nevertheless, it has proved difficult to establish its stability: that is, values of D 0 (HO-O 2 ), the HO-O 2 bond dissociation energy or, equivalently, Δ f H o 298 (HO 3 ), its standard enthalpy of formation. In this short article, a review is given of the results of experiments and calculations on HO 3 , culminating in experiments performed at low temperatures (55.9–99.8 K) in a CRESU apparatus, which establish both (i) rate constants for the formation of HO 3 in the three-body reaction, OH + O 2 + M → HO 3 + M, and (ii) equilibrium constants from which values of D 0 (HO-O 2 ) and Δ f H o 298 (HO 3 ) can be derived – making use of spectroscopic data for HO 3 , from which partition functions and standard entropies can be calculated. It is shown that these absolute experimental values for D 0 (HO-O 2 ) and Δ f H o 298 (HO 3 ) are: (i) in agreement with the limiting values determined in experiments by Lester and co-workers, and (ii) in fair agreement with the most recent theoretical results. Furthermore, these thermodynamic data make it clear that only very small fractions of OH will be bound with O 2 under the conditions found at all levels of the Earth’s atmosphere.

The hydroxyl radical is the principal oxidizing agent in the troposphere. It controls and determines the oxidizing power of the atmosphere and thus governs the atmospheric lifetime of many species, and their potential to contribute to climate change, air pollution and ozone formation. Owing to the development of new measurement techniques and the discovery of new reaction mechanisms, the understanding of the OH chemistry has witnessed significant advances during the last decades. This article presents a comprehensive review of the current understanding of OH radical chemistry with an outlook to future work in this active research area of atmospheric chemistry.Among the different OH radical initiation sources, e.g. O 3 photolysis, HCHO photolysis, ozonolysis of alkenes and the photolysis of nitrous acid (HONO), the latter has been shown in recent field and model studies to make a major contribution to tropospheric OH levels. New photochemical sources of HONO have been discovered in the laboratory which can explain unexpectedly high concentrations of HONO observed in the atmosphere during daytime. A detailed analysis of HONO sources in the atmosphere is presented. OH radicals react with volatile organic compounds (VOCs) in a complex cycle regenerating OH radicals in the propagation cycle. Recent field and model studies show that the OH radical propagation cycle is balanced for high NO x conditions. Thus, radical losses by reactions with VOCs are balanced by OH formation through the reaction HO 2 +NO. However, studies at low NO and high VOC levels imply that unknown radical sources of OH exist in rural and forested areas. Possible sources of OH radicals under these conditions are discussed.

Previous studies have suggested that the reaction of O 2 ( 1 Σ g + ) + N 2 O → NO + NO 2 may be a significant source of NO x in the atmosphere if the branching ratio of this channel is greater than ~1%. We have measured the overall rate coefficient (k 1 ) for the reaction of O 2 ( 1 Σ g + ) with N 2 O to be k 1 (295 K) = (1.06±0.14)×10 −13 cm 3 molecule −1 s −1 at 295 K and k 1 (T) = (7.48±1.66)×10 −14 exp[(87±40)/T] cm 3 molecule −1 s −1 as a function of temperature over the range 210–370 K. The yields of NO, NO 2 or O 3 that are possible reaction products from the title reaction were undetectably small. Also, the net loss of N 2 O from the title reaction was negligibly small. We report upper limits for the yields for the production of NO x , the production of O 3 and the loss of N 2 O (all at 298 K) to be < 2×10 −4 , < 1×10 −3 , and < 3×10 −3 , respectively. We conclude that the reaction of O 2 ( 1 Σ g + ) + N 2 O is neither a significant source of NO x in the atmosphere nor a significant sink for N 2 O.

The kinetics of the reaction between NH(X 3 Σ – ) + SO(X 3 Σ – ) have been studied over the range 295–703 K using pulsed laser photolysis / laser induced fluorescence in 100 Torr He. The rate coefficient is (5.0±2.0)×10 −11 cm 3 molecule −1 s −1 , and is independent of temperature within experimental scatter. Electronic structure calculations show that reaction proceeds via bound intermediates to form NS + OH and NO + SH as the major bimolecular products. An RRKM / master equation analysis shows that stabilization of the intermediates is negligible at the pressures investigated (<10 4 Torr), so that the overall rate coefficient is independent of pressure. The calculations show that the main product channel under combustion conditions is NS + OH. Attempts to observe NS under the experimental conditions were unsuccessful. The experimental decay data are consistent with the occurrence of the reaction NH 2 + Cl → NH + HCl at temperatures above 500 K; approximate Arrhenius parameters are provided.

The branching ratio β = k 1b /k 1a for the formation of iso-propyl nitrate, i-C 3 H 7 ONO 2 , in the gas-phase reaction of i-C 3 H 7 O 2 with NO, i-C 3 H 7 O 2 + NO → i-C 3 H 7 O + NO 2 (1a), i-C 3 H 7 O 2 + NO → i-C 3 H 7 ONO 2 (1b), was determined over the pressure range 55–500 Torr at 298 K using a high-pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass-spectrometer. The β coefficient was found to increase linearly with pressure from about 0.6% at 55 Torr to about 3% at 500 Torr. The atmospheric implication of the results obtained is briefly discussed.

An experimental investigation on the photolysis of the pesticide chloropicrin, (trichloronitromethane, CCl 3 NO 2 ), under atmospheric conditions was carried out at the outdoor European Photoreactor, (EUPHORE), in Valencia, Spain. The photodissociation rate coefficient, J obs (CCl 3 NO 2 ), was determined directly under sunlight conditions during spring and summer months. Values in the range J obs (CCl 3 NO 2 ) = (3.9–5.1)×10 −5 s −1 were obtained, corresponding to photolysis lifetimes of 7.1–5.4 hours. Absorption cross-sections for chloropicrin were determined over the wavelength range 260–370 nm, and together with the measured solar flux intensity, were used to calculate the maximum photolysis rate for chloropicrin, J max . Comparison of the observed photolysis rate with the calculated maximum photolysis rate showed that the effective quantum yield of photodissociation, Φ = J obs (CCl 3 NO 2 )/ J max , was 0.94±0.08. Photolysis of chloropicrin in air or nitrogen gave phosgene as the major carbon-containing product with a yield close to unity based on the loss of chloropicrin. The product yield data were shown to be consistent with a mechanism in which the photolysis channel produces a CCl 3 radical and NO 2 . Kinetic studies on the reactions of hydroxyl radicals and ozone with chloropicrin suggest that, as expected, loss of CCl 3 NO 2 by reaction with these species will be negligible under atmospheric conditions compared to photolysis. Photolysis of chloropicrin in air in the presence of isopropanol gave significant yields of ozone and is consistent with the generation of Cl atoms and NO x following the photodissociation of CCl 3 NO 2 . The atmospheric implications of the use of chloropicrin as a pesticide are discussed.

The gas-phase ozonolysis of the biogenic unsaturated compounds 1-penten-3-ol, ( Z )-2-penten-1-ol and 1-penten-3-one has been investigated in two atmospheric simulation chambers. The following rate coefficients (in units of 10 −17 cm 3 molecule −1 s −1 ) were determined at atmospheric pressure and 293±2 K using an absolute rate method: 1-penten-3-ol, (1.64±0.15); ( Z )-2-penten-1-ol, (11.5±0.66); 1-penten-3-one, (1.17±0.15). Reaction products were identified by in situ FTIR spectroscopy and gas chromatography – mass spectrometry (GC-MS). The major products and their average molar yields in the presence of a radical scavenger at relative humidity < 1% were: formaldehyde (0.49±0.02), 2-hydroxybutanal (0.46±0.03) and propanal (0.15±0.02) from 1-penten-3-ol; propanal (0.39±0.03) and glycolaldehyde (0.43±0.04) from ( Z )-2-penten-1-ol; formaldehyde (0.37±0.02) and 2-oxobutanal (0.49±0.03) from 1-penten-3-one. The formation of secondary organic aerosol was also observed with yields ranging from 0.13–0.17 for the unsaturated alcohols. Significantly lower yields of around 0.03 were measured for 1-penten-3-one. The results of this work are used to determine atmospheric lifetimes and reaction mechanisms for the gas-phase ozonolysis of 1-penten-3-ol, ( Z )-2-penten-1-ol and 1-penten-3-one. The broader atmospheric implications of this work are also discussed.

The HO 2 • + HOCH 2 OO • reaction was theoretically investigated, using various high-level, single-reference Complete Basis Set methods including CBS-QB3, CBS-QCI/APNO and CBS-Q(MPW1B95) and a new multi-reference CI-PT2 approach. Three major product channels under atmospheric conditions were identified and their molecular mechanisms elucidated in great detail by Intrinsic Reaction Coordinate Analyses (IRC) at the B3LYP/6–311G(d,p) level: (i) Direct head-to-tail H-atom abstraction from the hydroperoxy radical by the alkylperoxy, occurring on the triplet Potential Energy Surface (PES) leading to HOCH 2 OOH + O 2 ; (ii) A two-step rearrangement of the initial singlet HOCH 2 OOOOH tetroxide complex to form HC(O)OH + • OH + HO 2 • ; (iii) A multi-step rearrangement of the initial HOCH 2 OOOOH tetroxide to yield HC(O)OH + O 2 ( 1 Δ) + H 2 O, about twice as fast as the former channel on the singlet-surface. The findings provide an explanation for the observed hydroxyl radical formation in this reaction (Jenkin et al., Phys. Chem. Chem. Phys. 9 (2007) 3149) and rationalize the high overall rate and its pronounced negative temperature dependence (Veyret et al., J. Phys. Chem . 93 (1989) 2368).

The formation and growth of iodine oxide particles (IOPs), originating from molecular iodine precursor, has been studied at room temperature as a function of water vapour, and sulphuric and oxalic acid vapours. A linear variation in total IOP mass was observed over a wide range of iodine atom production rates under both dry and humid formation conditions. Particle formation was also observed in the absence of ozone, and was found to be temperature sensitive, with elevated temperatures resulting in reduced particle number and mass. Electronic structure calculations are used to show that particle formation is initiated by polymerization of I 2 O 4 with I 2 O 3 , or with itself. Formation of IOPs in humid conditions results in lower numbers and smaller particles than formed in the absence of water vapour, because H 2 O forms relatively stable complexes with molecules such as I 2 O 3 and I 2 O 4 , inhibiting their polymerization. Addition of H 2 O to particles formed under dry conditions shows the collapse of fractal-like, aggregate particle structures. The uptake of sulphuric acid vapour onto humidified particles was studied over a wide range of relative humidity (RH) at room temperature, with the calculated accommodation coefficient ( α ) for this process increasing with RH to a value of 0.75±0.05 at RH = 90%. In contrast, growth of particles exposed to oxalic acid vapour was not observed on the experimental timescales employed, indicating an upper limit for α of 10 −3 .

The HOCl heterogeneous reaction on frozen natural (NSS) and recrystallized (RSS) sea salt, KCl and NaCl solutions was studied using a low pressure flow reactor in order to measure the uptake coefficient γ and products of reaction. The HOCl sample used in these experiments always contained up to 25% Cl 2 O which was also studied separately as a pure gas in order to understand the heterogeneous chemistry of both gases. By performing HOCl uptake on frozen NSS solution at 200 K and a gas-phase residence time of (1.6±0.6) s we obtained a steady state uptake coefficient γ HOCl on NSS = (2.5±0.7)×10 -3 and γ Cl2O on NSS = (2.8±0.8)×10 -3 . On frozen KCl solution at 200 K we obtain γ HOCl on KCl = (2.8±1.3)×10 -3 , identical to NSS, and γ Cl2O on KCl = (4.6±0.8)×10 -4 . The main product formed during the uptake on frozen NSS solution is Cl 2 which is sustained for at least one hour. In contrast, only a transient Cl 2 flow (pulse) decreasing on the time scale of 100 s was observed on frozen KCl (NaCl) solution. 25±10 % of the HOCl taken up on all chloride-containing frozen substrates at 200 K react to produce Cl 2 at high HOCl concentration (4.5×10 11 molecule cm -3 ) and at a residence time of 1.6 s in comparison with twice that for Cl 2 O. For smaller concentrations such as [HOCl] = 3.7×10 10 molecule cm -3 and/or a shorter residence time (0.137±0.004s), HOCl uptake did not generate Cl 2 in contrast to Cl 2 O. A single Br 2 burst event was monitored when a Cl 2 O or HOCl/Cl 2 O mixture is taken up on fresh frozen NSS solution during the first uptake at 200 K. Further Cl 2 O or HOCl/Cl 2 O uptake on the same sample, even after annealing at 240 K does not show an additional Br 2 pulse. This Br 2 release may be significant in the autocatalytic ozone destruction mechanism in the troposphere during polar sunrise. Some of the atmospheric implications of the present results are highlighted with emphasis on the preequilibrium Cl 2 O(ads) + H 2 O(ice) ↔ 2 HOCl(ads) between adsorbed HOCl and Cl 2 O, with the latter being the gateway to reactive uptake of HOCl at low temperatures.

This experimental study deals with heterogeneous reactions of benzo(a)pyrene (BaP) with ozone, nitrogen dioxide and hydroxyl radicals. BaP was adsorbed on silica particles chosen here as a model of mineral atmospheric particles. Compound extractions were assisted by focused microwave and analyses were performed by gas chromatography coupled with mass spectroscopy in single ion monitoring mode. Pseudo-first order rate constants were obtained from the fit of experimental decays of particulate-BaP concentration versus reaction time. Second order rate constants were determined considering the different oxidant gaseous concentrations except for the case of hydroxyl radicals where only a pseudo-first order rate constant was proposed. Values obtained at room temperature are (2.1±0.5)×10 −15 cm 3 molecule −1 s −1 for (BaP + ozone), (5.8±1.4)×10 −16 cm 3 molecule −1 s −1 for (BaP + nitrogen dioxide) and (3.4±0.8)×10 −2 s −1 for (BaP + OH) reactions. Products have only been investigated for the NO 2 and the OH (in the presence of NO x ) reactions. 1-, 3- and 6-nitrobenzo(a)pyrenes were detected as degradation products and quantified. Reaction rate constants for product formation are (3.7±0.9)×10 −16 cm 3 molecule −1 s −1 for 6-NBaP, (2.2±0.6)×10 −17 cm 3 molecule −1 s −1 for 1-NBaP and (5.3±1.3)×10 −17 cm 3 molecule −1 s −1 for 3-NBaP. 1-, 3- and 6-nitroBaP account respectively for approximately 5%, 12% and 83% of total nitrated species. If in the presence of only nitrogen dioxide, BaP was totally degraded within few minutes, only 20 to 25 % of the initial BaP led to nitrated compounds when reacting with OH (in the presence of NO x ).

An aerosol flow reactor was coupled to an infrared absorption cell to study aerosol formation processes with high temporal resolution. The recorded infrared spectra were referenced using aerosol smog chamber experiments. Evaluation was done by studying the formation of secondary organic aerosol from α-pinene and catechol as precursors and ozone as oxidant. Three main infrared absorptions: ν(O-H), ν(C-H) and ν(C=O) were considered, and humic like properties of the secondary organic aerosol are mainly interpreted according to the formation and variations of carbonyl bands in the region between 1850 and 1600 cm −1 , especially the ν(C=O) of aryl carbonyls from catechol oxidation products below 1700 cm −1 . The relative intensities of two major ν(C=O) stretching vibrations at 1690 cm −1 and 1755 cm −1 were observed to depend strongly on the available ozone concentration. At high precursor/ozone ratios (2:1 or 1:1) the vibration at 1690 cm −1 predominates, indicating aryl carbonyl vibrations. With increasing ozone concentrations this vibration is replaced by the higher carbonyl vibration at 1755 cm −1 indicating unsaturated carbonyl-containing compounds. This is a strong hint at ring opening processes leading to unsaturated aliphatic compounds in the resulting particle. Aryl carbonyls and aromatic or olefinic ν(C=C) at 1620 cm −1 in aged particles remain visible, as aerosol smog chamber studies exhibit – thus a strong hint at humic like properties of the SOA from the spectroscopic point of view.

Comparative measurements on two or more optically trapped aerosol droplets can allow the hygroscopic properties of aerosol to be investigated in exquisite detail. In this paper we consider two methods for assessing the dependence on relative humidity of the water activity and solute concentration in the condensed phase. We demonstrate in this paper that using a control droplet as a microprobe of relative humidity in the vicinity of a second droplet of interest can allow the RH to be determined with an accuracy of between 0.01 to 0.1 %. Not only is such a probe highly responsive, but the accuracy of the RH determination improves with increase in GF or RH, counter to the performance of macroscopic RH probes. We also demonstrate that spontaneous Raman scattering recorded from excitation of the OH stretching vibration in supersaturated solution droplets of sodium chloride shows the considerable perturbation induced by the chloride ion on the hydrogen bonding network. Chloride is recognised as a hydrogen bonding structure breaker and these observations are shown to be consistent with measurements on more dilute solutions.

In this study the interaction of NO 2 with ethylene soot particles has been studied in a temperature from 300 to 670 K and NO 2 concentrations ranging from 5 · 10 10 molecules cm −3 to 5 · 10 12 molecules cm −3 . The adsorption of NO 2 takes place in the whole temperature range studied with a mean initial uptake coefficient of (5.9±1.0) · 10 −2 independent of temperature and gas phase concentration. At low temperatures HONO and NO are formed as gas phase products. At higher temperatures NO and CO 2 have been observed as gaseous products indicating the oxidation and gasification of soot. For the NO and CO 2 forming reaction an activation energy of E A = (44.7±8.2) · kJ mol −1 has been determined.

The uptake of gaseous HC(O)OH, CH 3 C(O)OH and CF 3 C(O)OH on ice films at temperatures corresponding to the upper troposphere (UT) has been investigated using a coated-wall flow tube with mass spectrometric measurement of gas concentration. Uptake was largely reversible and followed Langmuir-type kinetic behavior, i.e. surface coverage increased with trace gas concentration approaching a maximum surface coverage at N max ~2 to 3×10 14 molecules cm −3 , corresponding to ~25% monolayer (ML). The partition constants, K LinC , were obtained from the experimental data by analysis using the simple Langmuir model and also using a simple one-dimensional numerical model to simulate individual uptake profiles and retrieve partition constants for specific conditions of temperature and concentration, over the temperature range 208–238 K. The analysis showed that Langmuir constants decreased at high surface coverages, possibly due to adsorbate-adsorbate interaction or modification of the ice surface. At low coverage, the following expressions described the temperature dependence of the partition coefficients (K LinC ) for HC(O)OH ( K LinC = (1.5± 3.5 1.0 )×10 −8 exp((5143±268)/T) cm), for CH 3 C(O)OH ( K LinC = (0.55± 5.4 0.5 )×10 −8 exp((5703±536)/T) cm), and CF 3 C(O)OH ( K LinC = (512± 1903 404 )×10 −8 exp((309±331)/T) cm). For CF 3 C(O)OH there was an irreversible component of uptake, which was attributed to hydrate formation on the surface.

Laser photolysis technique was used to study the reactivity of nitrate radical towards four dicarboxylic acids. The temperature dependence of the reactions was investigated in the range from 278K to 318K. The effect of the acid-base equilibrium was examined by measuring the activation parameters of the dissociated and undissociated acids at different pH. For droplets or aerosols under moderate acidic conditions, the charge exchange reaction at the carboxylate group might compete with the oxidation by the hydroxyl radical. On liquid particles enriched in nitrate (such as a deliquescent ammonium nitrate), the lifetime of carboxylic groups towards the NO 3 radical may be as short as few hours (or less).

NO 3 oxidation reactions contribute to the chemical conversion processes of organic compounds in the tropospheric multiphase system. Substituted phenols are known pollutants in cloud droplets, rain, fog and (deliquescent) particles. This study presents rate constants obtained from kinetic investigations of NO 3 reactions with different substituted phenolic compounds in the aqueous solution. Effects of the temperature as well as the ionic strength on the reaction rates were measured using a laser flash photolysis – laser long path absorption (LP-LLPA) set-up. The following rate constants at T = 298 K in M -1 s -1 were obtained for reactions of the NO 3 radical with 4-nitrophenol (k = (8.8±4.6) · 10 8 ), 2-nitrophenol (k = (8.3±1.4) · 10 8 ), 4-hydroxybenzoic acid (k = (1.5±0.4) · 10 9 ), 4-methylphenol (k = (1.7±0.3) · 10 9 ), 4-methoxyphenol (k = (2.5±0.4) · 10 9 ), 4-aminophenol (k = (2.0±0.3) · 10 9 ), 2,6-dichlorophenol (k = (1.3±0.2) · 10 9 ), 2,6-dihydroxyphenol (k = (1.7±0.2) · 10 9 ), 2,6-dimethylphenol (k = (1.8±0.3) · 10 9 ), 2,6-dimethoxyphenol (k = (1.6±0.4) · 10 9 ), 2,6-dinitrophenol (k = (2.8±0.9) · 10 8 ), 4-hydroxy-3,5-dimethoxybenzaldehyde (k = (1.7±0.3) · 10 9 ), 4-hydroxy-3,5-dimethoxybenzoic acid (k = (1.4±0.6) · 10 9 ), 4-hydroxy-3-methoxybenzaldehyde (k = (1.1±0.2) · 10 9 ), 4-hydroxy-3-methoxybenzoic acid (k = (1.0±0.3) · 10 9 ), 3-hydroxy-4-methoxybenzoic acid (k = (1.3±0.4) · 10 9 ). Averaged activation energies of: para-substituted phenols are E Aߙpara = 11±6 kJ mol -1 ; 2,6-substituted phenols E Aߙ2,6 = 15±4 , poly-substituted phenols E Aߙpoly = 16±4 kJ mol -1 . From reactivity correlations it is concluded that 4-nitrophenol, 2,6-dinitrophenol and 4-hydroxybenzoic acid react via direct electron transfer. The majority of investigated compounds react in a mixed regime with contribution of both electron transfer and H-atom abstraction. Surprisingly, changes in ionic strength do not affect the reactions dominated by electron transfer but have an influence on the rate constants of reactions with a mixed mechanism.