Tropospheric OH and HO(2) radicals: field measurements and model comparisons

The hydroxyl radical, OH, initiates the removal of the majority of trace gases in the atmosphere, and together with the closely coupled species, the hydroperoxy radical, HO(2), is intimately involved in the oxidation chemistry of the atmosphere. This critical review discusses field measurements of local concentrations of OH and HO(2) radicals in the troposphere, and in particular the comparisons that have been made with numerical model calculations containing a detailed chemical mechanism. The level of agreement between field measurements of OH and HO(2) concentrations and model calculations for a given location provides an indication of the degree of understanding of the underlying oxidation chemistry. We review the measurement-model comparisons for a range of different environments sampled from the ground and from aircraft, including the marine boundary layer, continental low-NO(x) regions influenced by biogenic emissions, the polluted urban boundary layer, and polar regions. Although good agreement is found for some environments, there are significant discrepancies which remain unexplained, a notable example being unpolluted, forested regions. OH and HO(2) radicals are difficult species to measure in the troposphere, and we also review changes in detection methodology, quality assurance procedures such as instrument intercomparisons, and potential interferences.     

Daytime sources of nitrous acid (HONO) in the atmospheric boundary layer.

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH), the self-cleaning agent of the atmosphere and a key species in the formation of harmful photooxidants during summer smog. Recent field measurements using very sensitive HONO instruments have shown that daytime HONO concentrations are much higher than has been assumed previously and that the contribution of HONO to the radical formation was underestimated in the past. A strong photochemical HONO source has been proposed, which contributes to the primary OH radical production up to 56 %. These exciting results initiated new laboratory studies, in which new sources of HONO have been identified. It is demonstrated that HONO is photochemically formed 1) on surfaces treated with nitric acid, 2) by reduction of NO(2) on photosensitized organic surfaces like humic acids and c) in the gas phase photolysis of ortho-substituted nitroaromatics. Although significant uncertainties still exist on the exact mechanisms, these additional sources might explain daytime observations in the atmosphere and demonstrate that HONO should be generally measured in field campaigns, besides other radical sources.     

Formation of hydroxyl radical from the photolysis of frozen hydrogen peroxide

Hydrogen peroxide (HOOH) in ice and snow is an important chemical tracer for the oxidative capacities of past atmospheres. However, photolysis in ice and snow will destroy HOOH and form the hydroxyl radical (*OH), which can react with snowpack trace species. Reactions of *OH in snow and ice will affect the composition of both the overlying atmosphere (e.g., by the release of volatile species such as formaldehyde to the boundary layer) and the snow and ice (e.g., by the *OH-mediated destruction of trace organics). To help understand these impacts, we have measured the quantum yield of *OH from the photolysis of HOOH on ice. Our measured quantum yields (Phi(HOOH --> *OH)) are independent of ionic strength, pH, and wavelength, but are dependent upon temperature. This temperature dependence for both solution and ice data is best described by the relationship ln(Phi(HOOH --> *OH)) = -(684 +/- 17)(1/T) + (2.27 +/- 0.064) (where errors represent 1 standard error). The corresponding activation energy (Ea) for HOOH (5.7 kJ mol(-1)) is much smaller than that for nitrate photolysis, indicating that the photochemistry of HOOH is less affected by changes in temperature. Using our measured quantum yields, we calculate that the photolytic lifetimes of HOOH in surface snow grains under midday, summer solstice sunlight are approximately 140 h at representative sites on the Greenland and Antarctic ice sheets. In addition, our calculations reveal that the majority of *OH radicals formed on polar snow grains are from HOOH photolysis, while nitrate photolysis is only a minor contributor. Similarly, HOOH appears to be much more important than nitrate as a photochemical source of *OH on cirrus ice clouds, where reactions of the photochemically formed hydroxyl radical could lead to the release of oxygenated volatile organic compounds to the upper troposphere.     

Water effect on the OH + HCl reaction

Hydrochloric acid is a major reservoir for chlorine radicals in the atmosphere. Chlorine radicals are chemically reactivated by the relatively slow attack of OH radical on HCl. Through the formation of hydrogen-bonded complexes, water has a dramatic effect on the rate of this reaction. The introduction of water opens several new reaction pathways with rate coefficients that are faster than the "bare" reaction. Accounting for the low fraction of hydrogen bonded water complexes in the atmosphere, the present results suggest that these new mechanisms involving water can contribute, although modestly, to the total chemical reactivation of chlorine from HCl in the lower troposphere. The first reported value for the equilibrium constant for the formation of H(2)O·HCl complex, which is important in understanding the removal of HCl from the atmosphere by deposition, is presented.     

The lifetimes of organosulphur compounds in the troposphere

Dimethyl sulphide and other reduced sulphur compounds, produced by marine biogenic activity and other processes, play a significant role in the global biogeochemical cycling of the element. The rates of their reactions with atmospheric oxidants are reviewed and their lifetimes in the troposphere due to the various reactions are computed. Sufficient data are available on the tropospheric abundance of the hydroxyl radical (OH) and on the rates of its reactions with the sulphur compounds for reasonable estimates to be made of the sulphur lifetimes with respect to OH. Summertime lifetimes of 14–87 h for (CH₃)₂S are computed at different latitudes. In the case of the tropospheric concentrations of the nitrate radical (NO₃), few data are available. There is a similar paucity of data on its rates of reactions with the sulphur compounds, and so large uncertainties exist in the computed lifetimes. These are, in any case, much longer than those due to OH. The possibility exists that iodine photochemistry, producing iodoxyl (IO) radicals, may efficiently oxidize the reduced sulphur and other organic compounds in the marine troposphere leading to lifetimes of the order of hours. Few data exist on the rates or mechanisms of these reactions and these are identified as representing the greatest uncertainties in the estimates of organosulphur lifetimes in the troposphere.     

Do hydroxyl radical-water clusters, OH(H2O)n, n = 1-5, exist in the atmosphere?

It has been speculated that the presence of OH(H2O)n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.     

Chlorine atoms as a potential tropospheric oxidant in the marine boundary layer

In the troposphere, the hydroxyl radical (OH) has been assumed to be the major oxidant for organics and CO. However, a variety of evidence from both laboratory and field studies over the last decade strongly suggests that atomic chlorine may play a key role in marine areas. Potential reactions of sea salt particles, formed in marine areas by wave action, which could generate photolytic precursors to atomic chlorine are reviewed. The results of laboratory studies of NaCl reactions as well as the recent detection of Cl₂ in the marine troposphere indicate that oxidation of organics in marine areas by atomic chlorine may rival or even exceed that of OH under some conditions. Data from recent field studies which are suggestive of chlorine atom chemistry in air masses which have travelled over the Pacific Ocean are discussed. Finally, some future studies are suggested which would help to clarify and evaluate the importance of chlorine atoms in the chemistry of the marine boundary layer.     

Simple and sensitive determination of hydroxyl radical in atmosphere based on an electrochemically activated glassy carbon electrode

The hydroxyl radical (?OH) plays important roles in environment and health problems. However, the short life time and low concentrations of ?OH limited its detection. In this work, a simple method has been successfully performed for the sensitive detection of hydroxyl radical based on an activated glassy carbon electrode (AGCE).4-hydroxybenzoic acid (4-HBA) was used as a trapping agent for ?OH radicals, leading to the production of electroactive 3,4-dihydroxybenzoic acid (3,4-DHBA). Different procedures including polarisation and cyclic voltammetry in acid or base solutions have been used to activate the glassy carbon electrodes. The electrochemical behaviours of 3,4-DHBA on these activated electrodes were studied and compared. Experimental results showed that the glassy carbon electrode polarised in H₂SO₄ (AGCE-P/H₂SO₄) has the greatest sensitivity and reproducibility to 3,4-DHBA. 3,4-DHBA performed a linear relationship from 1.0 × 10^?7 to 1.0 × 10^?4 M on the AGCE-P/H₂SO₄. The detection limit was down to 6.2 × 10^?8 M. This method has been successfully applied for the detection of hydroxyl radical levels in atmosphere without separation and purification process.     

Quantitative analysis of the kinetic constant of the reaction of N,N'-propylenedinicotinamide with the hydroxyl radical using dimethyl sulfoxide and deduction of its structure in chloroform

N,N'-Propylenedinicotinamide (Nicaraven) is presently being developed for the treatment of cerebral stroke including subarachnoid hemorrhage. This drug is promising because some data suggest it to have an ability to scavenge the hydroxyl radical under physiological conditions in vivo, while it also has a high permeability through the blood brain barrier. Using the kinetic constant of the reaction between the hydroxyl radical and dimethyl sulfoxide, the formula derived by Babbs and Griffin (Free Rad. Biol. Med., 6 1989) was applied to obtain the kinetic constant of Nicaraven with the hydroxyl radical using a dimethyl sulfoxide-xanthine oxidase-hypoxanthine-Fe system, and this yielded the kinetic constant 3.4x10(9) M(-1) s(-1) (1 M=1 mol dm(-3)) for Nicaraven. Structurally related compounds were also investigated. The amide group of Nicaraven was thus found to play an important part in the reaction with the hydroxyl radical. Methanesulfinic acid, which was obtained from the reaction between dimethyl sulfoxide and the hydroxyl radical, was found to be stable under this adopted experimental condition and therefore was used to quantify the kinetic constant of Nicaraven. The structure of Nicaraven has also been investigated in CDCl3 using IR spectra, computer calculations and 1H-NMR analysis, and Nicaraven was thus shown to have an intramolecular hydrogen bond which forms a 7-membered ring that resembles a part of the 1H-1,4-benzodiazepines. This structure may play an important role in the penetration through the blood brain barrier.     

Rates of Reactive Removal of Anthropogenic Emissions in the Atmosphere

Anthropogenic emissions such as SO₂, CO, NO_x, and hydrocarbons constitute a perturbation of the natural equilibrium of the atmosphere. On a global scale, the extent of this perturbation does not significantly exceed the natural background levels as a result of direct or indirect photochemical “clean-up” processes in the atmosphere. Regionally, however, considerably increased concentrations of these trace components may occur. They are degraded by free radical reactions having widely differing yet substance-specific rates.     

Determination of antioxidant activity of phenolic antioxidants in a Fenton-type reaction system by chemiluminescence assay

The hydroxyl radical (*OH) has been implicated in various diseases, and it is therefore important to establish efficient methods to screen hydroxyl radical scavengers for antioxidant therapy. In this paper, a simple chemiluminescence assay was established to evaluate the *OH-scavenging capacity of phenolic compounds. This assay took advantage of the transient property of the Fenton reaction and the reaction between luminol and the hydroxyl radical, and effectively avoided the pro-oxidant action of some phenolic compounds. Fifteen phenolic compounds were assessed for their antioxidant activity in the Fenton reaction system, and even in the case of "pro-oxidants" that were excluded from the widely used deoxyribose (DR) assay. Since it overcomes the challenges that the traditional DR assay encounters, our method has promising applicative values: it is low-cost, time-saving, and reliable. It would also be more favorable than electron spin resonance (ESR) and radiolysis technology, which are known to be expensive and not commonly available to those specialized in free radical biology and medicine.     

OH-initiated oxidation mechanism and kinetics of organic sunscreen benzophenone-3: A theoretical study

Benzophenone-3 (BP-3), as an important organic UV filter, is widely used in the sunscreen, cosmetic, and personal care products. The chemical reaction mechanism and kinetics of BP-3 degradation initiated by hydroxyl (OH) radical was investigated in the atmosphere based on the density functional theory (DFT). The results showed that the OH radical is more easily added to the C3 position of the aromatic ring (pathway 3), while the H atom abstraction from the OH group on the aromatic ring (pathway 23) is an energetically favorable reaction pathway. At ambient temperature, 298 K, the overall rate constant for the primary reaction is about 1.50 × 10^?10 cm³ mol^?1 s^?1 with the lifetime of 1.92 h. OH addition reactions play the key role in the OH-initiated reaction of BP-3. The study is helpful for better understanding of the removal, transformation, and fate of BP-3 in the atmosphere.     

Cavity-enhanced measurements of hydrogen peroxide absorption cross sections from 353 to 410 nm

We report near-ultraviolet and visible absorption cross sections of hydrogen peroxide (H(2)O(2)) using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS), a recently developed, high-sensitivity technique. The measurements reported here span the range of 353-410 nm and extend published electronic absorption cross sections by 60 nm to absorption cross sections below 1 × 10(-23) cm(2) molecule(-1). We have calculated photolysis rate constants for H(2)O(2) in the lower troposphere at a range of solar zenith angles by combining the new measurements with previously reported data at wavelengths shorter than 350 nm. We predict that photolysis at wavelengths longer than those included in the current JPL recommendation may account for up to 28% of the total hydroxyl radical (OH) production from H(2)O(2) photolysis under some conditions. Loss of H(2)O(2) via photolysis may be of the same order of magnitude as reaction with OH and dry deposition in the lower atmosphere; these processes have very different impacts on HO(x) loss and regeneration.     

Formation of 3-Methylfuran from the gas-phase reaction of OH radicals with isoprene and the rate constant for its reaction with the OH radical

3-Methylfuran has been identified as a product of the gas-phase reaction of the OH radical with isoprene, and under simulated atmospheric conditions a formation yield of 0.044 ± 0.006 was determined. In an analogous manner, the OH radical reaction with 1,3-butadiene formed furan with a yield of 0.039 ± 0.011. Using a relative rate method, a rate constant for the reaction of the OH radical with 3-methylfuran of 9.35 × 10^?11 cm³ molecule^?1 s^?1 (with an estimated overall uncertainty of ±20%) at 296 ± 2 K was also determined. These data show that 3-methylfuran is a reactive compound which will be present in the troposphere at concentrations ?5% of those of its isoprene precursor.     

Hydroxyl radical detection with a salicylate probe using modified CUPRAC spectrophotometry and HPLC

Reactive oxygen species (ROS) may attack biological macromolecules giving rise to oxidative stress-originated diseases, so it is important to establish efficient methods to screen hydroxyl radical scavengers for antioxidant therapy. Since *OH is very short-lived, secondary products resulting from *OH attack to various probes are measured. As a low-cost measurement technique, we used a salicylate probe for detecting hydroxyl radicals generated from an equivalent mixture of Fe(II)+EDTA with hydrogen peroxide. The produced hydroxyl radicals attacked both the probe and the water-soluble antioxidants in 37 degrees C-incubated solutions for 2 h. The CUPRAC (cupric ion reducing antioxidant capacity) assay absorbance of the ethylacetate extract due to the reduction of Cu(II)-neocuproine reagent by the hydroxylated probe decreased in the presence of *OH scavengers, the difference being proportional to the scavenging ability of the tested compound. Attack by *OH radicals upon salicylate produced 2,3-dihydroxybenzoate, 2,4-dihydroxybenzoate, and 2,5-dihydroxybenzoate as major products. HPLC separation combined with CUPRAC spectrophotometry was used to identify and quantify hydroxylated salicylate derivatives in the presence of synthetic water-soluble antioxidants and green tea infusion. The developed spectrophotometric method for *OH detection was validated with HPLC, i.e., the concentrations of dihydroxybenzoates produced by radical attack from the probe were determined by HPLC, and the sum of (concentrationxabsorptivity) products of these components approximately agreed with the experimentally found CUPRAC absorbances, confirming the validity of Beer's law for the selected system. Statistical comparison of the results found with the proposed methodology and HPLC was made with two-way ANOVA (analysis of variance) test. Under optimal conditions, about 53% of the probe (salicylate) was converted into dihydroxybenzoate isomers in the absence of *OH scavengers, and these isomers were more specific markers of hydroxyl radicals than the non-specific malondialdehyde end-product of the TBARS test. Thus, the more costly and less speedy HPLC method could advantageously be substituted with the proposed spectrophotometric assay of *OH detection, which was also of much higher yield than the TBARS colorimetric assay.     

Long pathlength differential optical absorption spectroscopy (D O A S) measurements of gaseous HONO,NO2 and HCNO in the California South Coast Air Basin

A differential optical absorption spectrometer (DOAS) system was operated at Long Beach, CA during the 1987 SCAQS Fall episodes to measure atmospheric concentrations of nitrous acid (HONO), as well as ambient levels of nitrogen dioxide (NO₂) and formaldehyde (HCHO). The rapid scanning (-3000 spectra per min) spectrometer was interfaced to a 25 m basepath open, multiple reflection system operated routinely at a total optical path of 800 m. During several of the Fall episodes at Long Beach, levels of gaseous HONO were the highest (>15 ppb) reported to date by the DOAS technique. Although approximately half, to all, of the measured nighttime HONO concentrations could be accounted for by proposed heterogeneous formation pathways involving NO₂, HONO concentrations correlated well with primary pollutants such as CO and NO, suggesting that elevated nighttime HONO concentrations in the western end of the Los Angeles basin may be influenced by emissions of HONO from combustion sources. This has significant implications for models which assume HONO arises only from secondary formation, rather than a combination of direct emissions and atmospheric reactions. Estimates of hydroxyl (OH) radical production show that photolysis of HONO shortly after sunrise on these episode days produces a large pulse of OH radicals at a time of the day when OH production from photolysis of O₃ and HCHO is low. In terms of integrated OH radical production, HONO is of comparable importance to HCHO and much more important than O₃ during these Fall periods.     

Rapid Autoxidation Forms Highly Oxidized RO2 Radicals in the Atmosphere

Gas‐phase oxidation routes of biogenic emissions, mainly isoprene and monoterpenes, in the atmosphere are still the subject of intensive research with special attention being paid to the formation of aerosol constituents. This laboratory study shows that the most abundant monoterpenes (limonene and α‐pinene) form highly oxidized RO₂ radicals with up to 12 O atoms, along with related closed‐shell products, within a few seconds after the initial attack of ozone or OH radicals. The overall process, an intramolecular ROO→QOOH reaction and subsequent O₂ addition generating a next R′OO radical, is similar to the well‐known autoxidation processes in the liquid phase (QOOH stands for a hydroperoxyalkyl radical). Field measurements show the relevance of this process to atmospheric chemistry. Thus, the well‐known reaction principle of autoxidation is also applicable to the atmospheric gas‐phase oxidation of hydrocarbons leading to extremely low‐volatility products which contribute to organic aerosol mass and hence influence the aerosol–cloud–climate system.     

Determination of hydroxyl radical photoproduction rates in natural waters.

The photochemical formation rates of hydroxyl radicals (OH radicals) in river water and seawater were determined by a simple, rapid and sensitive benzene probe method, in which phenol formed by the reaction between benzene and photochemically-generated OH radicals was analyzed by on-line preconcentration HPLC. The OH radical formation rates from well-known OH radical sources, such as nitrate, nitrite and hydrogen peroxide, were in good agreement with those reported previously. River water samples containing high concentrations of nitrate and nitrite were found to show high OH radical formation rates. Ten to 80% of the OH radical formation in river water and seawater was due to the photolysis of nitrate and nitrite, but OH radical formation from hydrogen peroxide was negligible. The OH radical formation from unknown sources other than nitrate, nitrite and hydrogen peroxide was strongly correlated to the amount of fluorescent matter.     

Br2 production from the heterogeneous reaction of gas-phase OH with aqueous salt solutions: Impacts of acidity, halide concentration, and organic surfactants

This study reports the first laboratory measurement of gas-phase Br2 production from the reaction between gas-phase hydroxyl radicals and aqueous salt solutions. Experiments were conducted at 269 K in a rotating wetted-wall flow tube coupled to a chemical-ionization mass spectrometer for analysis of gas-phase components. From both pure NaBr solutions and mixed NaCl/NaBr solutions, the amount of Br2 released was found to increase with increasing acidity, whereas it was found to vary little with increasing concentration of bromide ions in the sample. For mixed NaCl/NaBr solutions, Br2 was formed preferentially over Cl2 unless the Br- levels in the solution were significantly depleted by OH oxidation, at which point Cl2 formation was observed. Presence of a surfactant in solution, sodium dodecyl sulfate, significantly suppressed the formation of Br2; this is the first indication that an organic surfactant can affect the rate of interfacial mass transfer of OH to an aqueous surface. The OH-mediated oxidation of bromide may serve as a source of active bromine in the troposphere and contribute to the subsequent destruction of ozone that proceeds in marine-influenced regions of the troposphere.     

Reactions in Tirapazamine Induced by the Attachment of Low‐Energy Electrons: Dissociation Versus Roaming of OH

Tirapazamine (TPZ) has been tested in clinical trials on radio‐chemotherapy due to its potential highly selective toxicity towards hypoxic tumor cells. It was suggested that either the hydroxyl radical or benzotriazinyl radical may form as bioactive radical after the initial reduction of TPZ in solution. In the present work, we studied low‐energy electron attachment to TPZ in the gas phase and investigated the decomposition of the formed TPZ^? anion by mass spectrometry. We observed the formation of the (TPZ–OH)^? anion accompanied by the dissociation of the hydroxyl radical as by far the most abundant reaction pathway upon attachment of a low‐energy electron. Quantum chemical calculations suggest that NH₂ pyramidalization is the key reaction coordinate for the reaction dynamics upon electron attachment. We propose an OH roaming mechanism for other reaction channels observed, in competition with the OH dissociation.