Radical Sink Research Articles

Chemical and physical properties of plumes of anthropogenic pollutants transported over the North Atlantic during the North Atlantic Regional Experiment

Plumes of photochemical pollutants transported from the industrialized regions of the northeast United States and Canada were sampled over the North Atlantic Ocean at distances up to 1000 km from the coast. The plumes were found in well defined layers up to 1 km thick and were usually isolated from the surface by a low altitude inversion. Plume composition was consistent with the occurrence of extensive photochemical processing during transit from source regions as indicated by high O3 concentrations (O3 maximum ∼150 parts per billion by volume (ppbv)), generally high fractional conversion (>85%) of NOx to its oxidation products, and high peroxide concentrations (median 3.6 ppbv; maximum 11 ppbv). These observations are in accord with processing times estimated from back trajectory analysis. CO and O3 concentrations were well correlated (r2 = 0.64) with a slope (0.26) similar to previous measurements in photochemically aged air. Good correlations were also observed between CO and accumulation mode particle number densities (r2 = 0.64), and CO and NOy (r2 = 0.67). O3 was found to depend nonlinearly on the NOx oxidation product concentration. At low values of (NOy‐NOx), the slope (14) was within the range of values measured previously in photochemically aged air masses, at higher concentrations the slope was much lower (4.6). The low slope at high concentrations is attributed to minimization of losses of NOx oxidation products in spatially well‐defined plumes during transport. A strong linear correlation (r2 = 0.73) was found between O3, and the concentration of radical sink species as represented by the quantity ((NOy‐NOx) + 2H2O2).

Measurements of peroxides in the atmosphere and their relevance to the understanding of global tropospheric chemistry

Peroxides are produced as termination products in atmospheric chain reactions involving peroxy radicals, both organic and inorganic. They are the principal sink for radicals produced in the troposphere from the photolysis of ozone in the presence of water vapour and as such are excellent indicators of the extent of free radical chemistry taking place at any given location. Their measurement is relatively simple and data on the concentration of peroxides in the atmosphere with respect to time and space can be collected easily and extensively.New data on peroxide measurements collected at different parts of the atmosphere, principally by the Meteorological Office C-130 Hercules aircraft are presented. They indicate that the extent of hydroxyl radical chemistry during the summer is controlled mostly by the water vapour content of the atmosphere. Both negative and positive correlations are observed between ozone and peroxide concentrations in vertical profiles over the North Atlantic Ocean and the equatorial Pacific. The negative correlations demonstrate that the ozone concentration throughout the troposphere is determined mostly by in situ photochemistry. This is borne out by the close correlation between calculated and measured concentrations of peroxides in vertical profiles. Positive correlations over the North Atlantic allow us to make an estimate of the amount of ozone, present there in the summer, which is formed from tropospheric as opposed to stratospheric chemistry.

Extinction of nonpremixed flames with halogenated fire suppressants

An experimental, analytical, and numerical study was performed to elucidate the influence of eleven gaseous agents, considered to be substitutes for CF 3Br, on the structure and critical conditions of extinction of diffusion flames burning liquid hydrocarbon fuels. The effectiveness of these agents in quenching flames was compared to those of CF 3Br and an inert diluent such as nitrogen. Experiments were performed on diffusion flames stabilized in the counterflowing as well as in the coflowing configuration. The fuels tested were heptane in the counterflowing configuration, and heptane, the jet fuels JP-8, and JP-5, and hydraulic fluids (military specifications 5606 and 83282) in the coflowing configuration. The oxidizing gas was a mixture of air and the agent. On a mass and mole basis CF 3Br was found to be most effective in quenching the flames and the mass-based effectiveness of the other eleven agents was found to be nearly the same as that of nitrogen. Experimental results were interpreted using one-step, activation-energy asymptotic theories and the results were used to provide a rough indication of the thermal and chemical influence of these agents on the flame structure. To understand in some detail the influence of CF 3Br on the structure and mechanisms of extinction of the flame, numerical calculations using detailed chemistry were performed. The calculated structure of counterflow heptane-air diffusion flames inhibited with CF 3Br was found to consist of three distinct zones including a CF 3Br consumption zone which appears to act as a sink for radicals. The calculated values of the critical conditions of extinction of counterflow heptane-air diffusion flames inhibited with CF 3Br were found to agree fairly well with measurements. The study suggests the need for refinement of the inhibition chemistry.

Radical-induced damage to bovine serum albumin: role of the cysteine residue.

The reactions of cerium(IV) and the hydroxyl radical [generated from iron(ii)/H2O2] with bovine serum albumin (BSA) have been investigated by EPR spin trapping. With the former reagent a protein-derived thiyl radical is selectively generated; this has been characterized via the anisotropic EPR spectra observed on reaction of this radical with the spin trap DMPO. Blocking of the thiol group results in the loss of this species and the detection of a peroxyl radical, believed to be formed by reaction of oxygen with initially-generated, but undetected, carbon-centred radicals from aromatic amino acids. Experiments with a second spin trap (DBNBS) confirm the formation of these carbon-centred species and suggest that damage can be transferred from the thiol group to carbon sites in the protein. A similar transfer pathway can be observed when hydroxyl radicals react with BSA. Further experiments demonstrate that the reverse process can also occur: when hydroxyl radicals react with BSA, the thiol group appears to act as a radical sink and protects the protein from denaturation and fragmentation through the transfer of damage from a carbon site to the thiol group. Thiol-blocked BSA is shown to be more susceptible to damage than the native protein in both direct EPR experiments and enzyme digestion studies. Oxygen has a similar effect, with more rapid fragmentation detected in its presence than its absence.

A thermodynamic appraisal of the radical sink hypothesis

Analysis of the radical sink hypothesis (C.C. Winterbourn, preceding paper) shows that all reactions are energetically feasible. The process is made irreversible by the formation of superoxide from the GSSG .− anion, which is an extremely favourable reaction, −31 kcal/mol. The overall process has a Gibbs energy change of approximately −50 kcal/mol, depending on the radical that is being “repaired” by glutathione.

Superoxide as an intracellular radical sink

A pathway is proposed for superoxide to act as a sink for intracellularly generated radicals. A variety of radicals, either directly or via reduced glutathione (GSH) as an intermediate, can transfer their unpaired electron to oxygen to give superoxide. It is proposed that in a cellular environment, superoxide can undergo chain reactions involving GSH with or without another redox cycling agent, converting GSH to oxidized glutathione (GSSG) and oxygen to hydrogen peroxide far in excess of the initial radical. This places an oxidative stress on the cell, depleting reducing equivalents and energy reserves. Superoxide dismutase is necessary to prevent this oxidative stress, as well as any direct damage by superoxide. Through this metabolic pathway, GSH and superoxide dismutase can be linked in antioxidant function, and superoxide dismutase, by reacting with superoxide, can provide general protection against radical reactions in the cell. The pathway also provides a mechanism for superoxide and superoxide dismutase to influence the redox state of the cell and regulate functions that are under redox control.

Degradation and regeneration of the cross-linking agent azo-t-butane

The ability of commercially available azo-t-butun (ATB) to facilitate immobilization of polysiloxane stationary phases in capillary columns deteriorates upon storage. Aged ATB produces columns with high bleed, solute retention shifts, erratic immobilization efficiency, and high activity. This deterioration is accelerated in the presence of light, suggesting free radical processes are involved. The loss of certain lowlevel impurities, which act as free radical transfer agents or radical sinks, may be responsible for the performance change. Regeneration can often temporarily be accomplished using platinum catalyzed reduction with trichlorosilane. The addition of small amounts of toluene or other radical transfer agents to aged ATB moderates the cross-linking process, allowing the reproducible production of columns with good immobilization efficiency, excellent intertness, low thermal bleed, and proper polarity.

The biogeochemical cycling of formic and acetic acids through the troposphere: an overview of current understanding

Despite remaining uncertainties, a general picture of the biogeochemical cycling of HCOOH and CH3COOH is beginning to emerge. These acids are ubiquitous vapor- and aqueous-phase constituents of the global troposphere, contributing significant fractions of the natural acidity in precipitation and cloudwater, particularly in continental regions of the tropics and subtropics. The similarity of concentrations in remote and impacted regions indicates that anthropogenic emissions of carboxylic acids or precursors are probably not important sources over broad geographic areas. In continental regions, seasonally varying emissions of acids and hydrocarbon precursors from vegetation are thought to represent major sources for both acids. In marine areas, the seasonality of wet deposition and apparent lack of transport from continents support the hypothesis of a marine biogenic source. Ratios of total (aqueous + vapor) concentrations of HCOOH versus CH3COOH in clouds vary as a function of H+. This observation is consistent with the hypothesis of a pH-dependent aqueous-phase source and sink for HCOOH in clouds which may act to stabilize aqueous-phase ratios against shifts in solution Ht. Large die1 periodicities in the concentrations of vapor-phase species over continents, together with a lack of evidence for long-distance transport, suggest that HCOOH and CH3COOH experience short atmospheric lifetimes of several hours to a few (at most) days. Because they are important controllers of free acidity and represent a potentially large sink for aqueous-phase OH radicals, the cycling of the carboxylic acids is expected to interact directly and indirectly in the chemical cycling of other atmospheric constituents.

Gas‐ and aqueous‐phase chemistry of HO2 in liquid water clouds

A model for reversible mass transport of HO2 between the gas and aqueous phases of liquid water clouds is used to examine the coupling of reaction kinetics of this species in the two phases. The Henry's law coefficient of HO2 necessary for this analysis is evaluated by means of a thermochemical cycle involving O2 −(aq) to be (1–3) × 103 M atm−1. The mass accommodation coefficient α for uptake of HO2 by liquid water is not known and is treated as an adjustable parameter. Results are expressed in terms of yields of HNO3(g), H2O2(g) and H2O2(aq) relative to the initial photochemical generation rate of OH(g). For large values of α (≳10−3) aqueous‐phase H2O2 formation may be a major radical sink process, but the rate of aqueous‐phase H2O2 production decreases strongly with α ≲10−3. Substantial difference, e.g., a factor of as much as 50 in gas‐phase HO2 concentration, is found between kinetic calculations where uptake of HO2 by cloud droplets is treated reversibly versus irreversibly. Such differences demonstrate the need to treat the dissolution process as reversible, even for reactive free‐radical species.

E.P.R. study of the reactivity of ClO with H2CO at 298 K

The reaction of ClO with H2CO has been studied by the discharge flow EPR technique. An upper limit of 10−15 cm³ molecule−1 s−1 has been found for the rate constant at 298 K. Then this reaction cannot be a sink for ClO radicals in the stratosphere. Besides, evidence has been shown for a noticeable reactivity between ClO and HCO at 298 K.