Kinetics Study of Heterogeneous Bromine Release from the Reaction between Gaseous Ozone and Aqueous Bromide Solution

Pollution-Derived Br2 Boosts Oxidation Power of the Coastal Atmosphere

The bromine atom (Br^•) has been known to destroy ozone (O₃) and accelerate the deposition of toxic mercury (Hg). However, its abundance and sources outside the polar regions are not well-known. Here, we report significant levels of molecular bromine (Br₂)─a producer of Br^•─observed at a coastal site in Hong Kong, with an average noontime mixing ratio of 5 ppt. Given the short lifetime of Br₂ (∼1 min at noon), this finding reveals a large Br₂ daytime source. On the basis of laboratory and field evidence, we show that the observed daytime Br₂ is generated by the photodissociation of particulate nitrate (NO₃^-) and that the reactive uptake of dinitrogen pentoxide (N₂O₅) on aerosols is an important nighttime source. Model-calculated Br^• concentrations are comparable with that of the OH radical─the primary oxidant in the troposphere, accounting for 24% of the oxidation of isoprene, a 13% increase in net O₃ production, and a nearly 10-fold increase in the production rate of toxic Hg^II. Our findings reveal that reactive bromines play a larger role in the atmospheric chemistry and air quality of polluted coastal and maritime areas than previously thought. Our results also suggest that tightening the control of emissions of two conventional pollutants (NO_x and SO₂)─thereby decreasing the levels of nitrate and aerosol acidity─would alleviate halogen radical production and its adverse impact on air quality.

Impact of Tetrabutylammonium on the Oxidation of Bromide by Ozone

The reaction of ozone with sea-salt derived bromide is relevant for marine boundary layer atmospheric chemistry. The oxidation of bromide by ozone is enhanced at aqueous interfaces. Ocean surface water and sea spray aerosol are enriched in organic compounds, which may also have a significant effect on this reaction at the interface. Here, we assess the surface propensity of cationic tetrabutylammonium at the aqueous liquid-vapor interface by liquid microjet X-ray photoelectron spectroscopy (XPS) and the effect of this surfactant on ozone uptake to aqueous bromide solutions. The results clearly indicate that the positively charged nitrogen group in tetrabutylammonium (TBA), along with its surface activity, leads to an enhanced interfacial concentration of both bromide and the bromide ozonide reaction intermediate. In parallel, off-line kinetic experiments for the same system demonstrate a strongly enhanced ozone loss rate in the presence of TBA, which is attributed to an enhanced surface reaction rate. We used liquid jet XPS to obtain detailed chemical composition information from the aqueous-solution-vapor interface of mixed aqueous solutions containing bromide or bromide and chloride with and without TBA surfactant. Core level spectra of Br 3d, C 1s, Cl 2p, N 1s, and O 1s were used for this comparison. A model was developed to account for the attenuation of photoelectrons by the carbon-rich layer established by the TBA surfactant. We observed that the interfacial density of bromide is increased by an order of magnitude in solutions with TBA. The salting-out of TBA in the presence of 0.55 M sodium chloride is apparent. The increased interfacial bromide density can be rationalized by the association constants for bromide and chloride to form ion-pairs with TBA. Still, the interfacial reactivity is not increasing simply proportionally with the increasing interfacial bromide concentration in response to the presence of TBA. The steady state concentration of the bromide ozonide intermediate increases by a smaller degree, and the lifetime of the intermediate is 1 order of magnitude longer in the presence of TBA. Thus, the influence of cationic surfactants on the reactivity of bromide depends on the details of the complex environment at the interface.

Reactive Uptake of Ozone to Simulated Seawater: Evidence for Iodide Depletion

The reaction of ozone with iodide in the ocean is a major ozone dry deposition pathway, as well as an important source of reactive iodine to the marine troposphere. Few prior laboratory experiments have been conducted with environmentally relevant ozone mixing ratios and iodide concentrations, leading to uncertainties in the rate of the reaction under marine boundary layer conditions. As well, there remains disagreement in the literature assessment of the relative contributions of an interfacial reaction via ozone adsorbed to the ocean surface versus a bulk reaction with dissolved ozone. In this study, we measure the uptake coefficient of ozone over a buffered, pH 8 salt solution replicating the concentrations of iodide, bromide, and chloride in the ocean over an ozone mixing ratio of 60-500 ppb. Due to iodide depletion in the solution, the measured ozone uptake coefficient is dependent on the exposure time of the solution to ozone and its mixing ratio. A kinetic multilayer model confirms that iodide depletion is occurring not only within ozone's reactodiffusive depth, which is on the order of microns for environmental conditions, but also deeper into the solution as well. Best model-measurement agreement arises when some degree of nondiffusive mixing is occurring in the solution, transporting iodide from deeper in the solution to a thin, diffusively mixed upper layer. If such mixing occurs rapidly in the environment, iodide depletion is unlikely to reduce ozone dry deposition rates. Unrealistically high bulk-to-interface partitioning of iodide is required for the model to predict a substantial interfacial component to the reaction, indicating that the Langmuir-Hinshelwood mechanism is not dominant under environmental conditions.

A kinetic model for ozone uptake by solutions and aqueous particles containing I- and Br-, including seawater and sea-salt aerosol

The heterogeneous interactions of gaseous ozone (O3) with seawater and with sea-salt aerosols are known to generate volatile halogen species, which, in turn, lead to further destruction of O3. Here, a kinetic model for the interaction of ozone (O3) with Br- and I- solutions and aqueous particles has been proposed that satisfactorily explains previous literature studies about this process. Apart from the aqueous-phase reactions X- + O3 (X = I, Br), the interaction also involves the surface reactions X- + O3 that occur via O3 adsorption on the aqueous surface. In single salt solutions and aerosols, the partial order in ozone and the total order of the surface reactions are one, but the apparent total order is second order because the number of ozone sites where reaction can occur is equal to the surficial concentration of X- ([X-]surf). In the presence of Cl-, the surface reactions are enhanced by a factor equal to , where and . Therefore, we have inferred that Cl- acts as a catalyst in the surface reactions X- + O3. The model has been applied to estimate ozone uptake by the reaction with these halides in/on seawater and in/on sea-salt aerosol, where it has been concluded that the Cl--catalyzed surface reaction is important relative to total ozone uptake and should therefore be considered to model Y/YO (Y = I, Br, Cl) levels in the troposphere.

The opposing effect of butanol and butyric acid on the abundance of bromide and iodide at the aqueous solution-air interface

The efficient oxidation of iodide and bromide at the aqueous solution-air interface of the ocean or of sea spray aerosol particles had been suggested to be related to their surface propensity. The ubiquitous presence of organic material at the ocean surface calls for an assessment of the impact of often surface-active organic compounds on the interfacial density of halide ions. We used in situ X-ray photoelectron spectroscopy with a liquid micro-jet to obtain chemical composition information at aqueous solution-vapor interfaces from mixed aqueous solutions containing bromide or iodide and 1-butanol or butyric acid as organic surfactants. Core level spectra of Br 3d, Na 2s, C 1s and O 1s at ca. 160 eV kinetic energy and core level spectra of I 4d and O 1s at ca. 400 eV kinetic energy are compared for solutions with 1-butanol and butyric acid as a function of organic concentration. A simple model was developed to account for the attenuation of photoelectrons by the aliphatic carbon layer of the surfactants and for changing local density of bromide and iodide in response to the presence of the surfactants. We observed that 1-butanol increases the interfacial density of bromide by 25%, while butyric acid reduces it by 40%, both in comparison to the pure aqueous halide solution. Qualitatively similar behavior was observed for the case of iodide. Classical molecular dynamics simulations failed to reproduce the details of the response of the halide ions to the presence of the two organics. This is attributed to the lack of correct monovalent ion parameters at low concentration possibly leading to an overestimation of the halide ion concentration at the interface in absence of organics. The results clearly demonstrate that organic surfactants change the electrostatic interactions near the interface with headgroup specific effects. This has implications for halogen activation processes specifically when oxidants interact with halide ions at the aqueous solution-air interfaces of the ocean surface or sea spray aerosol particles.

Microphysics of the aqueous bulk counters the water activity driven rate acceleration of bromide oxidation by ozone from 289-245 K

The reaction of ozone with bromide is an initiation process in bromine activation resulting in the formation of reactive bromine species with impacts on the fate of compounds in the lower atmosphere. Environmental halide sources often contain organics, which are known to influence aqueous bulk reactivity. Here, we present a study investigating the temperature dependence of bromide oxidation by ozone using a coated wall flow tube reactor coated with an aqueous mixture of citric acid, as a proxy for oxidized secondary organic matter, and sodium bromide. Using the resistor model formulation, we quantify changes in the properties of the aqueous bulk relevant for the observed reactivity. The reactive uptake coefficient decreased from 2 × 10-6 at 289 K to 0.5 × 10-6 at 245 K. Our analysis indicates that the humidity-driven increase in concentration with a corresponding increase in the pseudo-first order reaction rate was countered by the colligative change in ozone solubility and the effect of the organic fraction via increased viscosity and decreased diffusivity of ozone as the temperature decreased. From our parameterization, we provide an extension of the temperature dependence of the reaction rate coefficients driving the oxidation of bromide, and assess the temperature-dependent salting effects of citric acid on ozone solubility. This study shows the effects of the organic species at relatively mild temperatures, between the freezing point and eutectic temperature of sea as is typical for the Earth's cryosphere. Thus, this study may be relevant for atmospheric models at different scales describing halogen activation in the marine boundary layer or free troposphere including matrices such as sea-spray aerosol and brine in sea ice, snow, and around mid-latitude salt lakes.