- Observation of gas-phase peroxynitrous and peroxynitric acid during the photolysis of nitrate in acidified frozen solutions
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The photolysis of nitrate embedded in ice and snow can be a significant source of volatile nitrogen oxides affecting the composition of the planetary boundary layer. In this work, we examined the nitrogen oxides evolved from irradiated frozen solutions containing nitrate. Products were monitored by cavity ring-down spectroscopy (CRDS), NO-O3 chemiluminescence (CL), and chemical ionization mass spectrometry (CIMS). Under acidic conditions, the nitrogen oxides volatilized were mainly in the form of NOz, i.e., nitrous (HONO), nitric (HONO2), peroxynitrous (HOONO), and peroxynitric acid (HO2NO2). Identification of acidic nitrogen oxides by CIMS and possible HOONO, HONO2 and HO 2NO2 formation pathways are discussed.
- Abida, Otman,Mielke, Levi H.,Osthoff, Hans D.
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p. 187 - 192
(2011/10/05)
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- Reactivity of Peroxynitric Acid (O2NOOH): A Pulse Radiolysis Study
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Peroxynitrate (O2NOOH/O2NOO-) is formed within less than 2 ms after pulse irradiation of aerated solutions containing relatively low concentrations of formate and nitrate. The pKa for peroxynitric acid was determined to be 5.9 ± 0.1 both from the pH-dependent absorbance of the anion at 310 nm and from the dependence of the decay kinetics on pH. An absorption spectrum was measured for the anion giving εmax(290) = 1500 ± 100 M-1 cm-1. This method of generation of peroxynitrate is very useful for studying the mechanism of the oxidation of various substrates by peroxynitrate. The oxidation by peroxynitrate can take place either directly or indirectly. In the direct oxidation pathway, the reaction is first order in peroxynitrate and first order in the substrate, whereas in the indirect oxidation pathway, the reaction is zero order in the substrate. In both cases, the observed rate constants are highly pH-dependent. The results show that the direct oxidation pathway takes place through O2NOOH. We suggest that the indirect oxidation takes place through reactive intermediates that are formed during the decomposition of peroxynitrate. In the presence of sufficient concentrations of the substrates, the oxidation yields approach 100% through the direct and indirect oxidation pathways.
- Goldstein, Sara,Czapski, Gidon
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p. 4156 - 4162
(2008/10/09)
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- Global thermodynamic atmospheric modeling: Search for new heterogeneous reactions
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This article demonstrates quantitatively how far reactions are from chemical equilibrium over the full space of a two-dimensional atmospheric model. This method could be used with data where an instrument-equipped aircraft measures numerous species simultaneously. An atmospheric reaction is displaced from equilibrium by solar radiation and relocation of species by atmospheric motions. One purpose of this study is to seek additional stratospheric or tropospheric gas-phase chemical reactions that might undergo heterogeneous catalysis. Hypothetical cases can be rapidly screened in terms of their thermodynamic potential to react under measured or modeled atmospheric conditions of temperature and local species concentrations. If a reaction is interesting, is slow in the gas phase, and has a high thermodynamic tendency to react, it is a good candidate for a laboratory study to seek a heterogeneous catalyst. If the reaction is thermodynamically unfavorable, there is no catalyst that can cause the reaction to occur. If a reaction is thermodynamically favored to occur but also endothermic, it will tend to be slow at stratospheric temperatures. We find, as expected, that four heterogeneous reactions important in causing the Antarctic ozone hole have high thermodynamic tendencies to occur under atmospheric conditions, but one of these is only weakly thermodynamically allowed in some regions of the atmosphere. The reaction of SO2 and HNO3 to form HONO has a high thermodynamic potential to occur, is a well-known laboratory reaction at ice temperature, and may occur in nitric acid-rich sulfate aerosols. Throughout the troposphere and stratosphere, we find that formaldehyde has an extremely high thermodynamic potential to reduce nitric acid. Formaldehyde is known to stick to and remain in sulfuric acid solution, where it adds water to form H2C(OH)2. Near room-temperature H2C(OH)2 reacts with nitric acid in a two-step mechanism to form two molecules of HONO, but the rate of this process under conditions of stratospheric sulfuric acid aerosols is unknown.
- Fairbrother, D. Howard,Sullivan, Daniel J. D.,Johnston, Harold S.
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p. 7350 - 7358
(2007/10/03)
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