Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.

Aerosol Fragmentation Driven by Coupling of Acid–Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals

Abstract: A key uncertainty in the heterogeneous oxidation of carboxylic acids by hydroxyl radicals (OH) in aqueous-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chemistry. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimolecular decomposition of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of organic carbon between the gas and aqueous phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidation of citric acid aerosol at high relative humidity. The reaction scheme, containing both free-radical and acid-base elementary reaction steps with physically validated rate coefficients, accurately predicts the experimentally observed molecular composition, particle size, and average elemental composition of the aerosol upon oxidation. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimolecular fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equilibria activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low molecular weight, highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and volume growth that accompany oxidation. These results frame the oxidative lifecycle of atmospheric aerosol: it is governed by feedbacks between reactions that first increase the particle oxidation state, then eventually promote water uptake and acid-base chemistry. When coupled to free-radical reactions, acid-base channels lead to formation of low molecular weight gas-phase reaction products and decreasing particle size.

The Formation of O and H Atoms in the Reaction of CH2 with O2 at High Temperatures

Abstract: The formation of O and H atoms in the reaction of CH2 with O2 was investigated behind incident shock waves at temperatures between 1000 K and 1700 K and at total densities between 5 · 10−6 and 1 · 10−5 mol/cm3 by means of Atomic Resonance Absorption Spectroscopy (ARAS) at 130.5 nm and 121.6 nm, respectively. The contribution of O and H forming channels was found to be about 20%. The rates of formation of O and H atoms in this reaction were determined to be k1a = (4 ± 2) · 1010 cm3/mol · s k1e = (5 ± 2) · 1010 cm3/mol · s In the temperature range investigated an over all rate constant can be represented by k1 = (5 ± 3) · 1011 cm3/mol · s. A comparison with low temperature data leads to a change of the apparent energy of activation of the overall reaction in the temperature range between 300 and 1000 K.