scholarly journals Intramolecular Hydrogen Shift Chemistry of Hydroperoxy-Substituted Peroxy Radicals

2018 ◽  
Vol 123 (2) ◽  
pp. 590-600 ◽  
Author(s):  
Eric Praske ◽  
Rasmus V. Otkjær ◽  
John D. Crounse ◽  
J. Caleb Hethcox ◽  
Brian M. Stoltz ◽  
...  
Keyword(s):  
Peroxy Radicals ◽  
Hydrogen Shift ◽  
Intramolecular Hydrogen

The atmospheric oxidation of dimethyl, diethyl, and diisopropyl ethers. The role of the intramolecular hydrogen shift in peroxy radicals

2016 ◽  
Vol 18 (11) ◽  
pp. 7707-7714 ◽  
Author(s):  
Sainan Wang ◽  
Liming Wang
Keyword(s):  
Atmospheric Oxidation ◽  
Peroxy Radicals ◽  
Hydrogen Shift ◽  
Intramolecular Hydrogen ◽  
Clean Atmosphere

Ethers can be auto-oxidized with no O3 formation in a ‘clean’ atmosphere.

Calculated Hydrogen Shift Rate Constants in Substituted Alkyl Peroxy Radicals

2018 ◽  
Vol 122 (43) ◽  
pp. 8665-8673 ◽  
Author(s):  
Rasmus V. Otkjær ◽  
Helene H. Jakobsen ◽  
Camilla Mia Tram ◽  
Henrik G. Kjaergaard
Keyword(s):  
Rate Constants ◽  
Peroxy Radicals ◽  
Hydrogen Shift ◽  
Alkyl Peroxy Radicals

Hydrogen shift reactions in four methyl-buten-ol (MBO) peroxy radicals and their impact on the atmosphere

2016 ◽  
Vol 147 ◽  
pp. 79-87 ◽  
Author(s):  
Hasse C. Knap ◽  
Johan A. Schmidt ◽  
Solvejg Jørgensen
Keyword(s):  
Peroxy Radicals ◽  
Hydrogen Shift

Importance of isomerization reactions for the OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

2020 ◽  
Author(s):  
Anna Novelli ◽  
Luc Vereecken ◽  
Birger Bohn ◽  
Hans-Peter Dorn ◽  
Georgios Gkatzelis ◽  
...  
Keyword(s):  
Oh Radicals ◽  
Oh Radical ◽  
Peroxy Radicals ◽  
Atmospheric Conditions ◽  
Unimolecular Reactions ◽  
Model Calculations ◽  
Hydrogen Shift ◽  
Wide Range ◽  
Regeneration Efficiency ◽  
Simulation Chamber

<p>Theoretical, laboratory and chamber studies have shown fast regeneration of hydroxyl radical (OH) in the photochemistry of isoprene largely due to previously disregarded unimolecular reactions which were previously thought not to be important under atmospheric conditions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in the isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospheric conditions (i.e. NO between 0.15 and 2 ppbv and temperature between 25 and 41°C) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to one, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations and the best agreement was observed when at least 50% of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 parts per billion (ppbv). For these levels of NO, up to 50% of the OH radicals are regenerated from the products of the 1,6 α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO<sub>2 </sub>radicals through photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehyde hydrogen shift (rate constant ~10 s<sup>-1</sup> at 300K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO<sub>2</sub>), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy and organic peroxy radicals, carbon monoxide and the sum of methyl vinyl ketone, methacrolein and hydroxyl hydroperoxides) is nearly independent on the adopted yield of HPALD and di-HPCARP-RO<sub>2</sub> as both degrade relatively fast (< 1 h), forming OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the oxygenated products distribution, which affect radical levels and organic aerosol downwind of unpolluted isoprene dominated regions.</p>

scholarly journals A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol

2019 ◽  
Author(s):  
Kelvin H. Bates ◽  
Daniel J. Jacob
Keyword(s):  
Secondary Organic Aerosol ◽  
Transport Model ◽  
Oxidation Mechanism ◽  
Organic Aerosol ◽  
Atmospheric Oxidation ◽  
Peroxy Radicals ◽  
Chemical Transport Model ◽  
Total Size ◽  
Hydrogen Shift ◽  
Isomer Distribution

Abstract. Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone, the hydroxyl radical (OH), nitrogen oxide radicals (NOx), carbon monoxide (CO), oxygenated and nitrated organic compounds, and secondary organic aerosol (SOA). We analyze these effects in box models and in the global GEOS-Chem chemical transport model using the new Reduced Caltech Isoprene Mechanism (RCIM) condensed from a recently developed explicit isoprene oxidation mechanism. We find many similarities with previous global models of isoprene chemistry along with a number of important differences. Proper accounting of the isomer distribution of peroxy radicals following the addition of OH and O2 to isoprene influences the subsequent distribution of products, decreasing in particular the yield of methacrolein, and increasing the capacity of intramolecular hydrogen shifts to promptly regenerate OH. Hydrogen shift reactions throughout the mechanism lead to increased OH recycling, resulting in less depletion of OH under low-NO conditions than in previous mechanisms. Higher organonitrate yields and faster tertiary nitrate hydrolysis lead to more efficient NOx removal by isoprene and conversion to inorganic nitrate. Only 20 % of isoprene-derived organonitrates (excluding peroxyacyl nitrates) are chemically recycled to NOx. The global yield of formaldehyde from isoprene is 22 % per carbon and less sensitive to NO than in previous mechanisms. The global molar yield of glyoxal is 2 %, much lower than in previous mechanisms because of deposition and aerosol uptake of glyoxal precursors. Global production of isoprene SOA is about one third each from isoprene epoxydiols (IEPOX), organonitrates, and tetrafunctional compounds. We find a SOA yield from isoprene of 13 % per carbon, much higher than commonly assumed in models, and likely offset by SOA chemical loss. We use the results of our simulations to further condense RCIM into a Mini-Caltech Isoprene Mechanism (Mini-CIM) for less expensive implementation in atmospheric models, with a total size (108 species, 345 reactions) comparable to currently used mechanisms.

scholarly journals A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol

2019 ◽  
Vol 19 (14) ◽  
pp. 9613-9640 ◽  
Author(s):  
Kelvin H. Bates ◽  
Daniel J. Jacob
Keyword(s):  
Secondary Organic Aerosol ◽  
Transport Model ◽  
Oxidation Mechanism ◽  
Organic Aerosol ◽  
Atmospheric Oxidation ◽  
Peroxy Radicals ◽  
Chemical Transport Model ◽  
Total Size ◽  
Hydrogen Shift ◽  
Isomer Distribution

Abstract. Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone (O3), the hydroxyl radical (OH), nitrogen oxide radicals (NOx), carbon monoxide (CO), oxygenated and nitrated organic compounds, and secondary organic aerosol (SOA). We analyze these effects in box models and in the global GEOS-Chem chemical transport model using the new reduced Caltech isoprene mechanism (RCIM) condensed from a recently developed explicit isoprene oxidation mechanism. We find many similarities with previous global models of isoprene chemistry along with a number of important differences. Proper accounting of the isomer distribution of peroxy radicals following the addition of OH and O2 to isoprene influences the subsequent distribution of products, decreasing in particular the yield of methacrolein and increasing the capacity of intramolecular hydrogen shifts to promptly regenerate OH. Hydrogen shift reactions throughout the mechanism lead to increased OH recycling, resulting in less depletion of OH under low-NO conditions than in previous mechanisms. Higher organonitrate yields and faster tertiary nitrate hydrolysis lead to more efficient NOx removal by isoprene and conversion to inorganic nitrate. Only 20 % of isoprene-derived organonitrates (excluding peroxyacyl nitrates) are chemically recycled to NOx. The global yield of formaldehyde from isoprene is 22 % per carbon and less sensitive to NO than in previous mechanisms. The global molar yield of glyoxal is 2 %, much lower than in previous mechanisms because of deposition and aerosol uptake of glyoxal precursors. Global production of isoprene SOA is about one-third from each of the following: isoprene epoxydiols (IEPOX), organonitrates, and tetrafunctional compounds. We find a SOA yield from isoprene of 13 % per carbon, much higher than commonly assumed in models and likely offset by SOA chemical loss. We use the results of our simulations to further condense RCIM into a mini Caltech isoprene mechanism (Mini-CIM) for less expensive implementation in atmospheric models, with a total size (108 species, 345 reactions) comparable to currently used mechanisms.

scholarly journals Atmospheric autoxidation is increasingly important in urban and suburban North America

2017 ◽  
Vol 115 (1) ◽  
pp. 64-69 ◽  
Author(s):  
Eric Praske ◽  
Rasmus V. Otkjær ◽  
John D. Crounse ◽  
J. Caleb Hethcox ◽  
Brian M. Stoltz ◽  
...  
Keyword(s):  
Hydrogen Transfer ◽  
Peroxy Radical ◽  
Rate Coefficients ◽  
Peroxy Radicals ◽  
Molecular Configuration ◽  
Atmospheric Pollutant ◽  
Kinetic Isotope ◽  
Experimental Approaches ◽  
Intramolecular Hydrogen ◽  
American Cities

Gas-phase autoxidation—regenerative peroxy radical formation following intramolecular hydrogen shifts—is known to be important in the combustion of organic materials. The relevance of this chemistry in the oxidation of organics in the atmosphere has received less attention due, in part, to the lack of kinetic data at relevant temperatures. Here, we combine computational and experimental approaches to investigate the rate of autoxidation for organic peroxy radicals (RO2) produced in the oxidation of a prototypical atmospheric pollutant, n-hexane. We find that the reaction rate depends critically on the molecular configuration of the RO2 radical undergoing hydrogen transfer (H-shift). RO2 H-shift rate coefficients via transition states involving six- and seven-membered rings (1,5 and 1,6 H-shifts, respectively) of α-OH hydrogens (HOC-H) formed in this system are of order 0.1 s−1 at 296 K, while the 1,4 H-shift is calculated to be orders of magnitude slower. Consistent with H-shift reactions over a substantial energetic barrier, we find that the rate coefficients of these reactions increase rapidly with temperature and exhibit a large, primary, kinetic isotope effect. The observed H-shift rate coefficients are sufficiently fast that, as a result of ongoing NOx emission reductions, autoxidation is now competing with bimolecular chemistry even in the most polluted North American cities, particularly during summer afternoons when NO levels are low and temperatures are elevated.

Sign in / Sign up

Export Citation Format

Share Document