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Atmospheric Fate of Methyl Bromide

Joelsson, Magnus LU (2012) FYSM60 20112
Department of Physics
Combustion Physics
Abstract (Swedish)
Den fotokemiska reaktorn vid Copenhagen Centre for Atmospheric Research, Köpenhamns universitet, har i denna studie används för att bestämma nya värden på hastighetskonstanterna
k(i) för reaktionen:
CH3Br + OH -> produkter (i)
och k(ii) för reaktionen:
CH3Br + O(1D) -> produkter. (ii)
Radikalerna (O(1D) och OH) framställdes i studien genom att fotolysera ozonmolekyler med UV-C-ljus. Oxidationsprocessen undersöktes genom att alternera korta fotolysperioder med mätningar av koncentrationen av de olika reaktanterna i cellen. Mätningarna urfördes med en Fourier Transfer infrarödspektrometer. Hastighetskonstanten k(i) bestämdes med Relativ- Rate-metoden, vilket innebär att CH3Br åtföljdes av och jämfördes med ett referensämne.... (More)
Den fotokemiska reaktorn vid Copenhagen Centre for Atmospheric Research, Köpenhamns universitet, har i denna studie används för att bestämma nya värden på hastighetskonstanterna
k(i) för reaktionen:
CH3Br + OH -> produkter (i)
och k(ii) för reaktionen:
CH3Br + O(1D) -> produkter. (ii)
Radikalerna (O(1D) och OH) framställdes i studien genom att fotolysera ozonmolekyler med UV-C-ljus. Oxidationsprocessen undersöktes genom att alternera korta fotolysperioder med mätningar av koncentrationen av de olika reaktanterna i cellen. Mätningarna urfördes med en Fourier Transfer infrarödspektrometer. Hastighetskonstanten k(i) bestämdes med Relativ- Rate-metoden, vilket innebär att CH3Br åtföljdes av och jämfördes med ett referensämne. Hastighetskonstanten k(i) bestämdes därför först som en faktor av hastighetskonstanten k(iii), som beskriver reaktionen:
referens + OH -> produkter. (iii)
Hastighetskonstanten k(ii) bestämdes med simuleringsprogrammet Kintecus®. En kinetisk justerades så att dess resultat överensstämmer med de uppmätta koncentrationer av CH3Br. k(i) är enligt våra studier (3.53 ± 0.23) × 10-14 cm3/molecule s, vilket är högre än det för tillfället
rekommenderade värdet. k(ii) visade sig vara i intervallet (2.5 : 5.4) × 10-10 cm3/molecule s, även det högre än det enda tidigare värdet som har rapporterats. Atmosfäriska budgetmodeller kan därför behöva revideras för att erhålla bättre värden för koncentrationer på ämnen innehållande
Br och även indirekt ämnen som påverkas av dessa. CH3Br har två dominerande isotopsammansättningar, nämligen CH379Br och CH381Br. En möjlig Kinetisk Isotopeffekt för reaktionerna (i) och (ii) undersöktes och kvantifierades.Våra resultat tyder på att KIE(i) finns i intervallet 1:1.2. Reaktion (i) är därför antagligen snabbare för den lättare molekylen CH379Br än för den tyngre CH381Br. Även om ett värde på KIE(ii) inte kunde slås fast, så gick ett troligt värde att härleda till KIE(ii) < 1. Reaktion (ii) är därför i motsats till reaktion (i) förmodligen snabbare för CH381Br än för CH379Br. Så vitt vi vet så är dessa de första försöken att bestämma KIE(i) och KIE(ii). Resultatet kan bidra till en bättre förståelse för atmosfäriska källor för Br-reservoarer. (Less)
Abstract
The photochemical reactor in Copenhagen Centre for Atmospheric Research, University of Copenhagen, was used to determine new values for the rate constants k(i) and of the reaction:
CH3Br + OH −> products (i)
and k(ii) for the reaction:
CH3Br + O(1D) −> products. (ii)
The radicals (O(1D) and OH) were produced by photolysis of ozone molecules with UV-C light. The oxidation process was monitored by alternate short photolysis periods and measurements of the concentrations of the different reactants in the cell. These measurements were conducte with a Fourier Transfer Infrared Spectrometer. To obtain concentrations from the IR spectra the iterative non linear least square fitting program MALT5 was used. The rate constant k(i) was determined... (More)
The photochemical reactor in Copenhagen Centre for Atmospheric Research, University of Copenhagen, was used to determine new values for the rate constants k(i) and of the reaction:
CH3Br + OH −> products (i)
and k(ii) for the reaction:
CH3Br + O(1D) −> products. (ii)
The radicals (O(1D) and OH) were produced by photolysis of ozone molecules with UV-C light. The oxidation process was monitored by alternate short photolysis periods and measurements of the concentrations of the different reactants in the cell. These measurements were conducte with a Fourier Transfer Infrared Spectrometer. To obtain concentrations from the IR spectra the iterative non linear least square fitting program MALT5 was used. The rate constant k(i) was determined with the Relative Rate method, meaning that CH3Br was accompanied by and compared with a reference compound. The rate constant k(i) was therefore first obtained as a factor of the rate constant k(iii), describing the reaction
reference + OH −> products. (iii)
The rate constant k(ii) was determined with the simulation software Kintecus®. A kinetic model was first adjusted to fit measured reference compound concentration, whereupon k(ii) in the model was adjusted to fit measured CH3Br concentrations. k(i) was found to be (3.53 ± 0.23) × 10−14 cm3/molecule s, higher than the current recommended value. k(ii) was found to be in the range (2.5 : 5.4) × 10−10 cm3/molecule s, also higher than the only value previously stated in the literature. Atmospheric budget models might therefore have to be revised in order to give a better estimate of Br-containing compound concentrations and indirectly concentrations of all species affected by them. CH3Br has two dominant isotope configurations, namely CH379Br and CH381Br. A possible Kinetic Isotope Effect (KIE) for the reactions (i) and (ii) were examined and quantified by a method similar to the Relative Rate method described above. To quantify the amount of CH3Br lost to reaction (i) in the system designed to seek KIE(ii), Kintecus® was employed. KIE(i) were found to be in the range 0.98:1.24. Reaction (i) is thus most likely faster for the lighter molecule CH379Br than the heavier CH381Br. KIE(ii) could not be satisfyingly quantified, but was deduced to most likely be KIE(ii) < 1. The reaction (ii) is therefore in contrast to reaction (i) faster for CH381Br than CH379Br. To our knowledge, these are the first attempts to decide values for KIE(i) and KIE(ii). The results might contribute to a better understanding of the atmospheric sources of Br reservoirs. (Less)
Please use this url to cite or link to this publication:
author
Joelsson, Magnus LU
supervisor
organization
course
FYSM60 20112
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Methyl bromide, ozone
language
English
id
2430693
date added to LUP
2012-04-23 14:25:39
date last changed
2012-11-12 22:39:56
@misc{2430693,
  abstract     = {The photochemical reactor in Copenhagen Centre for Atmospheric Research, University of Copenhagen, was used to determine new values for the rate constants k(i) and of the reaction:
CH3Br + OH −> products (i)
and k(ii) for the reaction:
CH3Br + O(1D) −> products. (ii)
The radicals (O(1D) and OH) were produced by photolysis of ozone molecules with UV-C light. The oxidation process was monitored by alternate short photolysis periods and measurements of the concentrations of the different reactants in the cell. These measurements were conducte with a Fourier Transfer Infrared Spectrometer. To obtain concentrations from the IR spectra the iterative non linear least square fitting program MALT5 was used. The rate constant k(i) was determined with the Relative Rate method, meaning that CH3Br was accompanied by and compared with a reference compound. The rate constant k(i) was therefore first obtained as a factor of the rate constant k(iii), describing the reaction
reference + OH −> products. (iii)
The rate constant k(ii) was determined with the simulation software Kintecus®. A kinetic model was first adjusted to fit measured reference compound concentration, whereupon k(ii) in the model was adjusted to fit measured CH3Br concentrations. k(i) was found to be (3.53 ± 0.23) × 10−14 cm3/molecule s, higher than the current recommended value. k(ii) was found to be in the range (2.5 : 5.4) × 10−10 cm3/molecule s, also higher than the only value previously stated in the literature. Atmospheric budget models might therefore have to be revised in order to give a better estimate of Br-containing compound concentrations and indirectly concentrations of all species affected by them. CH3Br has two dominant isotope configurations, namely CH379Br and CH381Br. A possible Kinetic Isotope Effect (KIE) for the reactions (i) and (ii) were examined and quantified by a method similar to the Relative Rate method described above. To quantify the amount of CH3Br lost to reaction (i) in the system designed to seek KIE(ii), Kintecus® was employed. KIE(i) were found to be in the range 0.98:1.24. Reaction (i) is thus most likely faster for the lighter molecule CH379Br than the heavier CH381Br. KIE(ii) could not be satisfyingly quantified, but was deduced to most likely be KIE(ii) < 1. The reaction (ii) is therefore in contrast to reaction (i) faster for CH381Br than CH379Br. To our knowledge, these are the first attempts to decide values for KIE(i) and KIE(ii). The results might contribute to a better understanding of the atmospheric sources of Br reservoirs.},
  author       = {Joelsson, Magnus},
  keyword      = {Methyl bromide,ozone},
  language     = {eng},
  note         = {Student Paper},
  title        = {Atmospheric Fate of Methyl Bromide},
  year         = {2012},
}