Sulfur Oxides in Atmospheres

Benjamin Normann Frandsen

Sulfur Oxides in Atmospheres

Benjamin Normann Frandsen

Abstract

In this work the photochemistry of SO2, photolysis of H2SO4 and formation and spectroscopy of cis-OSSO + trans-OSSO was investigated, and their role in the atmospheres of Earth and Venus have been discussed. The reactivity of SO2 in its lowest lying triplet state (3B1) was studied experimentally and theoretically. SO2 was shown to react with a series of alkanes as well as water, but only when promoted to 3SO2 after absorbing UV light. Computational chemistry was used to assign the mechanism to a hydrogen abstraction reaction. For the methane and water reaction with 3SO2, the rate constants were calculated at a high computational level (coupled cluster) with canonical variational transition state theory and multidimensional tunneling. The hydrogen abstraction of water results in the formation of OH radicals, and the fact that 3SO2 can cause OH to form has interesting implications for atmospheric environments otherwise assumed to be depleted in OH, such as the anoxic atmospheres of Venus and Archean Earth (pre-oxygen era, >2.5 billion years ago). The experimental UV-VIS spectrum of H2SO4 is yet unknown, but its absorption cross-section is important to uncover, since H2SO4 seeds cloud formation and its photolysis is a source of SO2 in the atmosphere. To study the UV photolysis of H2SO4, the spectrum was calculated using a nuclear ensemble approach. This method was first benchmarked against experimental spectra of a set of sulfur compounds: SO2, SO3, H2S, OCS and CS2. The benchmark was shown to produce spectra in very good agreement with experimental spectra in terms of both absorption cross-sections and peak widths. By using this method to calculate the UV spectrum of H2SO4, along with the inclusion of a second H2SO4 conformer, it was shown that UV photolysis might play a bigger role in the breakdown of H2SO4 in Earth’s and Venus’ atmospheres than previously expected. The sulfur-rich Venusian atmosphere exhibits an absorption in the near-UV spectral region which has yet to be assigned, despite several suggestions in the literature. It has also been shown that the atmosphere has an unidentified reservoir of sulfur, which can interconvert into sulfur monoxide and SO2. Isomers of S2O2 are discussed as potential matches to the unidentified sulfur reservoir species and the two isomers, cis-OSSO and trans-OSSO, were identified as promising candidates to match to the enigmatic near-UV absorption in the Venusian atmosphere. These two isomers were shown to be formed from a sulfur monoxide self-reaction. The spectra of cis-OSSO and trans-OSSO were calculated using the benchmarked nuclear ensemble approach and are shown to match the enigmatic near-UV absorption on Venus. The challenges to the assignment of the two isomers of OSSO to the Venus near-UV absorption are discussed. The main issue is the high uncertainty on the sulfur monoxide concentration in the upper cloud layer of Venus, which translates into an uncertainty in the concentration of cis- and trans-OSSO. A matrix-isolation infrared spectroscopy setup was used to trap sulfur oxides at cryogenic temperatures in a matrix of a noble gas. This setup was modified to include a microwave discharge attachment on the matrix deposition line, which was used to form a plasma of the gas mixture during deposition onto the cold sample window. This was shown to effectively cause bond dissociation of the molecules in the noble gas mixture. The microwave discharge was used on SO2 to form new sulfur oxides, including cis-OSSO and trans-OSSO. An analysis of contaminants and spectral artefacts are included to be able to safely separate sample signals fromother sources. To complement the microwave discharge experiments, a matrix irradiation study was carried out where a 193 nm ArF excimer laser was used to photodissociate SO2 in the matrix. Calculated vibrational frequencies and intensities with anharmonic effects were used to aid in assignments of different sulfur oxides in the matrix-isolation experiments.

Original language	English

Publisher	Department of Chemistry, Faculty of Science, University of Copenhagen
Publication status	Published - 2019

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title = "Sulfur Oxides in Atmospheres",

abstract = "In this work the photochemistry of SO2, photolysis of H2SO4 and formation and spectroscopy of cis-OSSO + trans-OSSO was investigated, and their role in the atmospheres of Earth and Venus have been discussed. The reactivity of SO2 in its lowest lying triplet state (3B1) was studied experimentally and theoretically. SO2 was shown to react with a series of alkanes as well as water, but only when promoted to 3SO2 after absorbing UV light. Computational chemistry was used to assign the mechanism to a hydrogen abstraction reaction. For the methane and water reaction with 3SO2, the rate constants were calculated at a high computational level (coupled cluster) with canonical variational transition state theory and multidimensional tunneling. The hydrogen abstraction of water results in the formation of OH radicals, and the fact that 3SO2 can cause OH to form has interesting implications for atmospheric environments otherwise assumed to be depleted in OH, such as the anoxic atmospheres of Venus and Archean Earth (pre-oxygen era, >2.5 billion years ago). The experimental UV-VIS spectrum of H2SO4 is yet unknown, but its absorption cross-section is important to uncover, since H2SO4 seeds cloud formation and its photolysis is a source of SO2 in the atmosphere. To study the UV photolysis of H2SO4, the spectrum was calculated using a nuclear ensemble approach. This method was first benchmarked against experimental spectra of a set of sulfur compounds: SO2, SO3, H2S, OCS and CS2. The benchmark was shown to produce spectra in very good agreement with experimental spectra in terms of both absorption cross-sections and peak widths. By using this method to calculate the UV spectrum of H2SO4, along with the inclusion of a second H2SO4 conformer, it was shown that UV photolysis might play a bigger role in the breakdown of H2SO4 in Earth{\textquoteright}s and Venus{\textquoteright} atmospheres than previously expected. The sulfur-rich Venusian atmosphere exhibits an absorption in the near-UV spectral region which has yet to be assigned, despite several suggestions in the literature. It has also been shown that the atmosphere has an unidentified reservoir of sulfur, which can interconvert into sulfur monoxide and SO2. Isomers of S2O2 are discussed as potential matches to the unidentified sulfur reservoir species and the two isomers, cis-OSSO and trans-OSSO, were identified as promising candidates to match to the enigmatic near-UV absorption in the Venusian atmosphere. These two isomers were shown to be formed from a sulfur monoxide self-reaction. The spectra of cis-OSSO and trans-OSSO were calculated using the benchmarked nuclear ensemble approach and are shown to match the enigmatic near-UV absorption on Venus. The challenges to the assignment of the two isomers of OSSO to the Venus near-UV absorption are discussed. The main issue is the high uncertainty on the sulfur monoxide concentration in the upper cloud layer of Venus, which translates into an uncertainty in the concentration of cis- and trans-OSSO. A matrix-isolation infrared spectroscopy setup was used to trap sulfur oxides at cryogenic temperatures in a matrix of a noble gas. This setup was modified to include a microwave discharge attachment on the matrix deposition line, which was used to form a plasma of the gas mixture during deposition onto the cold sample window. This was shown to effectively cause bond dissociation of the molecules in the noble gas mixture. The microwave discharge was used on SO2 to form new sulfur oxides, including cis-OSSO and trans-OSSO. An analysis of contaminants and spectral artefacts are included to be able to safely separate sample signals fromother sources. To complement the microwave discharge experiments, a matrix irradiation study was carried out where a 193 nm ArF excimer laser was used to photodissociate SO2 in the matrix. Calculated vibrational frequencies and intensities with anharmonic effects were used to aid in assignments of different sulfur oxides in the matrix-isolation experiments.",

author = "Frandsen, {Benjamin Normann}",

year = "2019",

language = "English",

publisher = "Department of Chemistry, Faculty of Science, University of Copenhagen",

}

TY - BOOK

T1 - Sulfur Oxides in Atmospheres

AU - Frandsen, Benjamin Normann

PY - 2019

Y1 - 2019

N2 - In this work the photochemistry of SO2, photolysis of H2SO4 and formation and spectroscopy of cis-OSSO + trans-OSSO was investigated, and their role in the atmospheres of Earth and Venus have been discussed. The reactivity of SO2 in its lowest lying triplet state (3B1) was studied experimentally and theoretically. SO2 was shown to react with a series of alkanes as well as water, but only when promoted to 3SO2 after absorbing UV light. Computational chemistry was used to assign the mechanism to a hydrogen abstraction reaction. For the methane and water reaction with 3SO2, the rate constants were calculated at a high computational level (coupled cluster) with canonical variational transition state theory and multidimensional tunneling. The hydrogen abstraction of water results in the formation of OH radicals, and the fact that 3SO2 can cause OH to form has interesting implications for atmospheric environments otherwise assumed to be depleted in OH, such as the anoxic atmospheres of Venus and Archean Earth (pre-oxygen era, >2.5 billion years ago). The experimental UV-VIS spectrum of H2SO4 is yet unknown, but its absorption cross-section is important to uncover, since H2SO4 seeds cloud formation and its photolysis is a source of SO2 in the atmosphere. To study the UV photolysis of H2SO4, the spectrum was calculated using a nuclear ensemble approach. This method was first benchmarked against experimental spectra of a set of sulfur compounds: SO2, SO3, H2S, OCS and CS2. The benchmark was shown to produce spectra in very good agreement with experimental spectra in terms of both absorption cross-sections and peak widths. By using this method to calculate the UV spectrum of H2SO4, along with the inclusion of a second H2SO4 conformer, it was shown that UV photolysis might play a bigger role in the breakdown of H2SO4 in Earth’s and Venus’ atmospheres than previously expected. The sulfur-rich Venusian atmosphere exhibits an absorption in the near-UV spectral region which has yet to be assigned, despite several suggestions in the literature. It has also been shown that the atmosphere has an unidentified reservoir of sulfur, which can interconvert into sulfur monoxide and SO2. Isomers of S2O2 are discussed as potential matches to the unidentified sulfur reservoir species and the two isomers, cis-OSSO and trans-OSSO, were identified as promising candidates to match to the enigmatic near-UV absorption in the Venusian atmosphere. These two isomers were shown to be formed from a sulfur monoxide self-reaction. The spectra of cis-OSSO and trans-OSSO were calculated using the benchmarked nuclear ensemble approach and are shown to match the enigmatic near-UV absorption on Venus. The challenges to the assignment of the two isomers of OSSO to the Venus near-UV absorption are discussed. The main issue is the high uncertainty on the sulfur monoxide concentration in the upper cloud layer of Venus, which translates into an uncertainty in the concentration of cis- and trans-OSSO. A matrix-isolation infrared spectroscopy setup was used to trap sulfur oxides at cryogenic temperatures in a matrix of a noble gas. This setup was modified to include a microwave discharge attachment on the matrix deposition line, which was used to form a plasma of the gas mixture during deposition onto the cold sample window. This was shown to effectively cause bond dissociation of the molecules in the noble gas mixture. The microwave discharge was used on SO2 to form new sulfur oxides, including cis-OSSO and trans-OSSO. An analysis of contaminants and spectral artefacts are included to be able to safely separate sample signals fromother sources. To complement the microwave discharge experiments, a matrix irradiation study was carried out where a 193 nm ArF excimer laser was used to photodissociate SO2 in the matrix. Calculated vibrational frequencies and intensities with anharmonic effects were used to aid in assignments of different sulfur oxides in the matrix-isolation experiments.

AB - In this work the photochemistry of SO2, photolysis of H2SO4 and formation and spectroscopy of cis-OSSO + trans-OSSO was investigated, and their role in the atmospheres of Earth and Venus have been discussed. The reactivity of SO2 in its lowest lying triplet state (3B1) was studied experimentally and theoretically. SO2 was shown to react with a series of alkanes as well as water, but only when promoted to 3SO2 after absorbing UV light. Computational chemistry was used to assign the mechanism to a hydrogen abstraction reaction. For the methane and water reaction with 3SO2, the rate constants were calculated at a high computational level (coupled cluster) with canonical variational transition state theory and multidimensional tunneling. The hydrogen abstraction of water results in the formation of OH radicals, and the fact that 3SO2 can cause OH to form has interesting implications for atmospheric environments otherwise assumed to be depleted in OH, such as the anoxic atmospheres of Venus and Archean Earth (pre-oxygen era, >2.5 billion years ago). The experimental UV-VIS spectrum of H2SO4 is yet unknown, but its absorption cross-section is important to uncover, since H2SO4 seeds cloud formation and its photolysis is a source of SO2 in the atmosphere. To study the UV photolysis of H2SO4, the spectrum was calculated using a nuclear ensemble approach. This method was first benchmarked against experimental spectra of a set of sulfur compounds: SO2, SO3, H2S, OCS and CS2. The benchmark was shown to produce spectra in very good agreement with experimental spectra in terms of both absorption cross-sections and peak widths. By using this method to calculate the UV spectrum of H2SO4, along with the inclusion of a second H2SO4 conformer, it was shown that UV photolysis might play a bigger role in the breakdown of H2SO4 in Earth’s and Venus’ atmospheres than previously expected. The sulfur-rich Venusian atmosphere exhibits an absorption in the near-UV spectral region which has yet to be assigned, despite several suggestions in the literature. It has also been shown that the atmosphere has an unidentified reservoir of sulfur, which can interconvert into sulfur monoxide and SO2. Isomers of S2O2 are discussed as potential matches to the unidentified sulfur reservoir species and the two isomers, cis-OSSO and trans-OSSO, were identified as promising candidates to match to the enigmatic near-UV absorption in the Venusian atmosphere. These two isomers were shown to be formed from a sulfur monoxide self-reaction. The spectra of cis-OSSO and trans-OSSO were calculated using the benchmarked nuclear ensemble approach and are shown to match the enigmatic near-UV absorption on Venus. The challenges to the assignment of the two isomers of OSSO to the Venus near-UV absorption are discussed. The main issue is the high uncertainty on the sulfur monoxide concentration in the upper cloud layer of Venus, which translates into an uncertainty in the concentration of cis- and trans-OSSO. A matrix-isolation infrared spectroscopy setup was used to trap sulfur oxides at cryogenic temperatures in a matrix of a noble gas. This setup was modified to include a microwave discharge attachment on the matrix deposition line, which was used to form a plasma of the gas mixture during deposition onto the cold sample window. This was shown to effectively cause bond dissociation of the molecules in the noble gas mixture. The microwave discharge was used on SO2 to form new sulfur oxides, including cis-OSSO and trans-OSSO. An analysis of contaminants and spectral artefacts are included to be able to safely separate sample signals fromother sources. To complement the microwave discharge experiments, a matrix irradiation study was carried out where a 193 nm ArF excimer laser was used to photodissociate SO2 in the matrix. Calculated vibrational frequencies and intensities with anharmonic effects were used to aid in assignments of different sulfur oxides in the matrix-isolation experiments.

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/1ed7rpq/alma99123213868105763

M3 - Ph.D. thesis

BT - Sulfur Oxides in Atmospheres

PB - Department of Chemistry, Faculty of Science, University of Copenhagen

ER -

Sulfur Oxides in Atmospheres

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