Experimental and Theoretical Study of the Uptake of Peroxy Radicals by Organic Aerosol Surfaces PDF Download
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Author: Antoine Roose
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Many uncertainties are still associated to chemical reaction mechanisms implemented in atmospheric models, especially for ROx radicals (OH, HO2, RO2). Of particular interest, heterogeneous processes (uptake of radicals) occurring at the aerosol surface have yet to be better described in models. The objective of this work is to investigate the peroxy radical uptake onto organic aerosol surfaces. The uptake is investigated both experimentally (macroscale observation) and theoretically (molecular level description).In the first part of this thesis an aerosol flow tube coupled to a peroxy radical measurement system (chemical amplification) has been developed and characterized. This system allows the measurement of HO2 uptake onto organic aerosols and could be used to measure RO2 uptakes as well. In this work, the uptake coefficient of HO2 onto glutaric acid particles has been measured and compared to conflicting literature data.The second part of this work consisted in the computation of fundamental processes driving the HO2 uptake onto glutaric acid model particles, using molecular modelling tools (molecular dynamics (MM) and/or quantum mechanics (QM)). The accommodation coefficient has been determined and a preliminary investigation of heterogeneous reactions has been carried out using the hybrid QM/MM ONIOM method.
Author: Antoine Roose
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Many uncertainties are still associated to chemical reaction mechanisms implemented in atmospheric models, especially for ROx radicals (OH, HO2, RO2). Of particular interest, heterogeneous processes (uptake of radicals) occurring at the aerosol surface have yet to be better described in models. The objective of this work is to investigate the peroxy radical uptake onto organic aerosol surfaces. The uptake is investigated both experimentally (macroscale observation) and theoretically (molecular level description).In the first part of this thesis an aerosol flow tube coupled to a peroxy radical measurement system (chemical amplification) has been developed and characterized. This system allows the measurement of HO2 uptake onto organic aerosols and could be used to measure RO2 uptakes as well. In this work, the uptake coefficient of HO2 onto glutaric acid particles has been measured and compared to conflicting literature data.The second part of this work consisted in the computation of fundamental processes driving the HO2 uptake onto glutaric acid model particles, using molecular modelling tools (molecular dynamics (MM) and/or quantum mechanics (QM)). The accommodation coefficient has been determined and a preliminary investigation of heterogeneous reactions has been carried out using the hybrid QM/MM ONIOM method.
Author: Lucas Jonathan Neil
Publisher:
ISBN:
Category :
Languages : en
Pages : 138
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The importance of the hydroxyl radical (OH) to tropospheric chemistry is well known. The radicals' ability to react with most atmospheric trace gases allows it to act as the main removal mechanism for these gases. Due to the highly reactive nature of OH, the oxidizing capacity of the atmosphere is often defined simply by its concentration. Owing to its significant role, knowledge of all OH chemistry, homogeneous and heterogeneous, is important to understanding the chemistry of the troposphere. In order to accurately predict future levels of OH and other trace gases, a thorough understanding of all processes and variables involved in the emission and sequestering of these compounds is essential. The gas-phase chemistry of OH is well known and has been extensively characterized. The one process that scientists are still trying to fully understand is its involvement in heterogeneous chemistry. Some studies have suggested that the inclusion of OH heterogeneous chemistry is important to fully model tropospheric chemistry, while other studies have suggested that it can be neglected entirely. It is therefore important to study and understand the conditions in which heterogeneous chemistry is significant. In order to do this accurately, scientists must first understand the process and magnitude of the uptake of OH onto atmospherically relevant surfaces. The main objective of this work was the development of a new analytical tool for the study of heterogeneous hydroxyl radical reactions. To this end, experiments were conducted to determine the most efficient approach to couple a low pressure aerosol flow tube (LP-AFT) to a chemical ionisation mass spectrometer (CIMS). The use of CIMS allowed for the accurate detection and quantification of hydroxyl radicals. Through iterative experimentation the system was designed and became operational. Experimental work focused on laboratory studies of reaction kinetics, with data reported in this work representing the reactions of OH with model atmospheric aerosols. The uptake of OH on organic aerosols was examined using the newly developed LP-AFT-CIMS system at standard temperature. Liquid oleic acid particles were used to mimic atmospherically relevant particles. The uptake coefficient, [gamma], on oleic acid particles was determined to be 0.49 ± 0.08 for a log-normally distributed aerosol at ~400nm. This value is in very good agreement with currently published data. However, the overall error of this method (~16%) is observed to be lower than other currently available methods, which have errors ranging from 20 - 30%. It is postulated that the mass accommodation coefficient, [alpha], for OH radicals on organic surfaces approaches this value under standard atmospheric conditions. It's also suggested that under the correct conditions the heterogeneous loss of OH could contribute to the overall budget of the OH radical. Further atmospheric implications of this reaction are discussed.
Author: Yong Liu
Publisher:
ISBN:
Category :
Languages : en
Pages : 404
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Author: Maria Teresa Raventos-Duran
Publisher:
ISBN:
Category :
Languages : en
Pages : 146
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Author: Midhun George
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Hydroperoxyl (HO2) and organic peroxy (RO2, where R stands for any organic group) radicals are highly reactive molecules produced in the oxidation of many compounds in the troposphere. They participate in the catalytic cycle producing or destroying ozone (O3) in the troposphere. Thus, HO2 and RO2 measurements provide unique information about the chemical processing of an air mass. Over the last decades, the understanding of the role of HO2 and RO2 in the chemical processes in the planetary boundary layer (PBL) has improved through ground-based in-situ measurements. However, the number of unequivocal measurements of peroxy radicals in the free troposphere is still quite limited. Measurements from airborne platforms offer a unique opportunity to measure HO2 and RO2 together with other relevant trace gases to test and improve the understanding of their chemistry in the free troposphere. During this doctoral study, an extensive set of airborne RO2* (RO2* = HO2 + ∑RO2, where RO2 represents the organic peroxy radicals reacting with NO to produce NO2) measurements in the PBL and free troposphere was acquired, analysed and interpreted. The RO2* measurements were made using the Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS) developed at the Institut für Umweltphysik (IUP) of the University of Bremen. PeRCEAS has successfully deployed onboard the High Altitude LOng range research aircraft (HALO) in three research campaigns: the Oxidation Mechanism Observations (OMO) Asia and the Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales (EMeRGe) field missions in Europe and Asia. The PeRCEAS instrument was characterised and calibrated under atmospherically representative conditions in the laboratory to assure data quality, reproducibility, accuracy and to define optimal operating conditions for the airborne measurements. PeRCEAS successfully measured RO2* in 33 HALO flights. RO2* mixing ratios of up to 120 pmole mole-1 were measured in air masses having different origins, chemical compositions and physical conditions in Europe and Asia. The RO2* measurements, the simultaneous measurements of other relevant trace gases, aerosol concentration, photolysis frequencies and other meteorological parameters were synergistically analysed to identify the chemical processes controlling the amount of RO2*. From the analysis, it was found that RO2* is primarily produced following the photolysis of ozone (O3), formaldehyde (HCHO), glyoxal (CHOCHO), and nitrous acid (HONO) in the air masses investigated. The estimate for the contribution of O3 photolysis to RO2* production rate is > 40 % in the PBL and 40% in the free troposphere. This reduction is explained by the decrease in the water vapour concentration ([H2O]) as a function of altitude. Subsequently, the RO2* mixing ratios in the air masses measured during the EMeRGe in Asia and Europe campaigns were calculated assuming a photostationary steady-state (PSS) for RO2*. The RO2* production from precursor photolysis, the loss through HO2 - HO2, RO2 - RO2 and HO2 - RO2 reactions, the hydroxyl radical (OH) and organic oxy-radicals (RO) loss during the radical interconversion, and HO2 uptake on aerosol were considered for the calculation of RO2*. The calculations were constrained by the simultaneous measurements of photolysis frequencies, trace gas concentrations and aerosol particle number concentrations onboard HALO. Case studies confirmed the validity of the PSS assumption for air masses having different chemical compositions under different physical conditions. The RO2* calculated are generally in excellent agreement with the RO2* measurements. An experimental budget analysis was performed to estimate the main loss processes of RO2* by introducing the RO2* measurements in the PSS equation. Except for the measurements inside pollution plumes with NO 800 pmole mole-1 or aerosol particle number concentration > 800 particles cm-3, the HO2 - RO2 and HO2 - HO2 were the dominant RO2* loss process during both EMeRGe Asia and Europe. The RO2* losses through HO2 uptake on aerosol were higher in the pollution outflows measured in Asia than in Europe. This is attributed to the higher aerosol concentrations observed in the air masses probed during EMeRGe in Asia. The contribution from the HO2 uptake on aerosol increases up to 60 % for an assumed aerosol uptake coefficient of 0.24 inside pollution plumes in Asia, where the aerosol particle number concentration is > 1000 particles cm-3. In Europe, the OH - NOx reactions were the dominant RO2* loss process in the pollution outflow. This finding is explained by the EMeRGe in Europe measurements being typically closer to anthropogenic emissions sources than in Asia, except for the case study of Taipei and Manila.
Author: P. D. Lightfoot
Publisher:
ISBN: 9789282656815
Category : Peroxides
Languages : en
Pages : 157
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Author: P. D. Lightfoot
Publisher:
ISBN:
Category :
Languages : en
Pages : 5
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Author: Tamar Moise
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Category :
Languages : en
Pages :
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Author: Guy P. Brasseur
Publisher: Springer Science & Business Media
ISBN: 3642189849
Category : Science
Languages : en
Pages : 310
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Praise for Guy P. Brasseur's Atmospheric Chemistry in a Changing World American Meteorological Society "This volume summarizes and integrates more than a decade of atmospheric chemistry research. During the period under consideration, great progress has been made in computing, modeling, and observational techniques, and methods have also improved. Here, suggestions for the highest priority research for the next decade are made, and important information is related regarding impacts on the environment."
Author: Jonathan A. Bray
Publisher:
ISBN:
Category : Intermediates (Chemistry)
Languages : en
Pages : 318
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