Aerosols of nitrate solutions were irradiated in the presence of radical scavengers in an attempt to measure the yield of hydroxyl radical in both the aqueous phase and the gas phase. Carbon monoxide, benzoic acid, benzene and cyclohexane were used as scavengers to trap hydroxyl radical. The products from the reaction of these scavengers with hydroxyl radical were analysed with High Performance Liquid Chromatography and mass spectrometry. The radiant flux in the chamber was measured via ferrioxalate actinometry, both with bulk liquid and aerosol droplets. Many quantitative results were obtained but several anomalies were found. This suggests that Mie theory is not capable of predicting rates of photochemical reactions within droplets.

The potential of a condensate method for sampling atmospheric trace gases is investigated. A theoretical approach to the collection of condensate by a cooled surface is presented, followed by an experimental procedure and verification of the method. Heat and mass transfer problems are solved to give distributions of temperature, velocity and mass concentration. From these distributions, collection rates for water vapor, trace gases, and particulates are derived. It is shown that the collection of particulates is negligible in most cases when compared to the collection of gases in the condensate; highly soluble gases are sampled with the greatest efficiency. Experimentally, a plate, cooled to below the ambient dew point temperature, collected condensate which was subsequently analyzed for various components. Atmospheric concentrations were determined from the corresponding species in the condensate. Several trace gases have been detected by the condensate method. Concentrations of two of these, ammonia and nitric acid, are compared with filter methods, with reasonably good agreement. Comparison is also made with concentrations of ammonia, formaldehyde, nitric acid and nitrous acid found by others under similar conditions. Typical daily variations of trace components in more remote areas surrounding Tucson, Arizona, are presented in an attempt to estimate background Southwest U.S.A. levels; these are as follows: 1.0 ppb for ammonia, 0.5 ppb for formaldehyde, 0.1 ppb for nitrous acid, 0.4 ppb for nitric acid, 1.1 ppb for formic acid, 0.9 ppb for acetic acid, 0.3 ppb for sulfur dioxide, 0.3 ppb for hydrochloric acid and 1 ppb for hydrogen peroxide. The diurnal variation of these gases and relationship between them is discussed. Possible sources of error in the condensate method of sampling are considered. The collection rate of water vapor is measured and compared to that theoretically predicted; explanations are presented for the discrepancy between the two. The sensitivity to errors in the measurement of the collection temperature and dew point temperature is determined.

There are few published direct measurements of the atmosphere-surface exchange of volatile organic compounds (VOCs), particularly for biogenic VOCs (BVOCs). Global modelling of atmospheric chemistry and transport of BVOCs has large uncertainties due to the very small number of measurements in tropical regions, which are responsible for half the global BVOC emissions. This thesis presents direct measurements of concentrations and ecosystem fluxes of BVOCs in different regions (Tropics, Mediterranean) using the approach of virtual disjunct eddy covariance (vDEC) combined with proton transfer reaction mass spectrometry (PTR-MS) – a real-time BVOC sensor. The field measurements also included methodological developments of the vDEC/PTR-MS approach, which will be of value to the wider flux measurement community. A novel approach to determining the lag time between the vertical wind measurement and the air concentration measurement has been developed that will greatly reduce the uncertainty in the derived flux measurements. In the laboratory, the selectivity of PTR-MS was investigated by designing an alternating drift-voltage mode (AD-PTR-MS) to discriminate between structural isomers detected at the same m/z channel, with monoterpenes used as model compounds. The results of the measurements, particularly from the rainforest and oil palm plantations in Borneo, are novel and therefore provide important experimental constraints on models of atmospheric emissions, chemistry and transport. For example, although parameters which work reasonably well can be derived for model algorithms for the emission of isoprene from the rainforest, their performance over oil palms was less good, because of circadian controls of emissions from oil palms. However, the larger problem is the measured basal emission rates (BERs) which are significantly smaller than those used by default in the global MEGAN model. Another novel finding was the high deposition velocities of MVK and MACR (isoprene first order oxidation products) which at the oil palm plantation commonly exceeded 1 cm s-1; this result has implications for atmospheric modelling. The successful field results relied on significant developments in software for data acquisition and processing, and operational optimisation of the PTR-MS instruments in the extreme humidity encountered during the fieldwork in Borneo.

A química do selênio é um assunto de crescente interesse devido a sua presença em diversos ambientes químicos, em particular, na atmosfera terrestre. A ausência de estudos sobre espécies relativamente simples contendo 2-4 átomos motivou este projeto, que se concentrou na investigação teórica rigorosa de uma serie de espécies moleculares: SeF, SeCl, SeBr, HSeF, HFSe, HSeCl, HClSe, HSeBr, HBrSe e de vários isômeros na superfície de energia potencial 1[H, S, Se, Cl]. Propriedades espectroscópicas de um conjunto de estados eletrônicos e o calor de formação das moléculas SeF, SeCl e SeBr foram determinados. Juntamente com os novos resultados desta investigação, sugerimos uma revisão e correção de alguns dados teóricos e experimentais da literatura. Aspectos energéticos, estruturais e espectroscópicos associados aos pontos estacionários nas superfícies de energia potencial singleto [H, Se, X], X = F, Cl e Br, e [H, S, Se, Cl] também foram caracterizados, assim como determinados os calores de formação dos isômeros mais estáveis. Barreiras energéticas para os vários processos de isomerização foram estimadas bem como o gasto energético envolvido nas diferentes possibilidades de dissociação dos isômeros mais estáveis. No caso dos sistemas triatômicos ainda estimamos as constantes de velocidade para as reações de isomerizações direta e reversa. Nesse contexto, esperamos que este trabalho possa servir como uma referência para estudos teóricos e experimentais futuros desses sistemas e/ou de outros de complexidade idêntica. / The chemistry of selenium is a subject of increasing interest due to its presence in many chemical enviroments, specially in the Earth\'s atmosphere. The lack of studies of relatively simple species containing 2-4 atoms has motivated this project which was focused on a rigorous theoretical investigation of a series of molecular especies: SeF, SeCl, SeBr, HSeF, HFSe, HSeCl, HClSe, HSeBr, HBrSe, and the isomers on the 1[H, S, Se, Cl] potential energy surface. Spectroscopic properties of a set of electronic states and the heat of formation of SeF, SeCl, and SeBr were determined. Along with the new results from this investigation, we showed that some theoretical and experimental data reported in the literature be revised and corrected. Energetic, structural, and spectroscopic aspects associated with the stationary points on the singlet potential energy surfaces [H, Se, X], X = F, Cl e Br, and [H, S, Se, Cl] were also characterized, and the heats of formation of the most stable isomers evaluated. Energetic barriers for the various processes of isomerization were estimated, as well as the energy involved in the dierent possibilities of dissociation of the most stable isomers. In the case of triatomic systems, we still estimated the rate constants for the direct and reverse reactions. In this context, we expect that this work should serve as reference in future theoretical and experimental studies on these systems and/or others of similar complexity

<p>For decades there has been interest in understanding early prebiotic Earth, including its atmospheric chemistry. Saturn’s moon Titan is the only other body in our Solar System with an atmosphere thought to resemble that of early Earth’s, and for this reason it has garnered a lot of attention over the years. Much is now known about the smaller molecules present in that atmosphere, starting with the most abundant, N₂ and CH₄, and going up to slightly larger molecules such as cyanoacetylene and benzene. As the molecules get larger, however, so does the gap in knowledge, especially as it pertains to nitriles. This dissertation aims to add to the story of Titan’s nitriles by first characterizing a molecule thought to be the photochemical product of the reaction between cyanoacetylene and benzene, 3-phenyl-2-propyne-nitrile (PPN). The UV spectra of PPN proved immensely interesting due to the strong presence of in-plane and out-of-plane vibrations of b₂ and b₁ symmetry, respectively. This is possibly a result of strong vibronic coupling between several excited electronic states or Coriolis coupling between complementary b₁ and b₂ vibrational levels. The multi-layer extension of the multi-configuration time dependent Hartree (ML-MCTDH) algorithm was used to understand how the excited states and the vibrational levels might interact, and emission and absorption spectra were modeled and compared to the experimental spectra. The second group of molecules studied included the <i>ortho</i>-, <i>meta</i>-, and <i>para</i>-methyl PPN. Strong methyl rotor activity is seen in the <i>m</i>-methyl PPN, with some activity in the <i>p</i>-methyl PPN. The methyl rotor activity in the <i>m</i>-methyl PPN is similar to other <i>meta</i>-substituted toluenes, and allows us to describe the methyl rotor barrier height in both ground and excited electronic state. Additionally, in all three methylated PPNs we see evidence for strong vibronic coupling in the abundance of out-of-plane vibrations, as had been seen in PPN.</p>

Recent instrument development for aerosol measurement has focussed on small-scale, on-line measurements that can be incorporated into miniaturised sensor nodes as part of ambient or personal air quality monitoring networks. As a result, optical particle counters (OPCs) have risen in popularity given their ability to consistently size and count individual particles. OPCs have limitations, however, in their inability to detect ultrafine particles (considered the most influential to human health) or to measure particle mass directly (the standard metric for air quality). The growing field of microelectromechanical systems (MEMS) offers a potential alternative by implementing microresonators as mass sensors. MEMS resonators have high mass sensitivities and have recently seen implementation as particulate matter (PM) monitors. The field of MEMS PM instruments is still limited with a variety of implemented resonator topologies and sampling mechanisms. In general, however, they offer real-time, high sensitivity measurements at low flow rates. The aim of this thesis was to further examine the viability of implementing MEMS resonators for PM measurement with a focus on practical considerations for real-world applications. To this end, a new microresonator-based impactor was developed - the MEMS Impactor Stage (MIS) - capable of accommodating various nozzle and resonator combinations. Square lateral bulk acoustic resonators were the primary topology, but the results within the thesis are widely applicable. A series of laboratory studies covered the resonator lifetime, reusability, detection limits, and response to environmental changes. The resonator displayed a high sensitivity throughout, capable of detecting ultrafine particles, but is vulnerable to misinterpretation. Beyond mass measurement, studies introduced possible extensions to hygroscopicity and compositional applications. Ambient particle measurements with the MIS, simulating a real-world application to air quality monitoring, showed the capabilities as a PM instrument while highlighting concerns to be addressed for future instrument design. A microresonator-based impactor has potential as an alternative to OPCs, but its cross sensitivity to deposition patterns and environmental effects must be accounted for prior to implementation as PM monitor.

Atmospheric aerosols are tiny particles suspended in the atmosphere. They affect global air quality, public health and climate (Boucher et al., 2013; Myhre et al., 2013; Seinfeld and Pandis, 2016), thus playing a key role in the Earth system. However, due to the complexity of aerosol processes and climate change feedbacks, our understanding of aerosols in a changing world is still limited (Boucher et al., 2013). To understand the impact of organic aerosol volatility on particle microphysics and global climate, I developed a new aerosol microphysics scheme, MATRIX-VBS, and its evaluation and application are presented in this dissertation. MATRIX-VBS couples the volatility-basis set (VBS, Donahue et al., 2006) framework with the aerosol microphysical scheme MATRIX (Multiconfiguration Aerosol TRacker of mIXing state, Bauer et al., 2008) that resolves aerosol mass and number concentrations, size, and mixing state. With the inclusion of organic partitioning and photochemical aging of semi-volatile organic aerosols, aerosols are able to grow via organic condensation, a process previously not available in the original model MATRIX, where organic aerosols were treated as nonvolatile. Both MATRIX and MATRIX-VBS can be used as stand-alone box models or within a global model. After the development of MATRIX-VBS in the box model framework, both model’s simulations were performed and assessed on the box and global scales. On the box model scale, idealized experiments were designed to simulate different environments, clean, polluted, urban, and rural. I investigated the evolution of organic aerosol mass concentration and volatility distribution among gas and aerosol phases, and results show that semi-volatile primary organic aerosols evaporate almost completely in the intermediate-volatility range and stay in the particle phase in the low volatility range. I also concluded that the volatility distribution of organics relies on emission, oxidation, and temperature, and the inclusion of organic aerosol volatility changes aerosol mixing state. Comparing against parallel simulations with the original model MATRIX, which treats organic aerosols as nonvolatile, I assessed the effect of gas-particle partitioning and photochemical aging of semi-volatile organics on particle growth, composition, size distribution and mixing state. Results also show that the new model produces different mixing states, increased number concentrations and decreased aerosol sizes for organic-containing aerosol populations. Monte-Carlo type experiments were performed and they offered a more in-depth look at the impact of organic aerosol volatility on activated number concentration, which is the number concentration of aerosols that are activated but has not yet formed into a cloud droplet. By testing multiple parameters such as aerosol composition, mass concentration and number concentration, as well as particle size, I examined the impact of partitioning organic aerosols on activated aerosol number concentration. I found that the new model MATRIX-VBS produces fewer activated particles compared to the original model MATRIX, except in environments with low cloud updrafts, in clean regions at above freezing temperatures, and in polluted areas at high temperature (310K) and extremely low humidity conditions. I concluded that such change is caused by the differences in aerosol number concentration and size between the two models, which would determine how many particles could activate. On the global scale, MATRIX-VBS was implemented in the NASA GISS ModelE Earth systems model. I assessed and evaluated the new model by comparing aerosol mass and number concentrations, activated cloud number concentration, and AOD against output from the original MATRIX model. Further, I evaluate the two models against observations of organic aerosol mass concentration from the aircraft campaign ATom (Atmospheric Tomography Mission), and aerosol optical depth from ground measurement stations from AERONET (Aerosol Robotic Network) as well as satellite retrievals from MODIS (MODerate resolution Imaging Spectroradiometer) and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations). Results show that organics in MATRIX-VBS experience more distant long-range transport, and their mass concentration increase aloft and decrease at the surface as compared to those in MATRIX. There are still underestimations in the vertical profiles of mass concentration in both models, especially in the high latitudes in the Northern Hemisphere and South Pacific Ocean basin, possibly due to the application of universal distribution of mass-based emission factors among different volatilities that perhaps is not realistic in all climate zones, thus affecting organic aerosol lifetime and transport. Just as the box model results, there are more particles and generally more activated ones (except for rare cases such as the highly polluted Eastern China) in MATRIX-VBS than in MATRIX. As for AOD comparisons, MATRIX-VBS have generally lower AOD than MATRIX, which can be due to smaller aerosols and different aerosol composition in the new model, which is also underestimating biomass burning in the Amazon and Congo basins. Compared to satellite retrievals from MODIS and ground measurements from AERONET, both models overestimate aerosol optical depth over anthropogenic polluted regions and biomass regions such as central Africa. Overall, both models also underestimate AOD as compared to AERONET in the winter (DJF), whereas they generally overestimate or estimate it well in other seasons. Even though during its initial evaluation, MATRIX-VBS does not seem to have improved from MATRIX on the global scale in representing the real world, it made the first key step in improving our understanding of organic aerosols on the process level. Changes in mass, number concentration, size distribution, and mixing state (composition) have great implications and impact on climate. Further studies are needed in examining and improving factors linked to the new representation of semi-volatiles in an aerosol microphysics model, including but not limited to the treatment of mass-based emission factor distribution among different organic volatilities and the size distribution of tiny organic particles that have evaporated but not completely. Challenges in evaluations of organic aerosol against measurements remain in that remote regions of significant interest lack available measurements, and additional field campaigns will be important for us to better understand real world conditions and shed light on model performance.

Uma importante classe de poluentes do ar são os compostos orgânicos voláteis (VOCs), comumente encontrados na atmosfera de centros urbanos e industriais, em quantidades muito baixas. Os VOCs contribuem para episódios sérios de poluição. Eles participam ativamente na formação do smog fotoquímico, problema encontrado em muitos centros urbanos. Além do efeito tóxico à saúde humana, os VOCs podem apresentar impactos indiretos via produção fotoquímica de ozônio. Alguns VOCs encontrados no ar urbano são carcinogênicos. Diante do papel importante dos VOCs na química atmosférica, o objetivo geral deste projeto de pesquisa foi avaliar os VOCs emitidos por fontes antropogênicas e biogênicas na atmosfera urbana da cidade de São Paulo. As emissões antropogênicas foram avaliadas através de estudos de emissões veiculares em dois túneis da cidade de São Paulo. A atmosfera de um túnel fornece condições apropriadas para a medida da composição média das emissões veiculares. A caracterização química da atmosfera dos túneis mostra que as emissões veiculares se acumulam ao longo dos túneis causando níveis altos de poluição. Os freqüentes congestionamentos nas principais vias de acesso e nos túneis da Cidade de São Paulo tomam preocupante a exposição humana a poluentes tóxicos no ar. Para a avaliação das emissões biogênicas foram empregadas câmaras fechadas (sistema cuvette) para coletar os VOCs emitidos por plantas. Espécies de plantas que emitem uma quantidade significativa de isopreno e monoterpenos foram estudadas em alguns países do mundo. No entanto, nenhum estudo sobre emissão de VOCs por plantas foi feito na vegetação encontrada na cidade de São Paulo. Dessa maneira, foram realizadas amostragens de algumas plantas da vegetação típica da Mata Atlântica. Os valores de taxas de emissão de VOCs em &#181;g de carbono por hora e massa de folha seca (&#181;gC h-1 g-1) foram calculados para cada planta. Ocorreram variações nas taxas de emissão tanto entre as diferentes espécies de plantas (inter-espécies), como entre exemplares diferentes de uma mesma espécie (intra-espécies). / Volatile organic compounds (VOCs) are an important class of air pollutants, commonly found in urban and industrial atmospheres at low concentrations. VOCs contribute to serious episodes of air pollution. They play a major role in the formation of the photochemical smog, an air pollution problem often encountered in many urban centers. Besides the toxic effect to human health, VOCs may be responsible by indirect impacts, such as the photochemical production of ozone. Some VOCs found in urban air are carcinogenic. The goal of this study was to evaluate VOCs emitted by anthropogenic and biogenic sources in the urban atmosphere of São Paulo City since they have an important role in atmospheric chemistry. VOCs anthropogenic emissions were evaluated through vehicular emissions inside two urban road tunnels of São Paulo City. A tunnel atmosphere provides appropriate conditions for the measurement of the average composition of vehicular emission. The chemical characterization of the atmosphere of both tunnels showed that vehicular emissions accumulate inside the tunnels leading to high pollutants levels. VOCs biogenic emissions from plants were evaluated by using closed chambers (cuvette system). Species of plants that emit significant amounts of isoprene and monoterpenes were studied in some countries of the world. Nevertheless, no study about VOCs emissions had been done in the vegetation found in São Paulo City. Some typical plants of the Mata Atlântica were investigated. The VOCs emission rates expressed in &#181;g of carbon per hour and leaves dry weight (&#181;gC h-1 g-1) were calculated for each plant. Variations in the emissions rates were observed between different plants species (inter-species) and between different plants of the same species (intra-species).

Submitted by Sandra Pereira (srpereira@ufrrj.br) on 2017-01-20T11:19:55Z No. of bitstreams: 1 2016 - Michel Braga Garcia.pdf: 2199692 bytes, checksum: 4e890f95e489a3e9ea5a095b493dce78 (MD5) / Made available in DSpace on 2017-01-20T11:19:55Z (GMT). No. of bitstreams: 1 2016 - Michel Braga Garcia.pdf: 2199692 bytes, checksum: 4e890f95e489a3e9ea5a095b493dce78 (MD5) Previous issue date: 2016-04-06 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior - CAPES / Studies on Atmospheric Chemistry allow the understanding of chemical behavior of different atmospheric pollutants towards oxidant agents in troposphere. Among these agents, OH radicals, ozone and nitrate radicals are the most important. In coastal zones, chlorine atoms are also important specie for the chemical removal of primary and secondary pollutants. This work aims the kinetic study of the gas phase reaction between chlorine atom and formaldehyde, H2CO + Cl ? HCO + HCl. The reaction was described at HF, MP2, CCSD, QCISD and DFT (B2PLYP, MPW2PLYP and MPWKCIS1K) levels. Basis set effect was also explored and the aug-cc-pVDZ (ACCD) and aug-cc-pVTZ (ACCT) basis set were adopted. Geometry optimizations and vibrational frequencies calculations were performed for reactants and products. A prebarrier complex was located at all levels of theory, stabilized with respect the isolated reactants by ca. 4 kcal mol-1. Saddle points were located and characterized by their imaginary frequencies. Energy barriers were predicted as ca. 1 kcal mol-1, in agreement with experimental results (0.7 ? 1.0 kcal mol-1). Rate coefficients were calculated in the range from 100 to 500 K, adopting the canonical variational method, as available in the kcvt code. The calculated kinetic parameters, rate coefficients (k, 298 K, expressed in 10-11 cm3 molecule-1 s-1), activation energy (Ea, expressed in kcal mol-1) and Arrhenius preexponential factor (A, expressed in 10-11 cm3 molecule-1 s-1) were: k = 4.89, Ea = 0.25 and A = 7.45. The results suggest that a reaction dynamics is well described by a mechanism taking into account a prebarrier intermediate. Canonical variational rate coefficients were predicted in good agreement with experimental data, validating the proposal for the reaction mechanism introduced in this study / Os estudos em Qu?mica Atmosf?rica permitem entender o comportamento qu?mico de diversos poluentes atmosf?ricos frente a agentes oxidantes presentas na troposfera. Entre esses agentes, radicais OH, oz?nio e nitrato s?o os mais importantes. Ainda em zonas costeiras, ?tomos de cloro tornam-se tamb?m importantes esp?cies para a remo??o qu?mica de poluentes prim?rios e secund?rios. Este trabalho visa o estudo cin?tico da rea??o H2CO + Cl ? HCO + HCl em fase gasosa. A rea??o foi descrita em n?veis HF, MP2, CCSD, QCISD, DFT (B2PLYP, MPW2PLYP e MPWKCIS1K) Tamb?m o efeito da base foi explorado e, para tanto, as bases aug-cc-pVDZ (ACCD) e aug-cc-pVTZ (ACCT) foram adotadas. C?lculos de otimiza??o de geometria e frequ?ncia vibracionais foram feitos para reagentes e produtos. Foram vistos tamb?m para todos os n?veis de c?lculo a forma??o de um pr?-complexo estabilizado, em rela??o aos reagentes isolados, por cerca de 4 kcal mol-1. Ponto de sela tamb?m foram otimizados e identificados pela frequ?ncia vibracional imagin?ria. Barreiras de energia foram previstas em cerca de 1 kcal mol-1, em acordo com a previs?o da literatura, 0,7 - 1,0 kcal mol-1. Os coeficientes de velocidade foram calculados na faixa de 100 ? 500 K a partir da teoria do estado de transi??o variacional com auxilio do programa kcvt. Os par?metros cin?ticos encontrados, coeficientes de velocidade (k, 298 K, expressos em 10-11 cm3mol?cula-1s-1), energia de ativa??o (Ea, expresso em kcal mol-1) e fator pr?-exponencial (A, expresso em 10-11 cm3mol?cula-1s-1) foram: k298K = 4,89, Ea = 0,25 e A = 7,45. Os resultados sugerem que a din?mica da rea??o ? bem descrita a partir de um mecanismo que contempla a participa??o de um intermedi?rio pr?-barreira. Resultados de coeficientes de velocidade variacionais can?nicos obtidos neste trabalho se mostraram satisfat?rios quando comparados a dados experimentais validando a proposta de mecanismo trazida nesse trabalho.

The composition of the atmosphere has changed substantially over Earth's history, with important implications for past climate. A number of case studies will be presented which employ coupled climate model simulations to assess the strength of these chemical feedbacks on the climate. The eruption of Mount Samalas in 1257 led to the largest stratospheric volcanic injection of aerosol precursor gases in the Common Era, however climate model simulations of the last millennium typically overestimate the resulting climatic cooling when compared with tree-ring proxy records. A novel configuration of the Met Office UM-UKCA climate model is presented which couples an atmosphere-ocean general circulation model to a rigorous treatment of the relevant atmospheric chemistry and microphysical aerosol processes. This permits the climate response to a particular stratospheric injection of reactive volatile gases to be quantified and for the first time to date applied to a historical volcanic eruption. This model configuration compares favourably to observational data for simulations of the 1991Mount Pinatubo eruption. Results from an ensemble of model simulations are presented, with different assumptions about the sulfur dioxide and halogen loadings based on a recent geochemical reconstruction. These show a muted climate response, in reasonable agreement with tree ring records. Emissions of halogenated compounds lead to an increase in the sulfur dioxide lifetime, widespread ozone depletion and a prolonged climatic cooling. Strong increases in incident ultraviolet radiation at Earth's surface also occur. Oxygen levels may have varied fromas little as 10% to as high as 35% in the Phanerozoic (541Ma - Present). An increase in atmospheric oxygen increases atmospheric mass which leads to a reduction in incident shortwave radiation at Earth's surface due to Rayleigh scattering. However, this is offset by an increase in the pressure broadening of greenhouse gas absorption lines. Dynamical feedbacks also lead to increased meridional heat transport, warming polar regions and cooling tropical regions. An increase in oxygen content using the HadCM3-BL and HadGEM3-AO climate models leads to a global mean surface air temperature increase for a pre-industrial Holocene base case, in agreement with idealised 1D and 2D modeling studies. Case studies from past climates are investigated using HadCM3-BL which show that in the warmest climates, increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Maastrichtian (72.1 - 66.0Ma), increasing oxygen content leads to a better agreement with proxy reconstructions of surface temperature at that time irrespective of the carbon dioxide content. There is considerable uncertainty in the timing of the rise in atmospheric oxygen content from values around 1% in the Neoproterozoic (1000 Ma - 541 Ma) to the 10- 35% values inferred in the Phanerozoic with respect to two global glaciation episodes (717-635Ma). Results of simulations with HadCM3-BL which investigate the impact of oxygen content on the Neoproterozoic Snowball Earth glaciations are presented. These demonstrate that a smaller reduction in carbon dioxide content is required to initiate a Snowball Earth at low oxygen content. Geological evidence suggests the presence of a basaltic large igneous province before the Sturtian Snowball Earth episode. This could have caused episodes of paced explosive volcanism, injecting sulfate aerosol precursors into the stratosphere. Results of simulations to investigate the impact of different volcanic aerosol emission scenarios are presented. 500 Tg SO2 is investigated with a range of aerosol sizes. For aerosol size distributions consistent with the aerosol evolution in the aftermath of the Mount Pinatubo eruption, the Earth enters a Snowball Earth in between 30 and 80 years. Using a larger size of aerosols, consistent with a larger eruption, does not lead to a Snowball Earth. These simulations show that changes to the chemical composition of the atmosphere, whether reactive gases or bulk chemical composition may have played an important role in the past climate of Earth.