A comprehensive assessment of the influence of oxygenated volatile organic compounds on the atmospheric composition

Global atmospheric chemical models are an important tool to improve our understanding of the Earth’s atmospheric processes and to address the influence of anthropogenic activities on the Earth’s climate. In this context, one of the most important greenhouse gases is ozone (O3), whose photochemical production in the troposphere is fueled by volatile organic compounds (VOCs). An important sub-group of VOCs are oxygenated VOCs (OVOCs), which are photolabile and water soluble. Thus, a realistic simulation of tropospheric O3 in global atmospheric models also relies on the realistic representation of OVOCs. The overall objective of this thesis is to provide a comprehensive assessment of the influences of OVOCs on the atmospheric composition, by addressing three important aspects and their model representation. These aspects are: OVOCs’ photochemistry in the gas-phase, their uptake and transformations in the aqueous phase, and their emissions. With this aim, five studies are performed. Gas- and aqueous-phase mechanisms are built from chemical kinetic data, which are obtained from experiments, quantum chemical and theoretical kinetic calculations, or the literature. In order to investigate the importance of each mechanism on the atmospheric composition, they are implemented into the global ECHAM/MESSy Atmospheric Chemistry (EMAC) model. For analysing the impact of VOC emissions from biomass burning, a combination of the developed mechanisms is applied. The first study shows that EMAC underestimates gas-phase OVOC and hydroxyl radical (OH) concentrations, when ignoring isomerization reactions of isoprene peroxy radicals under low-NOx (NOx=NO+NO2) conditions. The second study demonstrates that in case of isocyanic acid (HNCO), its heterogeneous loss is far more important than its gas-phase chemical loss. In the third and fourth study, the development of the Jülich Aqueous-phase Mechanism of Organic Chemistry (JAMOC) allows to address the importance of in-cloud OVOC oxidation on tropospheric oxidants. This process leads to a significant reduction in gas-phase concentrations of OVOCs and HOx (HOx=OH+HO2). Elevated in-cloud HO2(aq) concentrations introduce an enhanced destruction in O3(aq) resulting in reduced gas-phase O3 concentrations. Thus, EMAC’s bias towards too high tropospheric O3 concentrations is diminished. Finally in the fifth study, the investigation of the 2015 Indonesian peatland fires reveals the significant impact of biomass burning VOC emissions on the regional tropospheric oxidation capacity. At the same time, enhanced phenol concentrations are predicted in the lower stratosphere leading to an enhanced destruction of O3 by phenoxy radicals, potentially contributing to the variability of O3 observed in satellite retrievals. The complete assessment demonstrates that a comprehensive and explicit representation of all OVOC fluxes and transformations in global models is one key to guide the activities solving humanity’s current and upcoming challenges related to climate change and air pollution. Especially, the development of JAMOC shows great potential to investigate the influence of aqueous-phase OVOC oxidation on acids and secondary organic aerosols (SOA) in future studies

How non-equilibrium aerosol chemistry impacts particle acidity: the GMXe AERosol CHEMistry (GMXe--AERCHEM, v1.0) sub-submodel of MESSy

Aqueous-phase chemical processes in clouds, fog, and deliquescent aerosols are known to alter atmospheric composition and acidity significantly. Traditionally, global and regional models predict aerosol composition by relying on thermodynamic equilibrium models and neglect non-equilibrium processes. Here, we present the AERosol CHEMistry (GMXe–AERCHEM, v1.0) sub-submodel developed for the Modular Earth Submodel System (MESSy) as an add-on to the thermodynamic equilibrium model (i.e. ISORROPIA-II) used by MESSy's Global Modal-aerosol eXtension (GMXe) submodel. AERCHEM allows the representation of non-equilibrium aqueous-phase chemistry of varying complexity in deliquescent fine aerosols. We perform a global simulation for the year 2010 by using the available detailed kinetic model for the chemistry of inorganic and small oxygenated organics. We evaluate AERCHEM's performance by comparing the simulated concentrations of sulfate, nitrate, ammonium, and chloride to in situ measurements of three monitoring networks. Overall, AERCHEM reproduces observed concentrations reasonably well. We find that, especially in the USA, the consideration of non-equilibrium chemistry in deliquescent aerosols reduces the model bias for sulfate, nitrate, and ammonium when compared to simulated concentrations by ISORROPIA-II. Over most continental regions, fine-aerosol acidity simulated by AERCHEM is similar to the predictions by ISORROPIA-II, but simulated aerosol acidity tends to be slightly lower in most regions. The consideration of non-equilibrium chemistry in deliquescent aerosols leads to a significantly higher aerosol acidity in the marine boundary layer, which is in line with observations and recent literature. AERCHEM allows an investigation of the global-scale impact of aerosol non-equilibrium chemistry on atmospheric composition. This will aid in the exploration of key multiphase processes and improve the model predictions for oxidation capacity and aerosols in the troposphere

Influence of in-cloud oxidation of organic compounds on tropospheric ozone

Large parts of the troposphere are affected by clouds, whose aqueous-phase chemistry differs significantly from gas-phase chemistry. Box-model studies have demonstrated that clouds influence the tropospheric oxidation capacity. However, most global atmospheric models do not represent this chemistry reasonably well and are largely limited to sulfur oxidation. Therefore, we have developed the J&#252;lich Aqueous-phase Mechanism of Organic Chemistry (JAMOC), making a detailed in-cloud oxidation model of oxygenated volatile organic compounds (OVOCs) readily available for box as well as for regional and global simulations that are affordable with modern supercomputers. JAMOC includes the phase transfer of species containing up to ten carbon atoms, and the aqueous-phase reactions of a selection of species containing up to four carbon atoms, e.g., ethanol, acetaldehyde, glyoxal. The impact of in-cloud chemistry on tropospheric composition is assessed on a regional and global scale by performing a combination of box-model studies using the Chemistry As A Boxmodel Application (CAABA) and the global atmospheric model ECHAM/MESSy (EMAC). These models are capable to represent the described processes explicitly and integrate the corresponding ODE system with a Rosenbrock solver.&#160;Overall, the explicit in-cloud oxidation leads to a reduction of predicted OVOCs levels. By comparing EMAC's prediction of methanol abundance to spaceborne retrievals from the Infrared Atmospheric Sounding Interferometer (IASI), a reduction in EMAC's overestimation is observed in the tropics. Further, the in-cloud OVOC oxidation shifts the hydroperoxyl radicals (HO2) production from the gas- to the aqueous-phase. As a result, the in-cloud destruction (scavenging) of ozone (O3) by the superoxide anion (O2-) is enhanced and accompanied by a reduction in both sources and sinks of tropospheric O3 in the gas phase. By considering only the in-cloud sulfur oxidation by O3, about 13 Tg a-1 of O3 are scavenged, which increases to 336 Tg a-1 when JAMOC is used. With the full oxidation scheme, the highest O3 reduction of 12 % is predicted in the upper troposphere/lower stratosphere (UTLS). Based on the IASI O3 retrievals, it is demonstrated that these changes in the free troposphere significantly reduce the modelled tropospheric O3 columns, which are known to be generally overestimated by global atmospheric models. Finally, the relevance of aqueous-phase oxidation of organics for ozone in hazy polluted regions will be presented. &#160;</p

Weather influence on aviation NOx climate impacts via ozone and methane

Aviation activities contribute substantially to the anthropogenic climate impact. Due to an increasing demand on aviation transport, multiple mitigation strategies have been established to reduce the contribution to climate change by aviation. One promising strategy is to re-route aircraft, such that climate sensitive atmospheric areas are avoided. This mitigation strategy, depends on the scientific understanding of all processes involved. The European project REACT4C assessed the feasibility of such a mitigation technique by simulating the climate impact of NOx, as well as other emissions and contrail formation for eight distinct weather pattern. For each weather pattern, unit emissions of NOx are emitted in the North Atlantic flight sector. Each air parcel, containing the emitted NOx, is tracked within the atmosphere. This unique model set-up allows to analyse concentration changes of O3 and CH4 along each trajectory. In general, due to the emitted NOx, O3 is produced and CH4 is lost. Most recent results showed that by just increasing the operation cost by 1%, the climate impact can be reduced by about 10%. By comparing climate cost functions (CCF), a metric of the climate impact per unit emission, to weather charts, a link between high pressure ridges and the total climate impact of NOx is observed. Therefore, this research focuses on identifying weather influences on the temporal development of O3 and CH4 due to aviation attributed NOx emission.In this thesis, the NOx chemistry, atmospheric transport processes and the model set-up of the REACT4C project is reviewed. The temporal development analysis of O3 is split-up into two parts, the O3 build-up and the O3 depletion. First, all data from the climate model are re-gridded and chemical production and loss rates are isolated from all other loss terms (i.e. diffusion). Certain characteristics of the temporal concentration changes of O3 are identified. A systematic analysis of the background chemical compounds and all important chemical reactions involved, provide insides to identify seasonal and emission altitude differences. With the help from literature and multiple statistical means, weather influences on those production and loss terms and thus the temporal development of O3 and CH4 , are identified. In a final step, inter-seasonal variations are analysed.In general, the chemical processes during the O3 build-up are dominated by the emitted NOx, whereas the chemical processes during the depletion of O3 are dominated by the high O3 concentration. Seasonal differences of the maximum O3 concentration and the total CH4 loss are caused by lower background concentrations of all chemicals involved during winter, which lead to lower production and loss rates of O3 and CH4 . At the same time altitude differences in the production and loss of O3 and CH4 are caused by altitude variations in all chemicals involved. The vertical transport within the atmosphere defines the time when the O3 maximum is reached. If an air parcel containing the emitted NOx, is transported fast to a lower altitude, the O3 maximum occurs sooner. If however the same air parcel would stay for a longer time at a high altitude, a late O3 maximum occurs. It could be identified that this downward motion is caused by the subsidence within a high pressure system. Airparcel with an earlier O3 maxima, experience high subsidence, which leads to a higher chemical activity based on higher temperatures. During summer a high O3 maximum can only be reached, if the background concentration of NOx is low during the O3 build-up. If the background NOx concentration is high, only very low O3 maxima occur. During winter the maximum O3 concentration is limited by the background concentration of HO2 . Only high HO2 background concentrations lead to high O3 maxima. The temporal development of CH4 is mainly influenced by the maximum O3 concentration as well as specific humidity. High O3 and H2O concentrations lead to high OH productions, which lead to a high CH4 losses. A high CH4 loss only occurs, if the maximum O3 concentrations and the specific humidity are high.This study shows that the weather situation each air parcel, containing NOx emissions, experiences has a direct influence on the resulting concentration changes of O3 and CH4 . Therefore, weather has a direct impact on the climate impact of NOx , since the concentration change of O3 and CH4 directly influences the resulting climate impact. The understanding of processes related to the climate impact of aviation attributed NOx emission is increased. This improved understanding shows great potential to improve possibilities to forecast local climate impact resulting from aviation NOx emissions, which is necessary for future re-routing mitigation strategies.Aerospace Engineerin

A large source of formic acid in the atmosphere mediated by cloud droplets

Formic acid (HCOOH) is a pervasive trace gas and the most abundant carboxylic acid in the troposphere. It is known to enhance cloud droplet activation and to contribute to the acidity of clouds and rainwater. Despite updated photochemical sources and revised emissions, knowledge and representation of formic acid remain incomplete as state-of-the-art models fail to reproduce the measured concentrations and considerably underestimate its burden. This indicates that major key sources still elude our understanding. Here we present experimental evidence and theoretical predictions of how formic acid is efficiently formed by oxidation of hydrated formaldehyde, methanediol (HOCH2OH), outgassing from cloud droplets. By representing explicitly these relevant processes in the global atmospheric chemistry model ECHAM5/MESSy (EMAC), we estimate that the amount of formic acid produced via this multiphase pathway could be 2-4 times larger than all the known chemical sources combined. Making use of worldwide observations provided by IASI/Metop satellite and ground-based FTIR instruments, we show that this additional production of formic acid can bridge the gap between model predictions and remote-sensing measurements. Moreover, it leads to an increase of the acidity of cloud and rainwater, in particular over the continents. The representation of this multiphase mechanism is important for advancing our understanding of the fate of organic carbon in the atmosphere. We also explore this oxidation pathway applied to higher carbonyl compounds, which could lead to the formation of more complex organic acids such as acetic and pyruvic acid.info:eu-repo/semantics/nonPublishe

Chemical impacts of aviation emissions

The presentation summarised the current status of the impact of NOx Emissions from aviation on the ozone enhancement and methane depletio