Modeling study of the interannual variability in global tropospheric hydroxyl radical and methane concentrations over the last two decades

Methane (CH4) is a major greenhouse gas whose global warming potential is 23 times more important than carbon dioxide (CO2). CH4 concentrations have steadily increased since the beginning of the industrial era, reaching an unprecedent level (almost 1800 ppb at present times). CH4 is currently considered as a one of the major driver of the climate change and its warming potential has leaded the Kyoto protocol to plan a significative reduction of its emissions. The objective of this thesis is to examine the factors that contribute to the CH4 global budget as well as its year-to-year variations. To that purpose, we have implemented a methane simulation in a global model of chemistry and transport. Conducting a CH4 simulation requires a comprehensive set of interannual varying CH4 emission inventories as well as a year-to-year varying 3-D fields of concentrations of hydroxyl radicals (OH) that is the main sink for CH4. The first part of this thesis examines the OH interannual variation, using results from a "full-chemistry" simulation that accounts for interannual variations in emissions of the main O3 precursors (including carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons) as well as the year-to-year variation in meteorology (e.g. atmospheric water vapor content), lightning emissions, and overhead O3 column. We calculated a global OH concentration of 1.22×106 molec cm-3 averaged over the period. We find that global OH concentrations steadily increased during the early 90's by 8% and reached a maximum in 1997. Afterward, concentrations decrease by 5%. We find that changes in water vapor concentrations, anthropogenic emissions of CO and NOx, lightning NOx emissions, and overhead ozone column all play a role in driving the OH year-to-year variations. The relative contributions of these parameters depend on the latitudinal and altitudinal regions of the troposphere. We examine in particular the influence of the 1997-1998 ENSO on the global OH concentrations. We find that the 1997-1998 ENSO resulted in a large increase in OH in the tropical areas and in the extra-tropical areas of the northern hemisphere (largely driven by an increase in water vapor) while it leads to a decrease in OH in the extra-tropical region of the southern hemisphere (that results from changes in the transport pathways that bring CO in the most southern latitudes and deplete OH). Our results (in terms of OH variability) are in contradiction to those found by other methods, especially those using inverse modeling approaches that are based on methyl chloroform (CH3CCl3) observations. This may results from poorly constrained sources. We then seek to implement a comprehensive set of CH4 emissions in our global model. We used anthropogenic CH4 emission data set of anthropogenic emissions from the International Institute of Technology (IIASA) whose emissions vary between 250 and 290 Tg/year between 1990 and 2000. We estimated biomass burning emissions, derived from an inventory of the total annual biomass burned area and emission factors that we constrained with measurements of the isotopic composition of atmospheric methane. This results in a global emissions rate of 60 Tg/year, which is much stronger than most of previous studies. We also developed a wetland scheme that accounts for soil carbon content, wetland fraction areas, soil temperature and humidity (with the three latter parameters varying interannually) and we evaluated the wetland areas with satellite-based estimates. Our global wetland emissions amount to about 150 Tg/year. We find a global CH4 lifetime of 10.6 years, which is in the range of values reported in previous studies. Comparing our results with different set of measurements gives promising results. The model reproduces well the year-to-year variation even if it slightly overestimates concentrations after 2000. By conducting different set of sensitivity simulations, we investigated the role of different parameters on the CH4 variations. We find that the long-term trend in CH4 is driven by a competition between anthropogenic emissions and tropospheric decay. Peaks of growth rate are driven by biomass burning emissions and to a lesser extend, wetland emissions. We find that even if we used increasing anthropogenic emissions during the 90s (that is in contradiction with some previous studies), reconstructing the observed year-to-year variation of CH4 concentrations was possible if one considers an increasing in OH during the 90s.

Interannual variability in tropospheric ozone and its precursors

Symeon Koumoutsaris

Tropospheric ozone (O3) is a harmful pollutant and an increasingly important green-house gas. Tropospheric O3 distributions are determined by complex and non-linear interactions between photochemistry and transport processes. The first objective of this thesis is to examine the factors that contribute to the interannual variation (IAV) of tropospheric O3 and related compounds, such as carbon monoxide (CO), for a period of 19 years (1987-2005), with a focus over the northern mid-latitudes. A series of long-term measurements, including ozonesondes, groud-based sites and satellites, are interpreted with a global chemical-transport model (GEOS-Chem) and a number of sensitivity simulations are performed to quantitatively assess the relative contribution of individual processes to the IAV of O3 and CO. We first assess the capability of the model to reproduce O3 and CO IAV. We find that the model is able to reproduce reasonably well many of the observed features of the O3 and CO IAV. The model however, fails to reproduce the observed variation in O3 in the northern mid-latitudes during the first half of the 1990s (negative anomaly in 1991-1993 followed by an upward trend in 1993-1996) which is likely due to an inadequate representation of stratospheric dynamics and stratospheric O3 concentrations in the model. This study has demonstrated that the principal drivers of O3 and CO IAV are meteorology and emissions. We examine in particular the 1998-1999 period. A large anomaly in O3 was observed at several sites over Europe and is fairly well reproduced by the model. Different processes are found to contribute to this long-lasting anomaly. The first period (spring 1998) was largely influenced by the strong preceding El Niño episode of 1997. El Niño Southern Oscillation (ENSO) affects the stratosphere-troposphere exchange (STE) and the Asian pollution export, through a change in convective activity and a strengthening of the subtropical jet. North American pollution export was also enhanced through an increase in temperature and in photochemical activity. During the second period (summer-fall 1998), the O3 budget over Europe was influenced by both boreal wildfires and Asian pollution. Finally, the O3 anomaly during the third period (spring 1999) was affected by enhanced biomass burning emissions in Southeast Asia. We further noticed that a significant correlation exists between the Southern Oscillation Index (SO1) in fall and the European tropospheric O3 column in the following spring (R= -0.65, N=19), throughout the period from 1987 to 2005. In other words, increasing tropospheric O3 column is seen over Europe the following spring of an El Niño year. In the second part of this thesis, we used inversion methods to derive monthly NOx, emissions from GOME and SCIAMACHY space-based retrievals of nitrogen dioxide (NO2) columns for a period of 10 years (1996-2005). We calculated emission trends over the highly industrialized regions and large cities around the globe. Decreasing emissions were found in the U.S. and Europe. However, negligible or even positive trends were seen in urban areas in northeastern U.S. (e.g. Boston) and southern Europe (e.g. Lisbon, Istanbul) which are most probably due to increasing motor vehicle NOx, emissions. Furthermore, a slowdown in the decreasing trend in NOx, combustion fuel emissions over North Europe has been noticed after 2000 and especially in 2003-2004. This is probably related to a slowdown in the decrease in road transport emissions. Increasing trends, on the other hand, have been found in the combustion fuel NOx, emissions over east China (8.3% year-1), due to the fast industrial and economical development of China. In addition, we found an upward trend in soil NOx, emissions which may be related to the increasing use of fertilizers in the northeastern part of the country. Finally, we examined the influence of using two different retrievals of NO2 columns on the resulting a posteriori inventories. The inversion was carried out twice for the year 2000, using two different retrievals, one from Dalhouse/SAO and one from BIRA/KNMI. The two resulting inventories were found to correspond well globally, but show important differences (more than 60%) at specific times and regions.

EPFL2008

Present state of global wetland extent and wetland methane modelling: conclusions from a model inter-comparison project (WETCHIMP)

Jed Oliver Kaplan

Global wetlands are believed to be climate sensitive, and are the largest natural emitters of methane (CH4). Increased wetland CH4 emissions could act as a positive feedback to future warming. The Wetland and Wetland CH4 Inter-comparison of Models Project (WETCHIMP) investigated our present ability to simulate large-scale wetland characteristics and corresponding CH4 emissions. To ensure inter-comparability, we used a common experimental protocol driving all models with the same climate and carbon dioxide (CO2) forcing datasets. The WETCHIMP experiments were conducted for model equilibrium states as well as transient simulations covering the last century. Sensitivity experiments investigated model response to changes in selected forcing inputs (precipitation, temperature, and atmospheric CO2 concentration). Ten models participated, covering the spectrum from simple to relatively complex, including models tailored either for regional or global simulations. The models also varied in methods to calculate wetland size and location, with some models simulating wetland area prognostically, while other models relied on remotely sensed inundation datasets, or an approach intermediate between the two. Four major conclusions emerged from the project. First, the suite of models demonstrate extensive disagreement in their simulations of wetland areal extent and CH4 emissions, in both space and time. Simple metrics of wetland area, such as the latitudinal gradient, show large variability, principally between models that use inundation dataset information and those that independently determine wetland area. Agreement between the models improves for zonally summed CH4 emissions, but large variation between the models remains. For annual global CH4 emissions, the models vary by +/- 40% of the all-model mean (190 Tg CH4 yr(-1)). Second, all models show a strong positive response to increased atmospheric CO2 concentrations (857 ppm) in both CH4 emissions and wetland area. In response to increasing global temperatures (+3.4 degrees C globally spatially uniform), on average, the models decreased wetland area and CH4 fluxes, primarily in the tropics, but the magnitude and sign of the response varied greatly. Models were least sensitive to increased global precipitation (+3.9% globally spatially uniform) with a consistent small positive response in CH4 fluxes and wetland area. Results from the 20th century transient simulation show that interactions between climate forcings could have strong non-linear effects. Third, we presently do not have sufficient wetland methane observation datasets adequate to evaluate model fluxes at a spatial scale comparable to model grid cells (commonly 0.5 degrees). This limitation severely restricts our ability to model global wetland CH4 emissions with confidence. Our simulated wetland extents are also difficult to evaluate due to extensive disagreements between wetland mapping and remotely sensed inundation datasets. Fourth, the large range in predicted CH4 emission rates leads to the conclusion that there is both substantial parameter and structural uncertainty in large-scale CH4 emission models, even after uncertainties in wetland areas are accounted for.

Copernicus Gesellschaft Mbh2013

Photocatalytic and Electrocatalytic Reduction of Carbon Dioxide in Pressurized Systems

Patrick Voyame

The depletion of carbon-based fossil fuels and the rise in atmospheric carbon dioxide concentration will force an inevitable change in the future global energy landscape. CO2 reduction presents the advantages of decreasing its atmospheric concentration and storing energy in chemical form in CO2 reduction products. With a predicted conversion to renewable energy such as solar or wind energy, energy storage will become a key process in the near future for buffering the fluctuating energy production. The objective of the present work was to study the efficiency towards CO2 reduction of different molecular catalysts. In particular, conducting experiments in high-pressure and supercritical conditions and observing the effect of CO2 pressure or concentration on the efficiency and selectivity of the reaction. As supercritical CO2 (scCO2) has poor solubilisation capabilities, experiments were conducted in biphasic systems or with addition of an organic co-solvent. To drive the reduction reaction of CO2, a catalyst is needed to overcome the kinetic limitations of the reaction, but an energy input is also necessary. Three different forms for this energy input were used in this work. In a first time a sacrificial product, decamethylferrocene (DMFc), was used to transfer electrons to CO2 in biphasic water/scCO2 systems. Complete oxidation of the DMFc was observed in presence of anion capable of transporting protons from water to the DMFc present in the supercritical phase. A photosensitization cycle was used to supply a water soluble catalyst, NI(II)Cyclam, in electron at the required potential to drive the reduction of CO2 into carbon monoxide in water/scCO2 system. The creation of the interface in the system appeared highly favourable to the efficiency of the catalyst. A second catalyst, a ruthenium polypyridyl carbonyl complex, was used for the photocatalytic reduction of CO2. Pressure had an important impact on the production of one of the two reduction product, carbon monoxide, while the production of formate was unaffected by CO2 pressure. As limitations in productivity in photocatalytic experiments were coming principally from the photosensitizer cycle or the sacrificial electron donor, photosensitizer was replaced by an electrode to provide the catalyst in electrons. Voltammetry in CO2-expanded liquids was described and determination of important parameters such as catalyst concentration and diffusion coefficient as a function of pressure was performed. Electrocatalytic reduction of CO2 by ruthenium and rhenium polypyridyl carbonyl complexes was studied in CO2-expanded liquids. The catalytic mechanisms were observed to be highly influenced by CO2 concentration.

EPFL2016