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Methane is the second most important anthropogenic greenhouse gas. Methane concentrations in the atmosphere have been increasing since the 1700s due to anthropogenic emissions. This increase slowed down in the past three decades, and the atmospheric growth rate showed variations that are not fully understood. This thesis aims to get a better understanding of the processes driving methane burden in the atmosphere by analyzing the variations in the methane growth rate in the years following the eruption of Mount Pinatubo on 15 June 1991. This period was marked by anomalous climate and photochemistry, which influenced both methane emissions, as well as the methane removal from the atmosphere. Methane is emitted from natural, anthropogenic and biomass burning sources, with a significant contribution from natural emissions from wetlands. The main removal mechanism of methane from the atmosphere is its reaction with the hydroxyl radical (OH) in the troposphere. OH concentrations are determined by complex photochemistry, sensitive to levels of ultra-violet (UV) radiation and water vapour. The eruption of Pinatubo emitted 20 Tg sulfur dioxide, forming sulfate aerosols that remained in the stratosphere for a few years. These aerosols caused a reduction in the amount of radiation reaching the troposphere, a 0.5°C decrease in the global mean temperature near the surface, and enhanced stratospheric ozone depletion. The Pinatubo stratospheric aerosols and the increased ozone depletion determined changes in the amount of UV radiation, which affected OH concentrations and the methane removal. The global cooling resulted in decreases in both the methane emission rates from wetlands and its removal by reaction with OH, both processes being temperature-dependent. OH production further responded to the decrease in water vapour associated with the temperature reduction. Other potential effects of the eruption on the methane budget include changes in the transport betw

The global methane growth rate showed large variations after the eruption of Mount Pinatubo in June 1991. Both sources and sinks of tropospheric methane were altered following the eruption, by feedback processes between climate and photo-chemistry. 5 Such processes include Ultra Violet (UV) radiative changes due to the presence of volcanic sulfur dioxide (SO2) and sulphate aerosols in the stratosphere, and due to stratospheric ozone depletion. Changes in temperature and water vapour in the following years caused changes in the tropospheric chemistry, as well as in natural emissions. We quantify the effects that these processes had on methane concentrations using a 10 one-dimensional chemistry model representative for the global tropospheric column. To infer the changes in UV radiative fluxes, we couple the chemistry model to a radiative transfer model. We find that the overall effect of the eruption on the methane growth rate is dominated by the effect of stratospheric ozone depletion. However, all the other processes are found to have non-negligible effects, and should therefore be 15 taken into account in order to obtain a good estimate of methane concentrations after the eruption. We find that the overall effect was a small initial increase in the methane growth rate after the eruption, then a decrease by about 8 ppbyr−1 by mid-1993. When changes in anthropogenic emissions are employed according to emission inventories, an additional decrease of about 5 ppbyr−1 in the methane growth rate is obtained be20 tween the years 1991 and 1993.