Variations in the atmospheric methane budget after the Mount Pinatubo eruption

Abstract

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 between the troposphere and the stratosphere, inhibition of wetland emissions due to sulfur deposition and changes in natural emissions of other compounds that react with OH. Methane concentrations in this period were also affected by natural variability not related to Pinatubo. A decrease in anthropogenic emissions was also hypothesized to occur after the collapse of the Soviet Union.

In the current thesis I quantified the combined effect of the above processes on methane variability. I first used a column chemistry model coupled to the radiative transfer model TUV. In the later stages, the three-dimensional chemistry and transport model TM5 was used.

I found that the global methane growth rate evolution could be reproduced to some extent by both models. Except for sulfur deposition and anthropogenic emission changes, all of the above processes were found to have had a significant contribution to the methane growth rate variations after the Pinatubo eruption. Both models found the minimum growth rate in the years following the eruption 6 to 9 months later than observed, which we attribute to uncertainties in emissions. The main remaining uncertainties are methane emissions from wetlands and biomass burning, as well as natural emission variations of NMVOC and the sensitivity of OH to these emissions.