Changing trends in tropospheric methane and carbon monoxide: A sensitivity analysis of the OH-radical

A sensitivity analysis is performed in order to study recently observed changes in atmospheric methane and carbon monoxide trends. For the analysis we have adapted a one-dimensional transport/chemistry model in order to comply with changes in vertical transport, stratosphere-troposphere flux of ozone, the water vapour cycle and the short-wave radiative transfer. In addition we have formulated an improved relationship which expresses the steady state OH concentration in terms of longer lived compounds which has a fair agreement with the one-dimensional model results. An analysis of the observed changes and trends in methane and carbon monoxide shows that both emissions and changes in global OH concentrations can be main causes for the observed changes. Average methane emissions have slowed down, particularly in the NH, in the last five years, though perhaps not very significantly. Carbon monoxide emissions are decreasing faster in the last couple of years than in the period 1983–1990. The study suggests that climate fluctuations (tropospheric water vapour, temperature and convective activity) and the stratospheric ozone depletion (tropospheric UV radiation) have a significant influence on tropospheric composition and thus on trends in methane and carbon monoxide concentrations.The IMAU is partner in the Netherlands Centre for Climate Research (CCR).