An analysis of atmospheric CH₄ concentrations from 1984 to 2008 with a single box atmospheric chemistry model

Abstract. We present a single box atmospheric chemistry model involving atmospheric methane (CH₄), carbon monoxide (CO) and radical hydroxyl (OH) to analyze atmospheric CH₄ concentrations from 1984 to 2008. When OH is allowed to vary, the modeled CH₄ is 20 ppb higher than observations from the NOAA/ESRL and AGAGE networks for the end of 2008. However, when the OH concentration is held constant at 10⁶ molecule cm⁻³, the simulated CH₄ shows a trend approximately equal to observations. Both simulations show a clear slowdown in the CH₄ growth rate during recent decades, from about 13 ppb yr⁻¹ in 1984 to less than 5 ppb yr⁻¹ in 2003. Furthermore, if the constant OH assumption is credible, we think that this slowdown is mainly due to a pause in the growth of wetland methane emissions. In simulations run for the Northern and Southern Hemispheres separately, we find that the Northern Hemisphere is more sensitive to wetland emissions, whereas the southern tends to be more perturbed by CH₄ transportation, dramatic OH change, and biomass burning. When measured CO values from NOAA/ESRL are used to drive the model, changes in the CH₄ growth rate become more consistent with observations, but the long-term increase in CH₄ is underestimated. This shows that CO is a good indicator of short-term variations in oxidizing power in the atmosphere. The simulation results also indicate the significant drop in OH concentrations in 1998 (about 5% lower than the previous year) was probably due to an abrupt increase in wetland methane emissions during an intense EI Niño event. Using a fixed-lag Kalman smoother, we estimate the mean wetland methane flux is about 128 Tg yr⁻¹ through the period 1984–2008. This study demonstrates the effectiveness in examining the role of OH and CO in affecting CH₄.