An analysis of atmospheric CH4 concentrations from 1984 to 2008 with a single box atmospheric chemistry model Z. Tan1 and Q. Zhuang1,2 1Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN, USA 2Department of Agronomy, Purdue University, West Lafayette, IN, USA
Received: 17 July 2012 – Accepted: 05 November 2012 – Published: 22 November 2012
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
Citation: Tan, Z. and Zhuang, Q.: An analysis of atmospheric CH4 concentrations from 1984 to 2008 with a single box atmospheric chemistry model, Atmos. Chem. Phys. Discuss., 12, 30259-30282, doi:10.5194/acpd-12-30259-2012, 2012.