HR: 14:55h
AN: A42D-06    [PDF]
TI: The Stable Isotopes of Atmospheric Methane: Long-Term Observations at Alert (Canada) and Neumayer (Antarctica)
AU: * Poss, C
EM: Christian.Poss@iup.uni-heidelberg.de
AF: Institut f\"{u}r Umweltphysik University of Heidelberg, INF 229, Heidelberg, D-69120 Germany
AU: Veidt, C
EM: Cordelia.Veidt@iup.uni-heidelberg.de
AF: Institut f\"{u}r Umweltphysik University of Heidelberg, INF 229, Heidelberg, D-69120 Germany
AU: Worthy, D E
EM: Doug.Worthy@ec.gc.ca
AF: Meteorological Service of Canada, 4905 Dufferin Street, Downsview Ontario, M3H 5T4 Canada
AU: Levin, I
EM: Ingeborg.Levin@iup.uni-heidelberg.de
AF: Institut f\"{u}r Umweltphysik University of Heidelberg, INF 229, Heidelberg, D-69120 Germany
AB: Ten years records of CH$_{4}$ concentration, $^{13}$C/$^{12}$C-CH$_{4}$ and $^{2}$H/H-CH$_{4}$ from the background stations Alert, Arctic, (82.45$\deg$N) and Neumayer, Antarctica, (70.65$\deg$S) are presented. High volume integrated (Alert) and spot (Neumayer) samples are collected at the respective sites and are analysed in Heidelberg using GC-FID technique for mixing ratios, IRMS for $^{13}$C/$^{12}$C and $^{2}$H/H ratios, and, in the last two years, a TDLAS system for analysis of the $^{2}$H/H ratio. At Neumayer we observe a seasonal CH$_{4}$ cycle with a peak-to-peak amplitude of about 28 ppb; minimum mixing ratios are ocurring in February/March, and maxima in August/September. The seasonalities of $^{13}$C/$^{12}$C and $^{2}$H/H are anticorrelated to the mixing ratio and average 0.12 $\permil$ ($^{13}$C/$^{12}$C) and 2.4 $\permil$ ($^{2}$H/H) peak-to-peak. Seasonal cycles at Neumayer can be explained by the seasonality of methane destruction in combination with a seasonally varying meridional circulation pattern in the atmosphere. The atmospheric OH sink (which contributes about 90% to the total sink of atmospheric methane) is stronger during summer. The long-term trends at Neumayer are 5.8 ppb/yr (mixing ratio), 0.04 $\permil$/yr ($^{13}$C/$^{12}$C) and 0.8 $\permil$/yr ($^{2}$H/H). At Alert we observe a seasonal amplitude of CH$_{4}$ about twice as large as at Neumayer, while the $^{13}$C/$^{12}$C and $^{2}$H/H seasonality is larger by a factor of three, with minimum mixing ratios in July, and maxima in January/February (isotopes almost anticorrelated, whereas minimum isotope ratios occur 2-3 month earlier than the maximum mixing ratios). Compared to Antarctica which is far away from any source region additional influences from methane sources lead to the larger seasonalities at Alert: The nearby wetland sources are emitting CH$_{4}$ during the summer season but they do not seem to affect much the mixing ratio. This is different for the isotope ratios. High methane concentrations during the winter months are caused by transport of stratified polluted air masses from mid latitudes to the Arctic station during this season. The long-term trends at Alert are 3.6 ppb/yr (mixing ratio), 0.03 $\permil$/yr ($^{13}$C/$^{12}$C) and 1.4 $\permil$/yr ($^{2}$H/H). At Alert and at Neumayer the mixing ratios are almost constant (except for seasonality) since about three years. From the difference between both stations a fractionation of -7.2 $\permil$ ($^{13}$C/$^{12}$C) and -234 $\permil$ ($^{2}$H/H) for the mean tropospheric methane sink can be derived. The same results are obtained from the analysis of the southern hemispheric seasonal cycle. With the two approaches one can estimate the isotopic signature of the mean source to -53.9 $\permil$ ($^{13}$C/$^{12}$C) and -275 $\permil$ ($^{2}$H/H). The continuing long term trend in both stable isotopes shows that in contrast to the mixing ratio the isotopes are still not in steady state with its sources and sinks.
DE: 0365 Troposphere--composition and chemistry
DE: 1610 Atmosphere (0315, 0325)
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting