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2.5. SATELLITE GROUP 75
2.5.4 Retrieval of methane from SCIAMACHY onboard ENVISAT
Participating scientist Christian Frankenberg, Ulrich Platt, Thomas Wagner
Abstract SCIAMACHY onboard ENVISAT features three near infrared spectrometers and thereby
enables the retrieval of atmospheric methane with high sensitivity to the lowermost atmospheric layers.
Using concurent retrievals of carbon dioxide and the application of atmospheric models, precise maps
of the global distribution of column averaged methane mixing ratios are derived.
Figure 2.38: Column averaged mixing ratio of methane averaged from August through November
2003.
Background Methane (CH4) is, after carbon
dioxide, the second most important anthropogenic
greenhouse gas, contributing directly 0.48 W m −2
to the total anthropogenic radiative forcing of
2.43 W m −2 by well-mixed greenhouse gases
(IPCC, 2001). In addition, it exhibits an indirect
effect of about 0.13 W m −2 through formation
of other greenhouse gases, most notably
tropospheric ozone and stratospheric water vapor
(Lelieveld et al., 1998).
A major limitation of present top-down methane
emissions inventories is the limited number of atmospheric
observation sites. SCIAMACHY now
offers the unique possibility of sensing methane
globally, retrieving methane abundances also in
remote areas.
Funding See satellite group overview.
Methods and results An algorithm (IMAP-
DOAS) was developed based on the principles
of differential optical absorption spectroscopy
(DOAS), that deals with the peculiarities of retrieving
strong absorbers in the near infrared,
thus allowing a precise retrieval of the respective
trace gases. It was shown that nonlinear iterative
schemes are necessary to account for saturation effects
and to avoid interdependencies of spectrally
overlapping strong absorbers.
Global measurements of the total columns of
methane with high precision were made possible
by use of concurent retrievals of the relatively homogenously
distributed carbon dioxide as proxy
for the light path of the recorded photons. The
highest abundances were found over areas of rice
cultivation in South-East Asia and can be considered
a direct proof of large scale methane emissions
in Asia.
In the time-period from August through November
2003, large discrepancies between measurements
and model were discovered over tropical
rainforest areas. Measured abundances showed
persistently higher abundances than those predicted
by the model. This led to the conclusion
that the tropical regions as methane source have
been hitherto underestimated in curent emissions
inventories.
An extension of the analysis to the years 2003 and
2004 showed that the discrepancies are highest
during the months of August through October.
The precision of these measurements now allows
their use in inversion models to quantify the temporal
and geographical distribution of methane
sources.
Main publications Frankenberg et al. [2005b],
Frankenberg et al. [2005a]