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要旨:
Carbon Monitoring Satellite (CarbonSat) is one of two candidate missions for ESA’s
Earth Explorer 8 (EE8) satellite – the selected one to be launched around the end of
this decade. The objective of the CarbonSat mission is to improve our understanding of
5 natural and anthropogenic sources and sinks of the two most important anthropogenic
greenhouse gases (GHG) carbon dioxide (CO2) and methane (CH4). The unique feature
of CarbonSat is its “GHG imaging capability”, which is achieved via a combination
of high spatial resolution (2 km×2 km) and good spatial coverage (wide swath
and gap-free across- and along-track ground sampling). This capability enables global
10 imaging of localized strong emission source such as cities, power plants, methane
seeps, landfills and volcanos and better disentangling of natural and anthropogenic
GHG sources and sinks. Source/sink information can be derived from the retrieved
atmospheric column-averaged mole fractions of CO2 and CH4, i.e. XCO2 and XCH4,
via inverse modeling. Using the most recent instrument and mission specification, an
15 error analysis has been performed using the BESD/C retrieval algorithm. We focus on
systematic errors due to aerosols and thin cirrus clouds, as this is the dominating error
source especially with respect to XCO2 systematic errors. To compute the errors
for each single CarbonSat observation in a one year time period, we have developed
an error parameterization scheme based on six relevant input parameters: we con20
sider solar zenith angle, surface albedo in two bands, aerosol and cirrus optical depth,
and cirrus altitude variations but neglect, for example, aerosol type variations. Using
this method we have generated and analyzed one year of simulated CarbonSat observations.
Using this data set we estimate that scattering related systematic errors are
mostly (approx. 85 %) below 0.3 ppm for XCO2 (<0.5 ppm: 99.5 %) and below 2 ppb for
25 XCH4 (<4 ppb: 99.3%). We also show that the single measurement precision is typically
around 1.2 ppm for XCO2 and 7 ppb for XCH4 (1-sigma). The number of quality
filtered observations over cloud and ice free land surfaces is in the range 33–47 million
per month depending on month. Recently it has been shown that terrestrial Vegetation Chlorophyll Fluorescence (VCF) emission needs to be considered for accurate XCO2
retrieval. We therefore retrieve VCF from clear Fraunhofer lines located at 755nm and
show that CarbonSat will provide valuable information on VCF. The VCF single measurement
precision is approximately 0.3mWm−2 nm−1 sr−1 (1-sigma). As a first appli-
5 cation of the one year data set we assess the capability of CarbonSat to quantify the
CO2 emissions of large cities using Berlin, the capital of Germany, as an example. We
show that the precision of the inferred Berlin CO2 emissions as obtained from single
CarbonSat overpasses is in the range 5–10 MtCO2 yr−1 (10–20 %). We found that systematic
errors could be on the same order depending on which assumptions are used
10 with respect to observational and biogenic XCO2 modeling errors.
