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Shangdianzi in Beijing) set up Wavelength-Scanned Cavity Ring Down Spectroscopy system(WS-
CRDS), and started a continuous high-precision observation from January 2009[4]. Since July 2010,
the Shangri-La Atmospheric Background Station in Yunnan Province has set up an online
observation system for CH
4
greenhouse gases and conducted continuous high-precision observations
to fill gaps in greenhouse gas observations in southwestern China
[5]. Higher accuracy of observation
can be obtained by ground-based observations; however, there are only about 300 global greenhouse
gas-observation stations in the world at present. Ground stations are unevenly distributed and the cost
of observation is too high to obtain the wide range data, which has become a major factor
constraining the research of carbon cycle
[6]. So the space-borne measurements are crucial.
In 1956, King et al. [7] proposed a technology of satellite infrared atmospheric detection, which
started the space-borne observation of atmospheric composition. Halogen Occultation Experiment
(HALOE) is mounted on the Upper Atmosphere Research Satellite (UARS), and its observations are
limited to the tropopause. The height of observations based on the Michelson interferometer and
Atmospheric Chemistry Experiments (ACE) on ENVISAT are similar to HALOE’s. Atmospheric
Infrared Sounder(AIRS) on board the EOS/Aqua platform was launched in May 2002, has 6 space -
borne sensors with 2378 channels covering the wavelength range of 649-1136, 1217-1613 and 2169-
2674 cm
-1
, providing vertical profiles of CH
4
[8]. The Greenhouse Gases Observing Satellite
(GOSAT), world's first spacecraft to measure the concentrations of carbon dioxide and methane, was
launched on January 23, 2009 in Japan. Methane profiles can be obtained by the thermal infrared
band of Fourier Transform Spectrometer (FTS). Zhang et al.
[9] analyzed the temporal and spatial
distribution of methane in the middle and upper troposphere in China from 2003 to 2008 using AIRS
CH
4
. The results show that AIRS CH
4
is consistent with ground-based CH
4
, with an error of less than
1.5%. Wang et al. [10] validated the near-surface CH
4
concentration of the AIRS V6.0 level 2
support product using the ground-based data at Waliguan in Qinghai, Lulinshan in Taiwan and Ulaan
Uul in Mongolia, and found that the error was less than 2% , the trend of change is consistent. Using
the vertical profiles of CH
4
provided by AIRS V6.0 product, Xiong et al. [11] compared the AIRS
retrieval results with simulation results of global tracer model version 3 (TM3) in South Asia from
2003 to 2007, and found that AIRS and TM3 have a consistent trend. Feng et al.[12] used the
ground-based CH
4
of Waliguan Atmospheric Background Station to validate the AIRS retrieval
CH
4
,and the results show that the AIRS retrieval CH
4
is consistent with ground-based CH
4
and there
is a significant positive correlation between them.
China's space-borne observations of greenhouse gas are in their infancy. At present, there are few
studies on the accuracy of methane profiles retrieved by space-borne instruments in China.
Comparison of the AIRS and GOSAT retrieval methane profiles has not been done yet. Therefore, in
this paper, AIRS Level 2 standard CH
4
products are validated by GOSAT inversions at China's five
major WMO/GAW atmospheric background stations, LAN, LFS, SDZ, WLG and XGLL site, and
the possible causes of AIRS retrieval error are also discussed.
2. Study area and data
2.1. Environment of atmospheric background stations
The five stations selected in this paper are located in the area with typical climate and ecological
environment in China and have regional characteristics. Waliguan station is located on the northeast
slope of the Qinghai-Tibet Plateau, slightly affected by human activities. The vegetation is dominated
by plateau meadows (Asakusa, arid and semi-arid desert, grasslands) and sandbanks, with grazing
mainly around the atmospheric background station. The dominant wind direction in winter is the
southwest, the northeast and the southeast in summer, with obvious plateau continental climate [13].
Waliguan station is the world's highest and the first atmospheric background station in the hinterland
of the mainland. Lin'an Atmospheric Background Station is located in China's subtropical monsoon
Space-based Validation of Methane Products from AIRS/AMSU in China
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