Characteristics of Spatial-Temporal Variation of Atmospheric Methane over the Tibetan Plateau

This paper presents a comparative analysis of retrievals of atmospheric methane (CH₄) by the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua platform and CH₄ observations from 2003 to 2012 at Waliguan atmospheric benchmark station. Characteristics of spatiotemporal distribution of tropospheric methane over the Tibetan Plateau (TP) are revealed based on AIRS retrievals. The results are as follows:1) The AIRS retrievals and the surface observation data at Waliguan show a consistent increasing trend. The time series of AIRS retrievals and surface observations are positively correlated and the correlation is statistically significant. The time of abrupt changes are consistent between the two data series. Therefore it is possible to analyze the characteristics of spatiotemporal distribution of CH₄ in the tropospheric over the TP using the AIRS retrievals. 2) A northwest-southeast oriented low concentration zone (LCZ) of CH₄ above the central area of TP and four permanent high concentration centers of CH₄ to the south and north of LCZ (Ngari, Nagqu, Lhoka, and Yushu) are found based on spatial analysis. 3) With the increase in altitude, an obvious decreasing trend of CH₄ concentration is found over the TB. In the middle and upper troposphere, the CH₄ concentration is lower in the LCZ but higher on both sides of it. In the top troposphere, CH₄ is higher to the south and lower to the north of LCZ. 4) CH₄ concentration shows a slowly rising trend with a rate of 0.0018 ppm/a over the TB with the fastest rise in the summer and the slowest in autumn. 5) CH₄ over the TB shows a clear seasonal and unimodal variation characteristic. It is higher in the summer and autumn than in winter, which is quite different to the summer-winter bimodal variation characteristic of CH₄ in East China. Seasonal variation of CH₄ over the TP increases as elevation increases.