Seven Years’ Observation of Mid-Upper Tropospheric Methane from Atmospheric Infrared Sounder

摘要

The Atmospheric Infrared Sounder (AIRS) on EOS/Aqua platform provides a measurement of global methane (CH₄) in the mid-upper troposphere since September, 2002. As a thermal infrared sounder, the most sensitivity of AIRS to atmospheric CH₄ is in the mid-upper troposphere with the degree of freedom of ~1.0. Validation of AIRS CH₄ product versus thousands of aircraft profiles (convolved using the AIRS averaging kernels) demonstrates that its RMS error (RMSE) is mostly less than 1.5%, and its quality is pretty stable from 2003 to 2009. For scientific analysis of the spatial and temporal variation of mid-upper tropospheric CH₄ (MUT-CH₄) in the High Northern Hemisphere (HNH), it is more valuable to use the AIRS retrieved CH₄ in a layer of about 100 hPa below tropopause (“Representative Layer”) than in a fixed pressure layer. Further analysis of deseasonalized time-series of AIRS CH₄ in both a fixed pressure layer and the “Representative Layer” of AIRS (only for the HNH) from 2003 to 2009 indicates that, similar to the CH₄ in the marine boundary layer (MBL) that was found to increase in 2007–2008, MUT-CH₄ was also observed to have a recent increase but the most significant increase occurred in 2008. MUT-CH₄ continued to increase in 2009, especially in the HNH. Moreover, the trend of MUT-CH₄ from 2006 to 2008 is lower than the trend of CH₄ in the MBL by 30–40% in both the southern hemisphere and HNH. This delay for the MUT-CH₄ increase of about one year than CH₄ in the MBL as well as the smaller increase trend for MUT-CH₄ suggest that surface emission is likely a major driver for the recent CH₄ increase. It is also found that the seasonal cycle of MUT-CH₄ is different from CH₄ in the MBL due to the impact of transport, in addition to the surface emission and the photochemical loss.