Mid‐upper tropospheric methane in the high Northern Hemisphere: Spaceborne observations by AIRS, aircraft measurements, and model simulations

Abstract

Spaceborne measurements by the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua satellite provide a global view of the methane (CH4) distribution in the mid‐upper troposphere (MUT‐CH4). The focus of this study is to examine the spatiotemporal variation of MUT‐CH4 in the high Northern Hemisphere (HNH) using AIRS retrievals, aircraft measurements, and simulations from a forward chemistry‐transport model (i.e., ACTM). Data from 2004 and 2005 focusing over two regions (Alaska and Siberia) are analyzed. An important feature in the seasonal variation of CH4 we found is the summer increase of MUT‐CH4, which is nearly opposite to the summer minimum of CH4 in the marine boundary layer (MBL). This study also demonstrated an apparent increase of CH4 over Alaska associated with the 2004 Alaska forest fire and a negative bias of the ACTM simulations in the HNH. The larger bias of the model simulations in the late winter to early spring may indicate possible unidentified CH4 emission sources (e.g., the use of energy or gas leakage) during this period, but more studies will be needed due to the retrieval uncertainties in the polar winter season. The summer increase of MUT‐CH4 is related to surface emission, but the enhanced convection in summer is likely the most important driver.