Derived methane in the stratosphere and lower mesosphere from Aura Microwave Limb Sounder measurements of nitrous oxide, water vapor, and carbon monoxide

Abstract

The global distribution of methane (CH4) in the stratosphere and lower mesosphere has been derived using coincident measurements of water vapor (H2O), carbon monoxide (CO), and nitrous oxide (N2O) from the Microwave Limb Sounder (MLS) instrument on the Aura satellite. The derivation method is based on empirical relationships between these species established using observations from the Atmospheric Chemistry Experiment—Fourier Transform Spectrometer (ACE-FTS) on the SCISAT I satellite. The observed correlation between CH4 and N2O from ACE-FTS is used to derive CH4 from MLS measurements of N2O in the lower stratosphere, extending from a pressure of 100 hPa to a range of 30–10 hPa, depending on atmospheric conditions. In the upper stratosphere and lower mesosphere, between 30–10 hPa and 0.1 hPa, correlations between CH4 and H2O are used to derive CH4 from MLS measurements of H2O. Coincident MLS measurements of CO are utilized to separate two distinct air mass regimes in the CH4 - H2O relationship. This new methane data set covers all seasons and latitudes observed by MLS over the course of the Aura mission from 2004 to 2014. Examples are shown demonstrating the consistency of MLS derived CH4 with other trace gas measurements.