Abstract
Abstract. We evaluate the uncertainties of methane optimal estimation retrievals from single-footprint thermal infrared observations from the Atmospheric Infrared Sounder (AIRS). These retrievals are primarily sensitive to atmospheric methane in the mid-troposphere through the lower stratosphere (∼2 to ∼17 km). We compare them to in situ observations made from aircraft during the HIAPER Pole to Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom) campaigns, and from the NOAA GML aircraft network, between the surface and 5–13 km, across a range of years, latitudes between 60∘ S to 80∘ N, and over land and ocean. After a global, pressure-dependent bias correction, we find that the land and ocean have similar biases and that the reported observation error (combined measurement and interference errors) of ∼27 ppb is consistent with the SD between aircraft and individual AIRS observations. A single observation has measurement (noise related) uncertainty of ∼17 ppb, a ∼20 ppb uncertainty from radiative interferences (e.g., from water or temperature), and ∼30 ppb due to “smoothing error”, which is partially removed when making comparisons to in situ measurements or models in a way that accounts for this regularization. We estimate a 10 ppb validation uncertainty because the aircraft typically did not measure methane at altitudes where the AIRS measurements have some sensitivity, e.g., the stratosphere, and there is uncertainty in the truth that we validate against. Daily averaging only partly reduces the difference between aircraft and satellite observation, likely because of correlated errors introduced into the retrieval from temperature and water vapor. For example, averaging nine observations only reduces the aircraft–model difference to ∼17 ppb vs. the expected ∼10 ppb. Seasonal averages can reduce this ∼17 ppb uncertainty further to ∼10 ppb, as determined through comparison with NOAA aircraft, likely because uncertainties related to radiative effects of temperature and water vapor are reduced when averaged over a season.
Highlights
Advances in remote sensing and global transport modeling and an increasingly dense network of surface measurements have led to substantive advances in evaluating the components and error structure of the global methane budget and the processes controlling this budget
The goal of this paper is to evaluate the uncertainties of new methane retrievals from Atmospheric Infrared Sounder (AIRS) single-footprint, original radiances using aircraft measurements from the HIAPER Pole-to-Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom) campaigns and National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Laboratory (GML) aircraft network, taken between 2006 and 2017
To summarize, averaging AIRS observations within 1 d reduces the error vs. aircraft, but correlated errors prevent daily averaged errors from dropping below 11.5 ppb
Summary
Advances in remote sensing and global transport modeling and an increasingly dense network of surface measurements have led to substantive advances in evaluating the components and error structure of the global methane budget and the processes controlling this budget. Methane retrievals have been applied to NIR radiances from the Greenhouse Gases Observing Satellite (GOSAT) instrument (e.g., Parker et al, 2011; Schepers et al, 2012), launched in 2009, and the TROPOspheric Monitoring Instrument (TROPOMI; e.g., Hu et al, 2018), launched in 2017. These data have sufficient accuracy to map regional surface methane enhancements (e.g., Kort et al, 2014; Wecht et al, 2014) and point source anomalies (Varon et al, 2019; Pandey et al, 2019). TIR methane measurements have been used to evaluate the role of fires (e.g., Worden et al, 2013b, 2017a), Asian emissions, and stratospheric intrusions (e.g., Xiong et al, 2009, 2013) in the global methane budget
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