Quantifying lower tropospheric methane concentrations using GOSAT near-IR and TES thermal IR measurements

Abstract

Abstract. Evaluating surface fluxes of CH 4 using total column data requires models to accurately account for the transport and chemistry of methane in the free troposphere and stratosphere, thus reducing sensitivity to the underlying fluxes. Vertical profiles of methane have increased sensitivity to surface fluxes because lower tropospheric methane is more sensitive to surface fluxes than a total column, and quantifying free-tropospheric CH 4 concentrations helps to evaluate the impact of transport and chemistry uncertainties on estimated surface fluxes. Here we demonstrate the potential for estimating lower tropospheric CH 4 concentrations through the combination of free-tropospheric methane measurements from the Aura Tropospheric Emission Spectrometer (TES) and XCH 4 (dry-mole air fraction of methane) from the Greenhouse gases Observing SATellite -Thermal And Near-infrared for carbon Observation (GOSAT TANSO, herein GOSAT for brevity). The calculated precision of these estimates ranges from 10 to 30 ppb for a monthly average on a 4 • × 5 • latitude/longitude grid making these data suitable for evaluating lower-tropospheric methane concentrations. Smoothing error is approximately 10 ppb or less. Comparisons between these data and the GEOS-Chem model demonstrate that these lower-tropospheric CH 4 estimates can resolve enhanced concentrations over flux regions that are challenging to resolve with total column measurements. We also use the GEOS-Chem model and surface measurements in background regions across a range of latitudes to determine that these lower-tropospheric estimates are biased low by approximately 65 ppb, with an accuracy of approximately 6 ppb (after removal of the bias) and an actual precision of approximately 30 ppb. This 6 ppb accuracy is consistent with the accuracy of TES and GOSAT methane retrievals.

Satellite observations of atmospheric methane and their value for quantifying methane emissions

“…Multispectral retrievals in the SWIR and TIR combine the advantages of both approaches and provide some vertical profile information, as demonstrated by Herbin et al (2013) using the combination of SWIR and TIR data from GOSAT, and by Worden et al (2015) using the combination of SWIR from GOSAT and TIR from TES. This could enable separation between the local/regional methane enhancement near the surface and the higher-altitude methane background (Bousserez et al, 2015).…”

Constraints on methane emissions in North America from future geostationary remote-sensing measurements

“…The effect of the interferences with this simultaneous retrieval approach is to reduce the overall sensitivity to methane but improve the posteriori errors. A proof of concept for combining near-IR and IRbased methane estimates to derive a lower tropospheric estimate is discussed in Worden et al (2015) using GOSAT and TES profile retrievals. Figure 2 shows the column averaging kernel for the TROPOMI and TES X CH 4 retrievals as well as the averaging kernels at three different levels for the multi-spectral retrieval.…”

Evaluation and attribution of OCO-2 XCO<sub>2</sub> uncertainties

“…The "column-averaged dry air mole fraction" of CO 2 or XCO 2 is then calculated by applying the column operator (e.g., Connor et al, 2008;Worden et al, 2015) to the estimated CO 2 profile. As discussed in Rodgers (2000), Worden et al (2004), Connor (2008), and Bowman et al (2006), when this nonlinear retrieval converges to a solution, the estimated XCO 2 can be written aŝ…”