Evaluation of single-footprint AIRS CH<sub>4</sub> profile retrieval uncertainties using aircraft profile measurements

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.

Quantification of CH<sub>4</sub> emissions from waste disposal sites near the city of Madrid using ground- and space-based observations of COCCON, TROPOMI and IASI

“…Compared to previous satellite instruments, TROPOMI is able to capture CH 4 enhancements due to emissions on fine scales and to detect large point sources (Varon et al, 2019;De Gouw et al, 2020;Schneising et al, 2020). Satellite retrievals using thermal infrared nadir spectra as observed by IASI (Infrared Atmospheric Sounding Interferometer) or TES (Tropospheric Emission Spectrometer) are especially sensitive to CH 4 concentrations between the middle troposphere and the stratosphere (e.g., Siddans et al, 2017;García et al, 2018;De Wachter et al, 2017;Kulawik et al, 2021;Schneider et al, 2021a). Schneider et al (2021a) developed an a posteriori method for combining the TROPOMI and IASI products to detect tropospheric CH 4 , which has a positive bias of ∼ 1 % with respect to the reference data.…”

Joint spectral retrievals of ozone with Suomi NPP CrIS augmented by S5P/TROPOMI

“…This algorithm has a flexible and generic Radiative Transfer Model (RTM) that covers the entire wavelength range from the UV to the TIR and a nonlinear retrieval algorithm based on the Optimal Estimation (OE) method (Rodgers, 2000). MUSES has a long heritage in retrieving atmospheric parameters from a number of different satellite missions and instruments, including AIRS, TES and OMI, as well as for performing joint spectral retrievals (Bowman et al, 2006;Kulawik et al, 2006;Fu et al, 2013;Luo et al, 2013;Fu et al, 2018;Worden et al, 2019;Kulawik et al, 2021).…”

The Atmospheric Infrared Sounder

“…The AIRS radiance spectra also contain information on spatial and temporal variability and in long-lived climate forcers such as methane (CH 4 ) [51][52][53][54][55][56][57], carbon dioxide (CO 2 ) [58][59][60][61][62][63][64][65][66][67], and nitrous oxide (N 2 O) [68]. AIRS observations have provided evidence of substantial increases in atmospheric ammonia (NH 3 ) concentrations from 2002 to 2016 over several of the world's major agricultural regions [69].…”