Ship- and aircraft-based XCH₄ over oceans as a new tool for satellite validation

<p>Satellite-based estimations of dry-air column-averaged mixing ratios of methane (<span class="inline-formula">XCH₄</span>) contribute to a better understanding of changes in <span class="inline-formula">CH₄</span> emission sources and variations in its atmospheric growth rates. High accuracy of the satellite measurements is required, and therefore, extensive validation is performed, mainly against the Total Carbon Column Observing Network (TCCON). However, validation opportunities at open-ocean areas outside the coastal regions are sparse. We propose a new approach to assess the accuracy of satellite-derived <span class="inline-formula">XCH₄</span> trends and variations. We combine various ship and aircraft observations with the help of atmospheric chemistry models, mainly used for the stratospheric column, to derive observation-based <span class="inline-formula">XCH₄</span> (obs. <span class="inline-formula">XCH₄</span>). Based on our previously developed approach for the application to <span class="inline-formula">XCO₂</span>, we investigated three different advancements, from a simple approach to more elaborate approaches (approaches 1, 2, and 3), to account for the higher tropospheric and stratospheric variability in <span class="inline-formula">CH₄</span> as compared to <span class="inline-formula">CO₂</span>. Between 2014 and 2018, at 20–40° N of the western Pacific, we discuss the uncertainties in the approaches and the derived obs. <span class="inline-formula">XCH₄</span> within 10° by 20° latitude–longitude boxes. Uncertainties were 22 <span class="inline-formula">ppb</span> (parts per billion) for approach 1, 20 <span class="inline-formula">ppb</span> for approach 2, and 16 <span class="inline-formula">ppb</span> for approach 3. We analyzed the consistency with the nearest TCCON stations and found agreement of approach 3 with Saga of <span class="inline-formula">1±12</span> <span class="inline-formula">ppb</span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">1</mn><mo>±</mo><mn mathvariant="normal">11</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="39pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="6a90032efed1f8b55483afe692e8b4b8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-17-1297-2024-ie00001.svg" width="39pt" height="10pt" src="amt-17-1297-2024-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">ppb</span> with Tsukuba for the northern and southern latitude box, respectively. Furthermore, we discuss the impact of the modeled stratospheric column on the derived obs. <span class="inline-formula">XCH₄</span> by applying three different models in our approaches. Depending on the models, the difference can be more than 12 <span class="inline-formula">ppb</span> (0.6 %), showing the importance for the appropriate choice. We show that our obs. <span class="inline-formula">XCH₄</span> dataset accurately captures seasonal variations in <span class="inline-formula">CH₄</span> over the ocean. Using different retrievals of the Greenhouse Gases Observing Satellite (GOSAT) from the National Institute for Environmental Studies (NIES), the RemoTeC full-physics retrieval operated at the Netherlands Institute for Space Research (SRON), and the full-physics retrieval of the University of Leicester (UoL-OCFP), we demonstrate the applicability of the dataset for satellite evaluation. The comparison with results of approach 3 revealed that NIES showed a difference of <span class="inline-formula">−</span>0.04 <span class="inline-formula">±</span> 13 <span class="inline-formula">ppb</span> and strong scatter at 20–30° N, while RemoTeC and OCFP have a rather systematic negative bias of <span class="inline-formula">−</span>12.1 <span class="inline-formula">±</span> 8.1 and <span class="inline-formula">−</span>10.3 <span class="inline-formula">±</span> 9.6 <span class="inline-formula">ppb</span>. Our new approach to derive <span class="inline-formula">XCH₄</span> reference datasets over the ocean can contribute to the validation of existing and upcoming satellite missions in future.</p>