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<p>Column-averaged dry-air mole fractions of carbon dioxide (<span class="inline-formula">XCO₂</span>) and methane (<span class="inline-formula">XCH₄</span>) measured by a solar viewing portable Fourier transform spectrometer (FTS, EM27/SUN) have been characterized and validated by comparison using in situ profile measurements made during the transfer flights of two aircraft campaigns: Korea-United States Air Quality Study (KORUS-AQ) and Effect of Megacities on the Transport and Transformation of Pollutants at Regional and Global Scales (EMeRGe). The aircraft flew over two Total Carbon Column Observing Network (TCCON) sites: Rikubetsu, Japan (43.46<span class="inline-formula">^∘</span>&thinsp;N, 143.77<span class="inline-formula">^∘</span>&thinsp;E), for the KORUS-AQ campaign and Burgos, Philippines (18.53<span class="inline-formula">^∘</span>&thinsp;N, 120.65<span class="inline-formula">^∘</span>&thinsp;E), for the EMeRGe campaign. The EM27/SUN was deployed at the corresponding TCCON sites during the overflights. The mole fraction profiles obtained by the aircraft over Rikubetsu differed between the ascending and the descending flights above approximately 8&thinsp;km for both <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span>. Because the spatial pattern of tropopause heights based on potential vorticity values from the ERA5 reanalysis shows that the tropopause height over the Rikubetsu site was consistent with the descending profile, we used only the descending profile to compare with the EM27/SUN data. Both the <span class="inline-formula">XCO₂</span> and <span class="inline-formula">XCH₄</span> derived from the descending profiles over Burgos were lower than those from the ascending profiles. Output from the Weather Research and Forecasting Model indicates that higher <span class="inline-formula">CO₂</span> for the ascending profile originated in central Luzon, an industrialized and densely populated region about 400&thinsp;km south of the Burgos TCCON site. Air masses observed with the EM27/SUN overlap better with those from the descending aircraft profiles than those from the ascending aircraft profiles with respect to their properties such as origin and atmospheric residence times. Consequently, the descending aircraft profiles were used for the comparison with the EM27/SUN data. The EM27/SUN <span class="inline-formula">XCO₂</span> and <span class="inline-formula">XCH₄</span> data were derived by using the GGG2014 software without applying air-mass-independent correction factors (AICFs). The comparison of the EM27/SUN observations with the aircraft data revealed that, on average, the EM27/SUN <span class="inline-formula">XCO₂</span> data were biased low by 1.22&thinsp;% and the EM27/SUN <span class="inline-formula">XCH₄</span> data were biased low by 1.71&thinsp;%. The resulting AICFs of 0.9878 for <span class="inline-formula">XCO₂</span> and 0.9829 for <span class="inline-formula">XCH₄</span> were obtained<span id="page5150"/> for the EM27/SUN. Applying AICFs being utilized for the TCCON data (0.9898 for <span class="inline-formula">XCO₂</span> and 0.9765 for <span class="inline-formula">XCH₄</span>) to the EM27/SUN data induces an underestimate for <span class="inline-formula">XCO₂</span> and an overestimate for <span class="inline-formula">XCH₄</span>.</p>