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. Column-averaged dry-air mole fractions of carbon dioxide (XCO2) andmethane (XCH4) measured by a solar viewing portable Fourier transformspectrometer (FTS, EM27/SUN) have been characterized and validated bycomparison using in situ profile measurements made during the transferflights 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 overtwo Total Carbon Column Observing Network (TCCON) sites: Rikubetsu, Japan(43.46∘ N, 143.77∘ E), for the KORUS-AQ campaign andBurgos, Philippines (18.53∘ N, 120.65∘ E), for the EMeRGecampaign. The EM27/SUN was deployed at the corresponding TCCON sites duringthe overflights. The mole fraction profiles obtained by the aircraft overRikubetsu differed between the ascending and the descending flights aboveapproximately 8 km for both CO2 and CH4. Because the spatialpattern of tropopause heights based on potential vorticity values from theERA5 reanalysis shows that the tropopause height over the Rikubetsu site wasconsistent with the descending profile, we used only the descending profileto compare with the EM27/SUN data. Both the XCO2 and XCH4 derivedfrom the descending profiles over Burgos were lower than those from theascending profiles. Output from the Weather Research and Forecasting Modelindicates that higher CO2 for the ascending profile originated incentral Luzon, an industrialized and densely populated region about 400 kmsouth of the Burgos TCCON site. Air masses observed with the EM27/SUNoverlap better with those from the descending aircraft profiles than thosefrom the ascending aircraft profiles with respect to their properties suchas origin and atmospheric residence times. Consequently, the descendingaircraft profiles were used for the comparison with the EM27/SUN data. TheEM27/SUN XCO2 and XCH4 data were derived by using the GGG2014software without applying air-mass-independent correction factors (AICFs).The comparison of the EM27/SUN observations with the aircraft data revealedthat, on average, the EM27/SUN XCO2 data were biased low by 1.22 %and the EM27/SUN XCH4 data were biased low by 1.71 %. The resultingAICFs of 0.9878 for XCO2 and 0.9829 for XCH4 were obtained forthe EM27/SUN. Applying AICFs being utilized for the TCCON data (0.9898 forXCO2 and 0.9765 for XCH4) to the EM27/SUN data induces anunderestimate for XCO2 and an overestimate for XCH4.

atmospheric measurement techniques

Validation of XCO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and XCH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; retrieved from a portable Fourier transform spectrometer with those from in situ profiles from aircraft-borne instruments