Validation of acetonitrile (CH 3CN) measurements in the stratosphere and lower mesosphere from the SMILES instrument on the International Space Station

Fujinawa, Tamaki; Sato, Tomohiro O.; Yamada, Takayoshi; Nara, Seidai; Uchiyama, Yuki; Takahashi, Kodai; Yoshida, Naohiro; Kasai, Yasuko

Acetonitrile (inline-formulaCH3CN) is a volatile organic compound (VOC) and a potential tracer of biomass burning. We evaluated the capability of using observations derived from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station (ISS) to measure inline-formulaCH3CN profiles. The error in a inline-formulaCH3CN vertical profile from the Level-2 research (L2r) product version 3.0.0 was estimated by theoretical error analysis and also compared with other instrumental measurements. We estimated the systematic and random errors to be inline-formula∼5.8 ppt (7.8 %) and 25 ppt (60 %), respectively, for a single observation at 15.7 hPa in the tropics, where the inline-formulaCH3CN measurements are enhanced. The major source of systematic error was the pressure-broadening coefficient, and its contribution to the total systematic error was approximately 60 % in the middle stratosphere (15.7–4.8 hPa). The random error decreased to less than 40 % after averaging 10 profiles in the pressure range of 28.8–1.6 hPa. The total error due to uncertainties in other molecular spectroscopic parameters (2.8 ppt) was comparable to that of inline-formulaCH3CN spectroscopic parameters. We compared the SMILES inline-formulaCH3CN profiles with those of the microwave limb sounder (MLS) on the Aura satellite (version 4.2). The SMILES inline-formulaCH3CN values were consistent with those from MLS within the standard deviation (inline-formula1σ) of the MLS observations. The difference between the SMILES and MLS inline-formulaCH3CN profiles increased with altitude and was within 20–35 ppt (20 %–260 %) at 15.7–1.6 hPa. We observed discrepancies of 5–10 ppt (10 %–30 %) between the SMILES inline-formulaCH3CN profiles observed by different spectrometers, and hence, we do not recommend merging SMILES inline-formulaCH3CN profiles derived from different spectrometers. We found that the SMILES inline-formulaCH3CN volume mixing ratio (VMR) in the upper stratosphere has a seasonal maximum in February.

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Fujinawa, Tamaki / Sato, Tomohiro O. / Yamada, Takayoshi / et al: Validation of acetonitrile (CH3CN) measurements in the stratosphere and lower mesosphere from the SMILES instrument on the International Space Station. 2020. Copernicus Publications.

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