Validation of acetonitrile (CH3CN) measurements in the stratosphere and lower mesosphere from the SMILES instrument on the International Space Station

Acetonitrile (CH3CN) 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 CH3CN profiles. The error in a CH3CN 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 similar to 5.8 ppt (7.8 %) and 25 ppt (60 %), respectively, for a single observation at 15.7 hPa in the tropics, where the CH3CN 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 CH3CN spectroscopic parameters. We compared the SMILES CH3CN profiles with those of the microwave limb sounder (MLS) on the Aura satellite (version 4.2). The SMILES CH3CN values were consistent with those from MLS within the standard deviation (1s) of the MLS observations. The difference between the SMILES and MLS CH3CN 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 CH3CN profiles observed by different spectrometers, and hence, we do not recommend merging SMILES CH3CN profiles derived from different spectrometers. We found that the SMILES CH3CN volume mixing ratio (VMR) in the upper stratosphere has a seasonal maximum in February.