Infrared characterisation of acetonitrile and propionitrile aerosols under Titan's atmospheric conditions

Abstract

Pure, crystalline acetonitrile (CH₃CN) and propionitrile (CH₃CH₂CN) particles were formed in a collisional cooling cell allowing for infrared (IR) signatures to be compiled from 50 to 5000 cm⁻¹. The cell temperature and pressure conditions were controlled to simulate Titan's lower atmosphere (80–130 K and 1–100 mbar), allowing for the comparison of laboratory data to the spectra obtained from the Cassini–Huygens mission. The far-IR features confirmed the morphology of CH₃CN aerosols as the metastable β-phase (monoclinic) ice, however, a specific crystalline phase for CH₃CH₂CN could not be verified. Mie theory and the literature complex refractive indices enabled of the experimental spectra to be modelled. The procedure yielded size distributions for CH₃CN (55–140 nm) and CH₃CH₂CN (140–160 nm) particles. Effective kinetic profiles, tracing the evolution of aerosol band intensities, showed that condensation of CH₃CH₂CN proceeded at twice the rate of CH₃CN aerosols. In addition, the rate of CH₃CH₂CN aerosol depletion via lateral diffusion of the particles from the interrogation volume was approximately 50% faster than that of CH₃CN. The far-IR spectra recorded for both nitrile aerosols did not display absorption profiles that could be attributed to the unassigned 220 cm⁻¹ feature, which has been observed to fluctuate seasonally in the spectra obtained from Titan's atmosphere.