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

Abstract

Pure, crystalline acetonitrile (CHCN) and propionitrile (CHCHCN) particles were formed in a collisional cooling cell allowing for infrared (IR) signatures to be compiled from 50 to 5000 cm. 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 CHCN aerosols as the metastable β-phase (monoclinic) ice, however, a specific crystalline phase for CHCHCN 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 CHCN (55-140 nm) and CHCHCN (140-160 nm) particles. Effective kinetic profiles, tracing the evolution of aerosol band intensities, showed that condensation of CHCHCN proceeded at twice the rate of CHCN aerosols. In addition, the rate of CHCHCN aerosol depletion via lateral diffusion of the particles from the interrogation volume was approximately 50% faster than that of CHCN. The far-IR spectra recorded for both nitrile aerosols did not display absorption profiles that could be attributed to the unassigned 220 cm feature, which has been observed to fluctuate seasonally in the spectra obtained from Titan's atmosphere.

Organic Ices in Titan’s Stratosphere

“…While a metastable b-phase (monoclinic) ice was identified for CH 3 CN, there was no such phase observed for C 2 H 5 CN. Ennis et al (2017) also reported the 200-250 cm −1 region was absent of an CH 3 CN spectral feature, but found the ν 13 band of crystalline C 2 H 5 CN ice at 110 K was located at 226 cm −1 , which is in agreement with Dello Russo and Khanna (1996).…”

“…Purifying the sample becomes very crucial, and unfortunately, most of the published laboratory work focused on Titan's stratospheric ices are impure. For example, some work efforts report CO 2 impurity bands in the infrared spectra of their synthesized ices, e.g., in the HC 3 N spectra reported in Moore et al (2010), in the C 4 N 2 ice spectrum of Khanna et al (1987), and in the CH 3 CN ice spectrum of Ennis et al (2017). This latter work effort also noted the cryopump oil impurities in their samples.…”

“…These effects are most noticeable in the low energy lattice modes, as recently detailed in a comprehensive laboratory study on the ice phases of C 2 H 5 CN ). C 2 H 5 CN ice was also recently studied by Ennis et al (2017), in which C 2 H 5 CN and CH 3 CN ice particles were generated at 95 K, 110 K, and 130 K under simulated Titan temperature and pressure conditions using collisional cooling cell experiments. The investigators measured transmittance spectra from 50 to 5000 cm −1 , with a focus in the far-IR to study the morphology of the icy particulates, identify their crystalline phases, and also attempt to spectrally match the Haystack.…”

Acetonitrile cluster solvation in a cryogenic ethane-methane-propane liquid: Implications for Titan lake chemistry

“…For example, Anderson et al 8 review the arguments for and against the presence of an essentially separate aerosol layer of cyanoacetylene ice in Titan's atmosphere. Very recently Ennis et al, 9 made laboratory IR spectra of pure ACN ice for comparison with Cassini data. While the match was poor, the authors note that one cannot distinguish between phases comprised of a mix of ACN with other species versus tiny grains of segregated pure ice.…”

Spectral curve fitting of dielectric constants

“…8 The optical properties of a material can be inferred from experimental spectra from X-ray frequencies through to the Far-Infrared/Terahertz regions, in either transmittance or absorbance or reflectance mode. 2,[9][10][11][12][13] From the measured spectra, it is possible to extract both the real and imaginary components of the complex optical constants by employing the Kramer-Kronig (K-K) relation, a mathematical expression that connects the real and imaginary parts. 14 However, due to the limited range of available spectral data, the optical constants extracted by direct K-K analysis contain truncation errors.…”