1D-coupled photochemical model of neutrals, cations and anions in the atmosphere of Titan

Abstract

International audienceMany models with different characteristics have been published so far to study the chemical processes at work in Titan's atmosphere. Some models focus on neutral species in the stratosphere or ionic species in the ionosphere, but few of them couple all the species throughout the whole atmosphere. Very few of these emphasize the importance of uncertainties in the chemical scheme and study their propagation in the model.We have developed a new 1D-photochemical model of Titan's atmosphere coupling neutral species with positive and negative ions from the lower atmosphere up to the ionosphere and have compared our results with observations to have a comprehensive view of the chemical processes driving the composition of the stratosphere and ionosphere of Titan. We have updated the neutral, positive ion and negative ion chemistry and have improved the description of N2 photodissociation by introducing high resolution N2 absorption cross sections. We performed for the first time an uncertainty propagation study in a fully coupled ion-neutral model.We determine how uncertainties on rate constants on both neutral and ionic reactions influence the model results and pinpoint the key reactions responsible for this behavior. We find very good agreement between our model results and observations in both the stratosphere and in the ionosphere for most neutral compounds. Our results are also in good agreement with an average INMS mass spectrum and specific flybys in the dayside suggesting that our chemical model (for both neutral and ions) provides a good approximation of Titan's atmospheric chemistry as a whole. Our uncertainty propagation study highlights the difficulty to interpret the INMS mass spectra for masses 14, 31, 41 and we identified the key reactions responsible for these ambiguities.Despite an overall improvement in the chemical model, disagreement for some specific compounds (HC3N, C2H5CN, C2H4) highlights the role that certain physical processes could play (meridional dynamics or sticking on aerosols). We find that some critical key reactions are important for many compounds including both neutrals and ions and should be studied in priority to lower the remaining model uncertainties. Extensive studies for some specific processes (including photolyses) are required

Seasonal changes in the middle atmosphere of Titan from Cassini/CIRS observations: Temperature and trace species abundance profiles from 2004 to 2017

“…If we focus on the 10-0.01 mbar region, our derived C 2 H 2 abundance profiles show a local maximum at ∼0.2 mbar (∼265 km) that it is not predicted by photochemical models (Krasnopolsky, 2014;Loison et al, 2015;Dobrijevic et al, 2016;Vuitton et al, 2019). At 0.2 mbar, the observed C 2 H 2 volume mixing ratio of ∼4×10 −6 is consistent within a factor of 2 with photochemical model predictions.…”

The Role of Nitrogen in Titan’s Upper Atmospheric Hydrocarbon Chemistry Over the Solar Cycle

“…Therefore, photochemical models that do not simulate N 2 photoabsorption with sufficient resolution systematically underestimate the hydrocarbon and nitrile densities, while they overestimate the densities of ion species ). An effect of the choice of N 2 cross section resolution on minor species was later confirmed by Dobrijevic et al (2016), although minors were affected to a different extent in their study than in Luspay-Kuti et al (2015). However, the latter disagreement between the results is not surprising due to the very different low-resolution cross sections employed in Dobrijevic et al (2016) compared with Luspay-Kuti et al (2015), which guarantees different results.…”

“…An effect of the choice of N 2 cross section resolution on minor species was later confirmed by Dobrijevic et al (2016), although minors were affected to a different extent in their study than in Luspay-Kuti et al (2015). However, the latter disagreement between the results is not surprising due to the very different low-resolution cross sections employed in Dobrijevic et al (2016) compared with Luspay-Kuti et al (2015), which guarantees different results. This significant effect of N 2 photoabsorption on secondary and higher-order hydrocarbon production, loss, and vertical abundance raises the question as to the role of nitrogen and its importance in Titan's hydrocarbon chemistry.…”

Titan's atmosphere and climate

“…Photochemical models are now able to reproduce the observed abundances of most of the small (less than 100 Da) molecules in Titan's atmosphere [see, e.g., Vuitton et al, 2007;Hörst et al, 2008;Lavvas et al, 2008aLavvas et al, , 2008bVuitton et al, 2008;Yelle et al, 2010;Krasnopolsky, 2012;Vuitton et al, 2012;Westlake et al, 2012;Hébrard et al, 2013;Dobrijevic et al, 2014;Li et al, 2015;Dobrijevic et al, 2016] but the formation of heavier molecules is HÖRST Figure 3. Shown here are the 10 most abundant photochemically generated molecules in Titan's equatorial stratosphere in approximately decreasing order of abundance with their major atmospheric production and loss pathways listed [see, e.g., Vuitton et al, 2014].…”