Applying Lab-Measured Critical Saturation Values for Benzene Nucleation onto Tholin to the Study of the Microphysics of Titan’s South Polar Benzene Cloud

The seasonal enhancement of benzene (C₆H₆) in Titan’s south polar stratosphere coupled with thermal cooling in the 350-500 km altitude range resulted in cloud formation in that region. A benzene ice cloud with a top near 280 km and an equivalent radius upper limit of ~1.5 mm for pure benzene ice particles was detected by the Cassini Composite Infrared Spectrometer (CIRS) [1]. We have been investigating the microphysical properties of this cloud through a collaboration between laboratory work and microphysical modeling by (i) measuring the saturation vapor pressure [2, 3] and critical saturation for nucleation for temperatures relevant to Titan’s stratosphere using the NASA Ames Atmospheric Chemistry Laboratory (ACL) and COsmic SImulation Chamber (COSmIC) and (ii) incorporating these values in the Community Aerosol and Radiation Model for Atmospheres (CARMA, [4]).

 

CARMA simulates the microphysical evolution of aerosol particles in a column of atmosphere, and includes the processes of vertical transport, coagulation, nucleation, condensation, and evaporation. For nucleation, we follow the classical theory for heterogeneous nucleation by vapor deposition, which requires a knowledge of the critical saturation ratio. In [2], no data was available for the critical saturation (S_crit) for benzene ice nucleation, so we considered a value of 1.35, approximated from previous laboratory measurements of methane, ethane, and butane. We have now measured S_crit for benzene ice at Titan-relevant temperatures (135 – 170 K). Measurements for benzene nucleation onto lab-produced tholin particles indicate S_crit is significantly higher than our initial estimate, ranging ~3-14. Additionally, our nucleation measurements over a range of temperatures allow us to derive a temperature dependence to S_crit, which increases with decreasing temperature.

 

In CARMA, the critical saturation is converted to the contact parameter in order to calculate the nucleation rate for the formation of benzene cloud particles. For a flat substrate as used in the laboratory environment,

                 f_c = ¼*(2 + m)*(1-m)²

 where m is the contact parameter, calculated by solving the nucleation rate equation for a rate, J=1, at the critical saturation and temperature:

The nucleation rate, J, is a function of the energy of germ formation, DF_g, which is related to the degree of saturation through the germ radius, a_g. The other terms shown are particle radius r, nucleation prefactor K, Boltzmann’s constant k, surface energy s, molecular weight M, gas constant R, and particle density r.  As S_crit decreases towards unity, m increases towards unity and the energy required for nucleation is minimized. Figure 1 shows the critical saturation measurements converted to contact parameters for use in the CARMA nucleation calculations.

Figure 1. Contact parameter values calculated from lab measurements of critical saturation as a function of temperature for C₆H₆ nucleation onto tholin (black points and error bars). A linear fit, including the error, was calculated in order to model nucleation with a temperature-dependent contact parameter. The dashed green line shows our previous estimate for contact parameter used in [2].

The higher S_crit values for benzene nucleation onto lab-produced tholin translate to a higher degree of supersaturation for nucleation of benzene to proceed in Titan’s stratosphere as compared to our previous estimate [2]. Microphysical modeling has shown that increasing S_crit by a factor of 5 translates to about a 20 km drop in the cloud top height. Additionally, when we consider the temperature dependence to the critical saturation (and hence the contact parameter) the range of altitudes over which nucleation occurs becomes a more significant factor in determining the cloud top altitude.  This is indicated by the cloud top levels shown in Figure 2. The solid lines show the highest, lowest, and linear fit to the data in Fig. 1. The lowest cloud top is seen using the linear fit because the altitude where nucleation begins is at a colder temperature than shown in the measurements. All of our lab measurements show that benzene nucleation in Titan’s atmosphere is less efficient than we modeled it in our previous work ([2], dashed curve).

Figure 2. C₆H₆ number density profiles for a variety of contact parameters. The solid lines show our new lab measurements of benzene nucleation onto tholin, including the temperature-dependent fit, a constant value at the highest measured S_crit, and a constant value at the lowest measured S_crit. The dashed line is the profile using our previous estimate [2].

We will present microphysical modeling results showing the effects of these higher S_crit values on the number density and size distribution of cloud particles formed, and implications for the benzene cloud seen at Titan’s south pole. Nucleation is a significant factor in the height of the cloud top – the lower cloud tops modeled here may be an indirect indication of the presence of co-condensation which would alter the saturation vapor pressure and critical saturation of the mixture, allowing for cloud formation at 280 km at the south pole.

                                                         

Funding for this project is provided through NASA CDAP.

 

[1] Vinatier et al. 2018, Icarus, 310, 89-104. [2] Dubois et al. 2021, Planet. Sci. J. 2, 121. [3] Hudson et al. 2022, PSJ, 3:120, 6pp. [4] Barth 2020, Atmosphere, 11(10), 1064.