Methane-Nitrogen Binary Nucleation: A New Microphysical Mechanism for Cloud Formation in Titan's Atmosphere

Abstract

It is known that clouds are present in the troposphere of Titan; however, their formation mechanism, particle size, and chemical composition remain poorly understood. In this study, a two-component (CH 4 and N 2 ) bin-microphysics model is developed and applied to simulate cloud formation in the troposphere of Titan. A new process, binary nucleation of particles from CH 4 and N 2 gases, is considered. The model is validated and calibrated by recent laboratory experiments that synthesize particle formation in Titan-like environments. Our model simulations show that cloud layers can be formed at about 20 km with a particle size ranging from one to several hundred μm and number concentration 10 −2 to over 100 cm −3 depending on the strength of the vertical updraft. The particles are formed by binary nucleation and grow via the condensation of both CH 4 and N 2 gases, with their N 2 mole fraction varying from <10% in the nucleation stage to >30% in the condensation growth stage. The locally occurring CH 4 -N 2 binary nucleation mechanism is strong and could potentially be more important than the falling condensation nuclei mechanism assumed in many current models.

Titan's atmosphere and climate

“…Unlike Earth, Titan's clouds form from a number of different volatiles. Tsai et al [2012] divided clouds into three categories: convective methane clouds [Griffith et al, 1998[Griffith et al, , 2000Brown et al, 2002;Roe et al, 2002;Gibbard et al, 2004b;Griffith et al, 2005;Roe et al, 2005;Schaller et al, 2006aSchaller et al, , 2006bGriffith et al, 2009;Rodriguez et al, 2009;Schaller et al, 2009;Turtle et al, 2009;Brown et al, 2010;Rodriguez et al, 2011;Turtle et al, 2011a], stratiform ethane clouds [Griffith et al, 2006;Brown et al, 2010;Le Mouélic et al, 2012], and high-altitude cirrus clouds (HCN, HC 3 N, etc.) [Samuelson et al, 2007;Anderson et al, 2010;Anderson and Samuelson, 2011].…”

“…It is generally assumed that haze particles produced higher in the atmosphere act as cloud condensation nuclei [see, e.g., Griffith et al, 2006;Bauerecker and Dartois, 2009]; liquid and solid CH 4 nucleate easily on Titan haze analogues [Curtis et al, 2008] but CH 4 -N 2 binary nucleation may also play an important role in cloud formation [Wang et al, 2010;Tsai et al, 2012].…”

Titan's Atmosphere and Climate

“…It is generally assumed that haze particles produced higher in the atmosphere act as cloud condensation nuclei (see e.g., Griffith et al [2006]; Bauerecker and Dartois [2009]); liquid and solid CH 4 nucleate easily on Titan haze analogs [Curtis et al, 2008] but CH 4 -N 2 binary nucleation may also play an important role in cloud formation [Wang et al, 2010;Tsai et al, 2012].…”

“…Much like Earth, Titan exhibits a number of types of clouds resulting from different formation processes and conditions. Unlike Earth, Titan's clouds form from a number of different volatiles Tsai et al [2012]. divided clouds into 3 categories: convective methane clouds[Griffith et al, 1998[Griffith et al, , 2000Brown et al, 2002;Roe et al, 2002;Gibbard et al, 2004b;Griffith et al, 2005;Roe et al, 2005; Schaller et al, 2006a, b;Rodriguez et al, 2009;Schaller et al, 2009;Turtle et al, 2009;Brown et al, 2010; Rodriguez et al, 2011;Turtle et al, 2011a], stratiform ethane clouds[Griffith et al, 2006; …”