Photochemical haze precursors

Clouds and photochemical hazes are ubiquitous across planetary atmospheres, but their microphysical formation is complex and uncertain. Rather than model particle growth directly, VULCAN tracks a set of gas-phase precursor species as proxies for haze formation (Section 2.8 of Tsai et al. 2021 ¹).

Precursor selection

The model preferentially considers precursors related to polycyclic aromatic hydrocarbon (PAH) or nitrile formation. The full precursor set is:

• Benzene (C\(_6\)H\(_6\)) is treated as the key PAH proxy: once the first aromatic ring forms, the thermodynamics of attaching further rings changes little, and benzene formation is argued to be the rate-limiting step in building complex hydrocarbons.
• Nitriles are represented by HCN together with the less abundant H\(_2\)CN, CH\(_2\)NH, CH\(_3\)CN, and HC\(_3\)N. HCN itself is rarely the limiting factor, so the rarer nitriles are more indicative of complex-nitrile formation.
• CS\(_2\) represents sulfur-bearing haze precursors, following laboratory work (He et al. 2020).

Benzene mechanism

The simplified benzene-forming pathway uses propargyl recombination,

with \(\mathrm{C_3H_3}\) produced from \(\mathrm{CH_3 + C_2H \rightarrow C_3H_3 + H}\) (Frenklach 2002 ²). The intention is to capture the main formation pathway at minimum network cost; benzene photodissociation branches are poorly constrained, so the dominant channel is assumed to yield phenyl (C\(_6\)H\(_5\)) with a small (~15%) fraction to C\(_3\)H\(_3\) (Kislov et al. 2004 ³). See chemical networks for the supporting species.

Diagnostic: column density above 1 mbar

The model does not grow particles; instead the column number density of each precursor above 1 mbar is used as the diagnostic of haze-forming potential. Across the irradiated H\(_2\)-dominated atmospheres studied, HCN is consistently the most abundant precursor, but this does not by itself imply complex-nitrile formation. HC\(_3\)N, C\(_4\)H\(_2\), and C\(_6\)H\(_6\) tend to increase with decreasing temperature, while CH\(_3\)CN shows the opposite trend; CS\(_2\) (containing no hydrogen) is most favored in hot Jupiter conditions.

Implementation in VULCAN

As with the Ti/V chemistry, haze precursors require no dedicated code path: the precursor species and the benzene mechanism are part of the network file and are integrated like any other species through chemdf and the photolysis routines. The benzene photodissociation branches and cross sections (Boechat-Roberty et al. 2004; Capalbo et al. 2016) are read by op.ReadRate.make_bins_read_cross and applied in compute_J.