Irradiated Atmospheres IV: Effect of Mixing Heat Flux on Chemistry

Abstract

Vertical mixing disrupts the thermochemical equilibrium and introduces additional heat flux that alters exoplanetary atmospheric temperatures. We investigate how this mixing-induced heat flux affects atmospheric chemistry. Temperature increase in the lower atmosphere by the mixing-induced heat flux alters species abundances there and modifies those in the upper atmosphere through vertical transport. In the lower atmosphere, most species follow thermodynamic equilibrium with temperature changes. In the upper layers, species mixing ratios depend on the positions of quenching levels relative to the regions exhibiting significant mixing-induced temperature variations. When the quenching level resides within such region (e.g. CO, $\rm CH_4$, and $\rm H_2O$ with strong mixing), the mixing ratios in the upper atmosphere are modified due to changes in the quenched ratios affected by the temperature variation in the lower atmosphere. This alters the mixing ratio of other species (e.g. NO and $\rm CO_2$) through the chemical reaction network, whose quenching occurs in the region without much temperature change. The mixing ratios of $\rm CH_4$, $\rm H_2O$, and $\rm NH_3$ decrease in the lower atmosphere with increasing mixing heat flux, similarly reducing these ratios in the upper atmosphere. Conversely, the mixing ratios of CO, $\rm CO_2$, and NO rise in the lower atmosphere, with CO and $\rm CO_2$ also increasing in the upper levels, although NO decreases. Weaker host star irradiation lowers the overall temperature of the planet, allowing a smaller mixing to have a similar effect. We conclude that understanding the vertical mixing heat flux is essential for accurate atmospheric chemistry modeling and retrieval.