The effect of metallicity on the CH4 and CO quenched abundance in H-dominated atmospheres

Exoplanets show astonishing diversity in their parameter space, including atmospheric metallicity, which significantly affects the atmospheric composition. The effect of metallicity on the thermochemical equilibrium of exoplanet atmospheres has been studied widely. However, the effect on the disequilibrium abundance in the presence of transport (transport abundance) is largely unexplored and has only been studied for some targeted exoplanets. There are many available methods to find the transport abundance. Among these methods, the quenching approximation is the most straightforward way to constrain the transport abundance. In the quenching approximation, the quench level is defined at a pressure level where the chemical and mixing time scales become equal. Above the quench level, the transport abundance is given by the equilibrium abundance at the quench level (quenched abundance). We studied the effect of metallicity on the chemical and vertical mixing timescales for a large parameter space of pressure, temperature, and metallicity (0.1 mbar to 1 kbar, 500-2500 K, 0.1-1000 x solar metallicity). We compute the thermochemical equilibrium abundance for our parameter range and use it to mark the rate-limiting step (RLS) in a reduced chemical network. For this task, we built a network analysis tool to find the RLS and conversion scheme for a given set of molecules. By comparing the calculated mixing time scale and chemical time scale, we find all possible quench level data points in our parameter range and study the effect of metallicity on the location of the quench level. Our equilibrium results are in good agreement with the literature. The CO-CH4 and CO-CO2 equal-abundance curves move to low-temperature and high-temperature, respectively, with increasing metallicity. The abundance of CO and H2O increases linearly, whereas CO2 increases as the square of the metallicity. However, the CH4 abundance increases with metallicity only in the low-temperature and high-pressure regions, where it is a major source of C. The chemical time scale of CO is a weak function of metallicity; however, the chemical time scale of CH4 decreases linearly with increasing metallicity. By comparing the chemical time scale with the mixing time scale, we find that the quenched level of CO moves deep in the atmosphere with increasing metallicity and the CH4 quench level is a complex function of metallicity. For the benchmarking of quenched abundance, we compared the output of an in-house developed full transport chemical kinetics model with the output of the quenching approximation for two test exoplanets. The quenching approximation is accurate within an order of magnitude (which can be further improved by using the mixing length instead of the scale height). We also take three test exoplanets, HR 8799 b, HD 189733b, and Gj 436-b, and find that the quench level data points constrain exoplanets mixing strength and atmospheric metallicity for these exoplanets. Thus, the quench level data points and observed abundance of molecules can be a powerful tool to constrain atmospheric parameters.