| Abstract: | The atmospheric characterization of exoplanets is a central objective in present-day astronomy, especially with the capabilities of JWST and other upcoming observatories. The spectra of exoplanet atmospheres offer insights into their atmospheric composition and thermal structure. Molecules like CO, CO2, CH4, H2O, NH3, and HCN, which are the building blocks for more complex organic molecules and significant reservoirs of elemental C-N-O, have been detected in exoplanet atmospheres. Among various parameters influencing atmospheric composition, metallicity plays a significant role. The effect of atmospheric metallicity on thermochemical equilibrium abundances is reasonably well-constrained, though its effect on the disequilibrium composition needs to be better constrained. This thesis provides a comprehensive study of the effect of atmospheric metallicity on the composition of H2O, CO2, CO, CH4, NH3, N2, and HCN over a large parameter space (temperature: 500-2500 K, pressure: 0.1 mbar - 1 kbar, metallicity: 0.1-1000×solar metallicity) in the presence of disequilibrium processes. We built a 1-D photochemistry-transport model to solve the mass continuity equation for each species at every atmospheric layer. The model includes eddy diffusion and molecular diffusion as transport processes and uses the two-stream approximation of radiative transfer to estimate the photon flux for calculating the photochemical rates. As a more flexible approach for a general study, the quenching approximation method is used to calculate the atmospheric abundance in the presence of vertical mixing. To apply the quenching approximation, we calculated chemical timescales for major HCNO-bearing molecules using a reduced chemical network, with vertical mixing derived via the methodology given by Smith (1998). I will present our findings on how molecular abundances shift with metallicity under thermochemical equilibrium and in the presence of vertical mixing. I will also discuss the implications of the quenching approximation for atmospheric retrieval, focusing on constraining vertical mixing strength and atmospheric metallicity. |
