Modeling the Role of Selected Light Nonmethane Hydrocarbons on the Chemical Composition of Natural and Perturbed Troposphere

The original Oslo 2-dimensional global tropospheric photochemical model is modified by extending its vertical domain to 24.5 km with new transport coefficients to deduce the annual global source strengths of parent hydrocarbons (C-1 to C-3, or LHCs) and to study the role of photochemistry of these hydrocarbons and isoprene (rm C _5H_8) on the chemical composition of natural and perturbed troposphere. Model transport features are studied by comparing simulated atmospheric distributions and trends of CFC-11, CFC-12 and ^{85}Kr with corresponding long term observations. Four different photochemical schemes PC-1, PC -2, PC-3, and PC-5, that include C-1, C-2, C-3 hydrocarbons and rm C_5H_8 respectively, are developed. OH radical distributions calculated using these schemes and averaged surface observation data of LHCs as their respective lower boundary conditions are validated by comparing simulated atmospheric distribution and trends of rm CH_3CCl_3 with ALE/GAGE observations. Annual steady state source strengths of LHCs and rm C_2Cl _4 are calculated from their surface observations and above stated OH distributions. Comparison of modeled concentrations of C-2 and C-3 hydrocarbons in the lowest model layer with their corresponding observations shows that the sources of these species are seasonal in nature. The effects of photochemistry of light nonmethane hydrocarbons (NMHCs) on distributions of selected tropospheric species and ratio distributions of key species and on the budgets of O_3, CO, NOx and HNO _3 are also evaluated. Simulations of multiple changes in individual source strengths of NOx, CH_4, CO, NMHCs suggest that per molecule injected, NOx from aircraft emissions is the most efficient, the magnitude of which decreases with increase in emissions, in changing the global averaged O_3 concentration. Among NMHCs, changes in propane and ethane emissions are the most effective in changing the global average O _3 concentration and steady state lifetime of CH_4 respectively. For same perturbation, relatively small differences are noticed among the chemical compositions calculated by schemes PC -1, PC-2 and PC-3 but not for PC-5. Calculations show that tropospheric chemistry is most sensitive to the variations in tropical surface NOx emissions. Model results show that expected future increases in atmospheric temperature and water vapor concentration due to global warming will lead to a decreased concentrations of O_3 and CH_4. It is argued that the changes in concentrations of (hydrocarbons and CO) and NOx have specific and localized effects on the distribution of rm H_2O_2/HNO_3 which can be used to identify the origin and cause of atmospheric pollution events on the global scale. The last three chapters deal with the modeling of ^{13}CH_4 and its use to investigate the recent changes in growth rate of atmospheric CH_4, stratospheric heavy ozone (^{50}O_3) and the ozonation of atmospheric nitrogen and consequences of this reaction on the source strengths of N_2 O and NOx.