The interaction of regional- and global-scale atmospheric chemistry, transport, and climate processes

This dissertation focuses on three scientific issues: (1) effects on atmospheric photochemistry of convective venting of the PBL to the free troposphere; (2) interactive effects of aerosols, ozone, and UV flux; and (3) radiative forcing of climate by aerosols and ozone. In order to perform these studies, I developed a single-column chemical transport model (SCCTM), two versions (uniform-grid and stretched-grid) of a 3-D global chemical transport model (UMD-CTM), and a numerical algorithm to account for radiative effects of aerosols in a regional air quality model (MODELS-3/CMAQ). The results from two distinct case studies (an ozone/aerosol pollution episode and a convection event) using the SCCTM showed a strong sensitivity of model calculations to convection and aerosol optical depths in the ozone/aerosol pollution episode, as well as increases in upper tropospheric ozone due to deep convection over the central U.S. The maximum instantaneous radiative forcing of the ozone increase reached 0.75 W/m2 at the tropopause level. The radiative forcing of observed aerosol in the east-coast U.S. pollution episode reached −100 W/m2 near the surface. The results from the MODELS-3/CMAQ simulations for the same ozone/aerosol pollution episode show a variety of radiative effects of aerosols on ozone photochemistry depending on aerosol optical and physical characteristics, as well as precursor concentrations. Global simulations were conducted using two different versions of the 3-D global UMD-CTM. An evaluation of the uniform-grid UMD-CTM shows that the model calculations are in reasonable agreement with numerous sets of observations from a variety of regions and years. Comparisons with the observations over the central U.S. indicate that the results provided by the stretched-grid UMD-CTM are in better agreement with the observations, both in terms of location and timing of the convection and in terms of chemical mixing ratios. The results show that the central U.S. convection increases precursor mixing ratios and results in increases in photochemical ozone production in the upper troposphere. The 3-D global model results suggest regional increases in radiative forcing by upper tropospheric ozone resulting from deep convection. Accurate representation of the interaction among particles, trace gases, radiation, and convection is essential to global climate models.