I present here an improved three-dimensional numerical model of atmospheric chemistry and transport for use in studies of global biogeochemical cycles, and assessments of human impacts on the natural atmosphere.

The University of California at Irvine (UCI) Chemistry Transport Model (CTM) simulates atmospheric transport and transformation of trace chemicals as they are emitted from biota, soils, oceans, or through anthropogenic activity, and is a useful tool for studies of the Earth system.

I conduct experiments that test CTM stratospheric transport and identify shortcomings in the meteorological data used to drive this version of the CTM. I evaluate the potential climatic impact of future High Speed Civil Transport (HSCT) aircraft, and investigate the hypothesis of extraterrestrial water influx from "small comets", showing that such influx must be limited to 2 Tg H2O/yr, or 100 times less than proposed.

I describe new parameterizations for turbulent mixing in the atmospheric boundary layer (ABL) and wet/dry deposition of gases and aerosols. ABL mixing is accomplished through use of a nested grid upon which turbulent fluxes are calculated. The new parameterization improves CTM comparison with surface observations of 222Rn and 210Pb. Dry deposition of aerosols and gases uses a
"resistance-in-series" scheme, which improves dry deposition fluxes slightly compared to a base case using fixed deposition velocities. Four different methods for parameterizing wet deposition are considered, with schemes based on a
"scavenging efficiency" and the surface precipitation rate best matching observed
wet deposition fluxes.

These new parameterizations, along with an improved algorithm for wet deposition of gases and a comprehensive photochemical scheme, are incorporated into the CTM for study of the oxidation of dimethylsulfide (DMS). I compare
CTM results against surface observations and vertical profiles of selected trace gases of interest, including oxidation products of DMS such as non-sea-salt sulfate and methanesulfonic acid (MSA). While the CTM accurately simulates the global distribution of DMS, there are significant differences between observations and
modeled concentrations of some important species (e.g. H2O2 and surface O3).

Further studies are needed to improve model comparison with observations and understand sensitivity to model parameterizations.

Complete text available at http://essgrad.ps.uci.edu/~hannegan/papers/