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The diffusion of OH, H[O.sub.2], and [O.sub.3] in He, and of OH in air, is examined by using a coated-wall flow tube reactor coupled to a chemical ionization mass spectrometry. It is shown that measured values for OH and H[O.sub.2] are in better agreement with measured values of their polar analogues ([H.sub.2]O and [H.sub.2][O.sub.2]) as compared to measured values of their nonpolar analogues (O and [O.sub.2]).

DFT and ab initio molecular orbital calculations are performed to examine the structural and energetics of the Cl-O2-isoprene peroxy radicals arising from the CL-initiated oxidation of isoprene. The findings indicate the isomeric branching between the six CL-O2-isoprene peroxy radicals play an important role in determining the reaction pathways and final product distributions of the Cl-isoprene reaction system.

Optical microscopy and differential scanning calorimetry are used to investigate heterogeneous freezing of aqueous particles with the morphology of solid inclusions of crystallized ammonium sulphate, ice and letovicite. Results revealed that the surface area and surface microstructure of crystalline solids present in aqueous aerosols could significantly change the heterogeneous freezing temperature and critical ice saturations.

The freezing behavior of aqueous nitric acid solutions was investigated in order to elucidate the formation mechanism of solid polar stratospheric clouds. The nucleation rate of the nitric acid hydrates was found to depend predominantly on the saturation ratio of the liquid with respect to the solid.

The quantum yields of O[(super 1)D] from photolysis of ozone (O3) were measured in the wavelength range 295-338 nm at temperatures of 226-298 Kappa using chemical ionization mass spectroscopy (CIMS). It is estimated that the utilization of quantum yields data leads to a significant enhancement in the calculated O[(super 1)D] and thus OH production in the lower atmosphere, especially at high solar zenith angles.

It has been possible to identify the conditions needed for homogeneous extraction of ice in (NH4)2SO4-H2O particles. It is concluded that both homogenous and heterogeneous nucleation are significant in upper tropospheric cloud formation.

The thermodynamics and the phase transition kinetics of aqueous NH4HSO4 solutions 0-60 wt% has been investigated using differential scanning calorimetry.

The differential scanning calorimetry was used to study the transition of emulsified binary HNO3/H2O and HNO3/H2SO4/H2O acid solutions from the liquid to the solid phase to determine the mechanism of ice formation from stratospheric aerosols. Results showed that binary solution droplets with less than 33 wt% HNO3 turn in to ice ar 37-65K, while those with higher concentrations form glasses instead of freezing. Results also indicated that the formation of ice particles from ternary aerosols at stratospheric temperatures require an ice saturation ratio of over 1.65.

The mass accommodation coefficient of H2SO4 vapor on aqueous sulfuric acid and the gas-phase diffusion coefficient of H2SO4 vapor in N2/H2O at 300 K was obtained empirically. Measurements were made under laminar flow conditions in a coated wall tubular flow reactor coupled to a chemical ionization mass spectrometer for gas-phase detection. A kinetic model based on the additivity of kinetic resistances that relates gas-phase diffusion and mass accommodation to the measured H2SO4 vapor loss rate, was applied on the data.

A new optical microscope methodology was developed to study phase transitions in micrometer-sized droplets. This method was utilized to analyze the nucleation of ice from aqueous H2SO4 aerosols0-35 wt% in composition in the temperature range from 273 to 170 K. A thermodynamic model was employed to apply the new freezing temperature data to the formation of clouds in the upper troposphere and lower stratosphere.

A surface coating dilution technique is employed to explain the impact of relative humidity on the OH uptake of surfaces that are of extreme atmospheric importance. The OH uptake is shown to be highly dependent on the water vapor, blocking of anions and the changes in pH of the surfaces.

A detailed investigation of the interaction of HCl with a variety of ice surfaces shows that the nature of the HCl-ice interaction is critically dependent on the characteristics of the ice substrate.

A systematic examination was conducted on binding of molecular HCl at a variety of surface sites on crystalline nitric acid trihydrate (NAT) using density-functional-plane-wave-based and localized computational methods. It was found out that NAT(001) and (001) faces are thermodynamically the most stable after optimizing six low index faces but only one surface site on the (001) face, exhibits a high affinity for HCl.

A study of the binding of molecular HCl to a number of extended surface models of the basal (0001) face hexagonal ice in addition to a cluster model representative of an unreconstructed adsorption site is presented. The results show that the binding energy and enthalpy of molecular HCl on hexagonal ice crystals at 165-200 K are 23 and 25 kJ/mol, for low HCl coverage, decreasing to 20 and 22 kJ/mol, as the coverage of HCl increases as these calculations are based on low surface density of dangling OH groups.

A study investigated the branching ratio of the reaction of OH with ClO conducted at pressures between 100 and 200 Torr and at a range of temperatures extending to those found in the lower stratosphere using a novel, laboratory-developed technique. It also conducted statistical rate theory calculations on the OH + lO reaction system. Experimental and theoretical calculations were compared. Experimental methods, findings and conclusions are discussed.