The importance of radical-water complexes in the atmosphere is explored in this dissertation. Radicals, although present in small concentrations in the atmosphere, play a significant role in creating and removing atmospheric pollution. As the atmosphere warms and consequently gets wetter, it is essential to understand the effects of water vapor on radical chemistry. This dissertation reports studies on the effects of water vapor on the kinetics of the self-reaction of β-hydroxyethyl peroxy radical (β-HEP), a prominent organic peroxy radical in the atmosphere. Both experimental and computational studies have been performed to examine the effects of water vapor on the kinetics of the self-reaction. The influence of water vapor and temperature on the reaction rate constant is presented. The rate of the self-reaction increases between 2 to 6 times with an increase in water vapor and decrease in temperature. The products of the self-reaction in the presence and absence of water vapor have been computed using high level ab initio calculations. Major products include alkoxy radicals, peroxides, aldehydes, alcohols and oxygen. A new reaction pathway leading to formation of hydroperoxy radical (HO2) from the self-reaction of β-HEP in the presence of water vapor was identified. In the presence of high NOx concentration HO2, forms tropospheric ozone, which is classified as a harmful pollutant by the Environmental Protection Agency (EPA). Like tropospheric ozone, aerosols are also classified as harmful pollutants by the EPA. Sulfuric acid-water complexes are estimated to be the primary reason for new aerosol formation in the atmosphere. However, the sulfuric acid concentration in the atmosphere alone is not sufficient to account for observed aerosol concentrations. Classical nucleation theory is used to explain new particle formation (NPF), which is initiated by the formation of a nucleating site (a highly polar complex). This dissertation explores the role of various radical-molecule complexes acting as the nucleating site. Experimentally, the HO2-water complex is studied as a possible nucleating site for NPF. A new instrument was developed to create and measure radical-water complex initiated particle formation. The instrument incorporates two scanning mobility particle sizers (SMPS) to measure the size distribution and number density of the aerosol particles formed. The experimental setup uses UV absorption spectroscopy and wavelength modulated spectroscopy to measurethe HO2 radical and water vapor concentrations in the reaction cell. No significant particle formation was observed at room temperature and pressure. Particle formation from the HO2-water complex, may occur at lower temperatures. Additional radical-molecule complexes have been studied computationally in an effort to identify other possible nucleating sites for particle formation. In particular, the complexes of sulfuric acid, nitric acid, acetic acid and formic acid with ammonia, amidogen radical (NH2) and imidogen radical (NH) have been studied. H2SO4-NH2 and HNO3-NH2 complexes show the potential to act as nucleating sites for formation of aerosol particles in the atmosphere. In summary, water mediated chemistry plays a significant role in the atmosphere and must be included in scientific models to better predict pollution levels in the atmosphere.