The formation of new particles from gas phase condensation has been shown to significantly enhance concentrations of cloud condensation nuclei (CCN) in the Earth's atmosphere. Particles that have grown to CCN size contribute significantly to Earth's radiation balance. And particle nucleation has been observed throughout the atmosphere in varying meteorological conditions. Yet the chemical processes involved in particle nucleation are not well understood. Sulfuric acid has long been recognized as a contributor to new particle formation. However, sulfuric acid condensation alone cannot account for high particle production and growth rates observed in many regions of the atmosphere. Scientific understanding of these processes has been limited by available instrumentation and the chemical complexity of the atmosphere. A novel Chemical Ionization Mass Spectrometer (cluster-CIMS) has been developed, in collaboration with colleagues at the National Center for Atmospheric Research, to characterize homogeneously nucleated molecular clusters produced in a controlled laboratory environment. The cluster-CIMS gently ionizes neutral molecular clusters for quadrupole mass filtration with a minimum of disturbance to cluster composition. It is capable of characterizing particles from molecular sizes up to 1.5 nm in diameter with a resolution of +/− 1 amu. A climate controlled photochemical reaction chamber, designed as a 1000 L batch reactor, was built to produce nucleated molecular clusters at atmospherically relevant conditions. Laboratory experiments were conducted to elucidate potential molecular candidates for particle nucleation. The role of amines in particle formation was experimentally examined after atmospheric observations revealed enhanced sulfuric acid nucleation rates in the presence of amine compounds. Experimental results obtained with the cluster-CIMS, as well as other aerosol instrumentation, support the hypothesis that amines enhance sulfuric acid nucleation rates.