Research that was supported by this contract has contributed substantially to progress in our understanding of new particle formation in the atmosphere. Objectives included the development of new measurement methods, the application of those new instrument systems in atmospheric field studies, and the interpretation of results from those studies. We developed the "Nano TDMA" to measure the hygroscopicity and volatility of 4-20 nm particles. We used this instrument system to characterize properties of atmospheric particles in the Atlanta atmosphere in July/August 2002 as well as to study properties of diesel exhaust particles. We also developed the thermal desorption chemical ionization mass spectrometer (TDCIMS) to measure the chemical composition of nanoparticles as small as 7 nm with a time resolution of 10-20 minutes. The TDCIMS is currently the only instrument that can perform such measurements. Atmospheric field measurements were carried out in Atlanta (July/August 2002; we refer to this as the ANARChE study) and in Boulder, CO (2003/04). In the ANARChE study we measured, for the first time, the composition of freshly nucleated particles as small as 7 nm using the TDCIMS. The ANARChE study also included the first nano-TDMA measurements of the volatility and hygroscopicity of freshly nucleated particles as small as 4 nm. Other parameters that were measured included particle size distributions (3 nm-2 μm), and sulfuric acid and ammonia concentrations. Key discoveries from the ANARChE study are: (1) freshly nucleated particles in Atlanta consist primarily of ammonium and sulfate; evidence for significant amounts of other species such as organics and nitrates was not found; (2) new particle formation occurs when rates of cluster loss to preexisting particles are small compared to rates of lost to the next larger cluster size by growth; a dimensionless parameter L describes the ratio of these rates, and measurements showed that new particle formation was always observed when L was less than one and not when L was greater than one; (3) growth rates of freshly nucleated particles could be explained by condensation of sulfuric acid and coagulation of the newly formed nucleation mode in the mornings when particles were small (