Both the AMS and the ATOFMS have been key to the analysis of ambient aerosols. For nucleation mode particles under 100 nm, there has been relatively less study. Field-portable instruments are critical to gain direct insight into the composition of particles in the natural environment, and these same instruments are also useful for sampling from well-controlled atmospheric simulation experiments such as CLOUD. The APiTOF with various CI sources is widely used to determine the composition of neutral sub-3 nm particles, and without CI, it can detect naturally charged species in the atmosphere. These measurements have provided invaluable data for ongoing debates about the relative roles of neutral and charged NPF, suggesting that neutral species outnumber charged ones by around an order of magnitude, but that charged particles are associated with faster growth rates. From field studies that used the APiTOF, we have a better understanding of the role of a wide array of highly oxygenated organic molecules (HOMs) in NPF,(13) mechanism of volatile organic compounds formation, the observation that nitrogen oxide-containing species such as organo-nitrates drive nighttime particle formation, or to being to pick apart the complicated role of diverse biogenic vapors in particle growth and aging. Here, the ability to unambiguously determine the chemical composition of new particles is critical to understanding what chemical properties are associated with growing particles. In all of these measurements, it is important to consider the effects of fragmentation in the atmospheric pressure interface as described above as well as the different transport efficiencies of particles of widely varying masses, both of which can bias interpretation of the mass spectra.