Various in situ measurements show evidence of new particle formation over a wide range of latitudes in the middle to upper troposphere and lower stratosphere. However, the exact mechanism of new particle formation is still uncertain. Using a combination of satellite-derived brightness temperature, air parcel backward trajectory information, in situ measurements of aerosol and precursor gases and an aerosol microphysical model driven by parameterized ion-induced nucleation, we investigate the mechanism responsible for, and factors leading to, new particle formation in the middle to upper troposphere during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment. The number concentrations of modeled ultrafine particles of diameters 3 to 4 nm (N3--4nm) and 3 to 8 nm (N3--8nm) are consistent with in situ measurements indicating that new particle formation likely occurred by ion-induced nucleation a day prior to the time of measurement. A reduction in preexisting aerosol surface area in a region of cloud outflow probably triggered particle nucleation. These studies indicate that under typical middle to upper troposphere conditions, the ion mechanism is likely an important source of ultrafine particles, and these newly formed particles can grow to act as cloud condensation nuclei.