The second Ice Nuclei Spectroscopy (INSPECT-II) campaign was conducted at Storm Peak Laboratory in northwestern Colorado in April and May 2004. The physical and chemical characteristics of springtime atmospheric aerosols, including those which act as heterogeneous ice nuclei (IN), were investigated. The ice formation activity of submicron particles was measured with a continuous-flow diffusion chamber. The concentrations of heterogeneous ice nuclei () active at -15 to -50¿C and water supersaturations -25% < SSw < 0 typically ranged from 1 to 10 std l-1. Aerosol mass spectrometry measurements indicated that the composition of IN had much higher contributions from mineral dust/fly ash and metallic particle types compared to ambient particles of similar sizes. While IN concentration and composition measurements are similar to observations from the same site during INSPECT-I in November 2001, there was considerably more variability in during INSPECT-II associated with periods of sporadically high . A relation of to concentrations of larger particles was noted; however, the predictive utility of such a relationship proved limited during non-dust-related increases in accumulation mode number concentrations. We conclude that the observed high variability and extremes in during spring are attributable primarily to variations in airborne dust concentrations and that the observed in spring 2004 represent lower bounds on the expected at SPL in the spring. On the basis of modeled dust vertical profiles, strong impacts of dust on might be expected at the higher altitudes of cold clouds.