The original MOUDI (Micro-Orifice Uniform Deposit Impactor) cascade impactor, reported in the literature in 1991, used an external gear system to achieve a uniform deposit and was intended for industrial hygiene studies with sample times in the range of minutes to a few hours. To facilitate much longer run times, a second generation MOUDI, MOUDI-II, which uses internal electric motors to rotate the impaction plates, was developed. Three model 120 MOUDI-IIs were used in a 32-month program to sample ambient atmospheric aerosols at several industrial and urban locations in Minnesota. For these sampling locations, each 120 MOUDI-II operated continuously for a minimum of five, and optimally 7 days, to collect a sample at a site. During these community sampling events, the three 120 MOUDI-IIs logged 4007, 2637, and 3230 h of operating time, respectively. A laboratory side-by-side comparison of the three 120 MOUDI-IIs showed good agreement amongst the three 120 MOUDI-IIs and, thus, the particle size distributions were independent of the 120 MOUDI-II used. Application of the 120 MOUDI-IIs for long-term ambient sampling was demonstrated by comparing size distributions from the background locations: Minneapolis, Duluth and Ely, Minnesota, representing urban, light industrial, and pristine area type of aerosols, respectively. PM2.5 averages from Minneapolis and Duluth compare well with three-year averages from state regulatory sampling.