We have made long-term observations of particle size distributions in a semi-rural continental environment of the United States (Kent, Ohio) over a 4−year period, to investigate the seasonal trend of new particle formation (NPF). NPF events occurred throughout the year, but more frequently during the spring (∼40%) and fall (∼32%) than in the summer (∼19%) and winter (∼17%). The particle nucleation rates were also highest in the spring (with the median and standard deviation value of 6.8±1.3 cm−3 s−1) and lowest in the winter (1.0±0.9 cm−3 s−1). Particle growth rates were highest in the summer (6.4±0.4 nm h−1) and lowest in the winter (4.2±0.3 nm h−1), a trend different from nucleation rates, indicating that different chemcial species may be involved in particle nucleation and growth processes. Backward trajectory calculations show that NPF events were often associated with air masses originated from the east-southeast where numerous large size power plants are located, containing high sulfur dioxide (SO2). The NPF events in general had lower relative humidity, lower surface area of pre-existing particles, and higher solar radiation.