The size and composition of individual atmospheric particles were assessed using a single particle aerosol mass spectrometer in Ningbo, China, from December 30, 2016, to January 12, 2017. The particles primarily comprised carbonaceous and inorganic particles. In total, 60% of the particles were carbonaceous, thus making them the principal particles. All particle types included internally mixed secondary species, such as nitrate and sulfate. Temporal trends of particle number concentrations and particulate matter with an aerodynamic diameter of less than 2.5 μm (PM2.5) mass concentrations demonstrated that secondary formation in Ningbo had a severe effect on particle pollution. The sampling period was divided into three subperiods based on PM2.5 mass concentrations. In period I, local emissions and secondary formation primarily contributed to pollution. During periods II and III, particle abundance under high wind speed indicated that regional transport was a primary source of the two pollution processes. During the sampling period, high PM2.5 mass concentrations were often observed under high relative humidity and low temperature conditions at night. The results showed that stagnant meteorological conditions increased pollution in winter. This study enhanced the understanding of the size and composition of individual particles in the southern Yangtze River Delta region and provided useful information on the formation and growth of atmospheric aerosols.