Aerosols and new particle formation were studied in the western part of the Yangtze River Delta (YRD), at the SORPES station of Nanjing University. Air ions between 0.8 and 42 nm were measured using an air ion spectrometer; a DMPS provided particle size distributions between 6 and 800 nm. Additionally, meteorological data, trace gas concentrations, and PM_2.5 values were recorded. During the measurement period from 18 November 2011 to 31 March 2012, the mean total particle concentration was found to be 23 000 cm^−3. The mean PM_2.5 value was 90 μ g m^−3, well above national limits. During the observations, 26 new particle formation events occurred, typically producing 6 nm particles at a rate of 1 cm^−3 s^−1, resulting in over 4000 cm^−3 new CCN per event. Typical growth rates were between 6 and 7 nm h^−1. Ion measurements showed the typical cluster band below 2 nm, with total ion concentrations roughly between 600 and 1000 cm^−3. A peculiar feature of the ion measurements were the heightened ion cluster concentrations during the nights before event days. The highly polluted air of the YRD provides both the potential source (SO₂) and the sink (particulate matter) for sulfuric acid, leaving radiation as the determining force behind new particle formation. Accordingly, a good correlation was found between new particle formation rate and radiation values.