Cluster analysis of roadside ultrafine particle size distributions

► Ultrafine particles were measured for five years near a major roadway. ► The k-means clustering algorithm was applied to particle size distributions (PSD). ► Eight PSD types were found to show different temporal patterns. ► The PSD types had differing correlations with NO, NO2, PM2.5 and wind speed. ► Physical interpretations of the conditions yielding each PSD type were elucidated.