Spatiotemporal variability of submicrometer particle number size distributions in an air quality management district

► Submicrometer particle number size distributions at urban, coastal, mountain and downwind site. ► Modest spatial heterogeneity and strong diurnal variability of the number size distributions. ► The variability is largely driven by traffic and to a lesser extent by secondary aerosol formation. ► Measurements with size discrimination and multiple sites are critical in a diverse environment.