A nucleus counter was operated continuously at a field site during the 20-day period surrounding the political time change from Eastern Daylight Saving Time to Eastern Standard Time. A careful calculation shows that the time of diurnal rise did not change as a result of the 1 hr shift of human activity. This indicates that, in sparsely populated areas, the conversion of gases to particles by solar radiation is a major source of Aitken nuclei and the dominating influence in the diurnal trend. © 1968.