Monthly averaged size distributions for a) hours with rBC <0.71 cm^-3 and b) hours with rBC >0.71 cm^-3, where 0.71 cm^-3 is the median of the rBC measurements over JJAS. Green represents June, red is for July, black is for August and blue is for September. The number of hours and days in the averages are indicated in the legends. The shaded sections in a) indicate the range of the activation diameters for particles nucleating cloud droplets assuming a hygroscopicity parameter²⁶ range of 0.2–0.5 and an updraft speed of 0.4 m s^-1. The updraft is based on spring observations of clouds over north Alaska²⁷, and the hygroscopicity range reflects particle chemistry dominated either by organics or by sulphates, as observed in summer Arctic elsewhere²⁸ and at Alert during March-May of 2011 (Supplemental Material Figure S1).