Transmission electron microscopy was used to examine the morphology and some physical and chemical properties of individual particles collected north of latitude 80 ¿N by impaction or electrostatic precipitation during the period July 21 to September 18, 1996. Particles smaller than 50 nm in diameter showed no evidence of the presence of sulfuric acid, previously believed responsible for new particle formation. Many were crystalline or semicrystalline often with pentagonal or hexagonal habit and more heat resistant than ammonium salts. Most were unaffected by decane or xylene vapor, but some were wholly or partly dissolved, indicating that they were chemically different. Sulfuric acid, ammonia, and probably methane sulfonic acid appeared to be mainly responsible for growth of these particles to the sizes that could become involved in cloud drop formation, about 80--100 nm in diameter. Much greater diversity was present in larger particles, the most numerous class of which usually appeared to be very variable mixtures of sulfuric acid, methane sulfonic acid, and their ammonium salts. All contained significant organic material. Particles containing sea salt were generally larger than 250 nm in diameter and contained an amazing variety of other material, much of it organic. On days with sunshine unusual numbers of particles <5 nm and in the range 10--50 nm in diameter occurred. At the same time, concentrations of three different types of particle with diameters of the order of 100 nm were enhanced. One group was liquid when collected, had a large organic content, and wet the collecting surface. The other two were bacteria and flat insoluble plates some of which contained fragments of diatoms. It is proposed that they were ejected into the air by bubbles bursting on the open leads. ¿ 2001 American Geophysical Union