We report on enhanced levels of 3--5 nm particles in the remote summer Arctic. Simultaneous increases in particle number occurred in certain size ranges 100nm, marine in origin, were also present. Stable airmasses with at least 4 days residence over the ice, a surface mixed layer <140m deep, capped by a temperature inversion and cloud-free stable layer ~1km in depth excluded a tropospheric source. Instead a surface source was indicated. The most vigorous nucleation was associated with sudden reductions of humidity (<80%) causing rapid dissipation of fogs. However, particles <50nm contained no detectable H2SO4 implying recent formation or growth from material other than the acid. It is proposed that the marine particles were derived from bubbles bursting on open leads and provided the material for both nucleation and larger particle formation. Nucleation is attributed to oxidation of an amino acid, L-methionine. ¿ 1999 American Geophysical Union