The surface ionization of sodium and potassium salt particles with diameters of 1-100 nm is studied at atmospheric pressure. Particles impact on a resistively heated and positively biased Pt filament. The particles melt and dissociate at the filament and alkali ions are emitted, giving rise to a positive current at a closely situated collector. Salt particles with diameters below 5 nm are concluded to melt and ionize completely at the filament. Larger particles melt and ionize partially, yielding an ion signal that is proportional to the particle surface area. The ionization efficiency decreases with increasing particle size and reaches values around 1% for 0.1 mu m particles, with small variations depending on the stability of the alkali salt.