Fig. 1. New particle formation mechanisms over the pack ice shown for 17 September 2018.

a Evolution of particle size distribution; also shown are the iodic and sulfuric acid monomer concentration measured with a nitrate chemical ionization mass spectrometer. b Negative-ion size distribution from neutral cluster and air ion spectrometer measurements and naturally charged sulfuric acid and iodine clusters measured with the negative atmospheric pressure interface time of flight (APi-TOF) mass spectrometer. The legend indicates the number of iodine atoms per cluster where clusters with the same number of iodine atoms were summed up. The concentration is given in counts per second (cps). Grey shaded areas indicate periods with fog (here associated to a visibility below 2 km). c Mass defect plot of the negatively charged ions measured with the APi-TOF. The size of the markers is proportional to the logarithm of the concentration. The iodine clusters reported in the mass defect plot are the same as shown in the bottom panel of Fig. 1b. Squares indicate peaks for which it has not been possible to unambiguously identify their chemical composition, however their mass defect is compatible with iodine containing species.