We have studied the size distribution of 210Pb aerosols collected by a cascade impactor during a cruise from Marseille (France) to Amsterdam Island (South Indian Ocean). It was shown that 70 to 90% of the activity is measured in particles with a diameter smaller than 1.2 &mgr;m either in a continental atmosphere or in open ocean. This result confirms that 210Pb is a good tracer for aerosols produced by gas-to-particle conversion.