The chemical composition of marine aerosols was studied during the Flux, Etats de la mer, T¿l¿d¿tection en Conditions de fetch variable (FETCH) experiment that took place in the northwestern Mediterranean Sea in March/April 1998. Coastal marine aerosols were collected using a 13-stage low-pressure impactor and analyzed by ion chromatography. Samples were characterized according to their continental or marine origin in order to better understand the processes affecting the chemical composition of particles during advection over a coastal zone. Strong anthropogenic influence was observed up to 100 km from the coast illustrated by (1) NH4/nss-SO4 association with ratios ranging from 0.9 to 2.1, and for some cases NH4/nss-SO4-NO3 bicorrelation; (2) chloride depletion associated with supermicron NO3, indicating previous reaction between nitric acid and NaCl; and (3) elevated NO3 fraction in the submicron range when the air mass was rain-washed. New particle formation was not detected during the campaign from either bubble bursting or from dimethylsulfide oxidation products. Nss sulfate and methanesulfonic acid appear to condense on the numerous preexisting particles rather than undergoing gas-to-particle conversion. Formation of sea-salt supermicron aerosol takes place rapidly to represent 80% of the total ionic species as little as 25 km from the coast. ¿ 2001 American Geophysical Union