Aerosol samples were collected from Punta del Hidalgo in the Canary Islands and Sagres in Portugal during June--July 1997 as part of the Aerosol Characterization Experiment-2 (ACE-2) over the North Atlantic Ocean. We studied individual aerosol particles using transmission and scanning electron microscopy. The major aerosol types include fresh and partly or completely reacted sea salt that consist of NaCl, mixed-cation (Na, Mg, K, and Ca) sulfate, Na2SO4, and NaNO3; particles of industrial origin that include (NH4)2SO4, soot, flyash, silica, Fe oxide, and CaSO4; and minor terrestrial mineral dust. Because of their different geographic locations, samples from these two sites showed different degrees of impact from anthropogenic emissions from Europe. At Sagres, depending on the samples, between 0 and 30% of sea salt particles remain unreacted, while 0 to 50% were partly reacted, and 20 to 100% were completely converted to sulfate and nitrate through reactions with pollutants from the European continent. In contrast, at Punta del Hidalgo, the sea salt particles were much less affected by industrial pollution. Even during polluted periods, only less than 5% were completely reacted. The dramatic difference between the sea salt particles from Punta del Hidalgo and Sagres indicates dilution of pollution, rapid reaction, and exhaustion of reactive pollutants as they were transported to the open ocean.