Preliminary interpretation of pH and sodium concentration data for marine rains collected during February and July, 1981 at American Samoa (14°15′S, 170°34′W) as part of the Sea/Air Exchange (SEAREX) Program indicate that rain is usually slightly more acidic than would be expected on the basis of atmospheric CO2‐pure water equilibrium, in this remote region of the tropical South Pacific Ocean. The data are consistent with calculations of pH using the alkalinity equation for "model" solutions containing only sea‐salt and traces of strong acids in equilibrium with atmospheric CO2. Alkalinity due to sea‐salt appears to affect pH significantly only when the salinity of the rain is greater than 0.01 ‰ or 3000 µg Na/kg. The observed range of pH values is much smaller than has been observed recently at subtropical marine sites: this, along with the relatively high average pH value, suggests that climatic factors influence levels of acidity in rain over remote ocean areas.
