Sea‐salt and the acidity of marine rain on the windward Coast of Samoa

Abstract

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.

Precipitation composition at Cape Grim, 1977-1985

“…For pure water in equilibrium with carbon dioxide at 20°C we calculated a pH of 5.67 compared with 5.65 obtained by Pzenny et al (1982). When we repeated their calculation for the mean composition of Samoan rain at 20°C we obtained a pH of 5.50, identical with their value of 5.50.…”

“…For example other southern hemispheric studies have reported mean rainwater sodium concentrations of 95 peq/l for 31 samples from Samoa (Pzenny et al, 1982), 225 peq/l for 79 samples from Amsterdam Island (Keene et al, 1986) and 483 peq/l for 33 samples from New Plymouth, New Zealand (Ayers et al, 1986a). The much higher sea-salt content of Cape Grim rainwater, evidenced by the mean sodium concentration of 1167 peq/l, can be attributed to the rather high wind speeds commonly encountered at the Cape (Table I), since it is well known that production of sea-salt aerosol increases with wind speed (Erickson et al, 1986).…”

“…Thus the volume weighted mean pH of 6.01 deduced from Table 4 is probably higher than that of truly maritime rain in the vicinity of Cape Grim. To get an indication of the pH that would be expected in the absence of local soil debris in the rainwater we repeated the calculation performed by Pzenny et al (1982), in which pH is calculated for a diluted seawater system on the assumption that total seawater alkalinity is a conservative property. Like Pzenny et al (1982) we included nitrate and excess sulfate in the calculation as strong acid anions, but we also made the minor addition of including ammonia in the calculation in analogy to the way in which the borate equilibrium is included.…”

Covariations in oceanic dimethyl sulfide, its oxidation products and rain acidity at Amsterdam Island in the Southern Indian Ocean

“…Table I presents the monthly average precipitation over the period March 1987-February 1988and February-November 1989 Rainfall is frequent and fairly equally distributed throughout the year. To ascertain if the seasonal variation in the pH could be explained by a higher neutralization during winter when high winds are observed, we repeated the calculation performed by Pszenny et al (1982) in which the pH was calculated for a diluted seawater system assuming total seawater alkalinity to be a conservative property. Column 6 indicates the H + calculated assuming that the carbonic acid and the bicarbonate in the rainwater are in equilibrium with the atmospheric CO 2.…”

Methanesulfonic acid and non-sea-salt sulfate in Pacific air: regional and seasonal variations

“…To avoid contamination from local sources, the air samplers were controlled by wind speed and direction sensors that activated the pumps only when the winds blew from the sea at speeds of greater than 5 km/hr (Uematsu et al, 1983). Detailed descriptions of the sites at Shemya, Midway and Fanning were reported by Uematsu et al (1983) and at American Samoa by Pszenny et al (1982). At Norfolk Island, the sampler is located on the southeast coast on the edge of a steep slope, 80 m above sea level The New Caledonia site is located on the beach near Yate on the east coast.…”