Gas exchange and CO2 flux in the tropical Atlantic Ocean determined from 222Rn and pCO2 measurements

Measurements of 222 Rn vertical profiles and p CO 2 in the surface water and the atmosphere were made simultaneously in the tropical Atlantic ocean as part of the TTO/TAS program. The gas exchange rate or piston velocity was determined from the 222 Rn profiles, and the Δ p CO 2 between the surface ocean and the atmosphere was determiend from the p CO 2 measurements. The net flux of CO 2 across the sea‐air interface was calculated from these two data sets. The piston velocity ranged from 1.4 to 6.9 m/d and was correlated with wind speed. The slope of piston velocity versus wind speed was estimated to be between 0.3 and 1.1 (m/d)/(m/s). The Δ p CO 2 ranged from −35 μatm at 15°N, 55°W to +64 /zatm at 5°S, 28°W, with the zero Δ p CO 2 isopleth located at about 10°N. The high Δ p CO 2 values can be explained by lateral advection of surface water from the east with heating and biological consumption of CO 2 and alkalinity during transit. The net flux of CO 2 was into the ocean north of 10°N latitude with values reaching a maximum of 1.4 mol m −2 yr −1 at 15°N, 50°W. South of 10°N, the net flux was out of the ocean, reaching a maximum value of 2.7 mol m −2 yr −1 at 8°S, 28°W. The average net flux from 10°N to 10°S was 1.3 mol m −2 yr −1 out of the ocean, which is equivalent to 0.15 gigatons of carbon per year if the flux determined applied throughout the year.