Radon loss across the water‐air interface (Gulf of Thailand) estimated experimentally from ²²²Rn‐²²⁴Ra

We used 222 Rn and 224 Ra measured in the Upper Gulf of Thailand to estimate 222 Rn atmospheric evasion across the water‐air interface. The Chao Phraya estuary represents a steady‐state source of these tracers and we looked at their horizontal distribution on a transect leading away from the estuary into the Gulf. The isotopes 222 Rn and 224 Ra have very similar half‐lives and are affected in the same manner by mixing processes but only radon will emanate to the atmosphere. We thus are able to estimate the radon air‐water exchange rate from the difference in the slopes of the 222 Rn and 224 Ra horizontal distributions. Estimated gas exchange velocities ( k ) based on our results were 1.1 cm hr −1 in the dry season (January 2004) and 2.1 cm hr −1 during the wet season (July 2004). These rates agreed reasonably well with some theoretical models developed for lakes, estuaries and coastal systems.