Liquid-air interface (LAI) corrosion is regarded as a potential issue for interim storage tanks of high level liquid radioactive waste. Insightful studies have been carried out and several hypotheses have been proposed so far, while none has been directly confirmed experimentally. A sealed container has been developed to investigate the effect of atmosphere on LAI corrosion of partially immersed A537 steels, in which the composition and temperature of air can be changed accordingly, i.e. CO2 concentration varied between 0.05 wt% (natural air) and 1.97 wt%, and temperature varied between 25 degrees C and 34 degrees C. It has been found that the pitting initiation process under both anodic polarization and free immersion conditions can be largely accelerated by the increase of CO2 concentration. More importantly, a planar pH electrode has been applied to measure pH at the interface during anodic polarization and free immersion, and the occurrence of LAI corrosion can be correlated with the same interface pH at a certain temperature regardless of CO2 concentration, indicating a "critical pH" for LAI corrosion determined by this method. This finding suggests that LAI corrosion in high level liquid radioactive waste may be investigated through determination of a critical pH in certain circumstances. (C) The Author(s) 2019. Published by ECS.