The paper presents observational evidence from balloon measurements of two water vapor transport mechanisms in the TTL. The two mechanisms, horizontal in-mixing from the extra-tropics and vertical overshooting convection, lead to hydration of the tropical lower stratosphere. The results are supported by trajectory model simulations and remote sensing observations. The paper is well written and easy to follow, and the results shed light on the TTL transport processes from an observational perspective. I recommend publication in ACP after the following minor suggestions and comments are addressed. C1
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