A single water molecule is known to significantly lower the potential energy barrier in the atmospheric reactions of hydroxyl radical with volatile organic compounds but except at extremely low temperatures, it has never been observed to accelerate the reactions investigated so far. This quantum-mechanical computational work reveals a probable case of observing a single water molecule accelerating the atmospheric reactions of Trifluoroacetic acid and Trifluoroacetaldehyde at temperatures near 200 K under tropospheric conditions. (C) 2017 Elsevier B.V. All rights reserved.