Can a Single Water Molecule Really Catalyze the Acetaldehyde plus OH Reaction in Tropospheric Conditions?

In recent publications there has been considerable speculation about the possible role of a single, water molecule in the catalysis of reactions between organic volatile compounds and OH radicals. In this work we reanalyze the effect of water in the acetaldehyde + OH reaction, using quantum chemistry and computational kinetics calculations in a pseudosecond order mechanism, at average atmospheric water concentrations and temperatures. We show that one water molecule definitely does not accelerate the acetaldehyde + OH reaction under atmospheric conditions. The apparent rate coefficient is considerably smaller than the one in the absence of water, regardless of the method of calculation. According to present results, the possible role of water molecules has been overestimated and new experiments are needed.