The heterogeneous reaction of NO2 with water on diverse surfaces is broadly considered as a possible source of atmospheric HONO in dark conditions, but the associated mechanisms are not fully understood. We report data from first-principles simulations showing that the lifetime of the putative reactive NO2 dimer on the surface of pure water droplets is too small to host the whole process. One infers from our results that the hydrolysis of NO2 in clouds must be catalyzed by organic or inorganic species adsorbed on the droplets.