In the present study, we investigated the correlation between the hydroxyl radical formation rate (R.(OH)) and the degradation of a pesticide (mesotrione) in synthetic cloud water solutions and in two real atmospheric cloud waters collected at the top of puy de Dome station (France). Using terephthalic acid as the hydroxyl radical chemical probe, we established the linear correlation between the photogenerated hydroxyl radical under polychromatic wavelengths and the pesticide degradation rate: R-meso (M s(-1)) = (1.61 +/- 0.15) x 10(-1) R.(OH) (m s(-1)). Moreover, the formation rate of hydroxyl radical in two natural cloud waters was estimated considering H2O2 and NO3- and the difference between the predicted values and those experimentally obtained could be attributed to the presence of other photochemical sources: iron-complexes and total organic matter. The organic constituents could play a dual role of sources and scavengers of photoformed hydroxyl radicals in the aqueous phase.