A better understanding of hydroxyl radical photochemical sources in cloud waters collected at the puy de Dôme station – experimental versus modelled formation rates

The oxidative capacity of the cloud aqueous phase is investigated during three field campaigns from 2013 to 2014 at the top of the puy de Dôme station (PUY) in France. A total of 41 cloud samples are collected and the corresponding air masses are classified as highly marine, marine and continental. Hydroxyl radical (HO•) formation rates (R_HO•^f) are determined using a photochemical setup (xenon lamp that can reproduce the solar spectrum) and a chemical probe coupled with spectroscopic analysis that can trap all of the generated radicals for each sample. Using this method, the obtained values correspond to the total formation of HO• without its chemical sinks. These formation rates are correlated with the concentrations of the naturally occurring sources of HO•, including hydrogen peroxide, nitrite, nitrate and iron. The total hydroxyl radical formation rates are measured as ranging from approximately 2 × 10^−11 to 4 × 10^−10 M s⁻¹, and the hydroxyl radical quantum yield formation (Φ_HO•) is estimated between 10^−4 and 10⁻². Experimental values are compared with modelled formation rates calculated by the model of multiphase cloud chemistry (M2C2), considering only the chemical sources of the hydroxyl radicals. The comparison between the experimental and the modelled results suggests that the photoreactivity of the iron species as a source of HO• is overestimated by the model, and H₂O₂ photolysis represents the most important source of this radical (between 70 and 99 %) for the cloud water sampled at the PUY station (primarily marine and continental)