Iron photochemistry in cloud droplets at the Puy de Dôme

Iron play a significant role in atmosphere. Its reactivity depends on oxidation state and speciation. ln order to study photochemical cycle of iron in atmospheric waters, we collected cloud samples at the research station of puy de Dôme. We found very low levels: iron concentrations are ranging from 0.2 $\mu$M to 3.4 $\mu$M. The light is not an important factor towards affecting the oxidation state, in contrast with many field studies, where clear dependence from light intensity was shown. Contrary to recent studies and to chemical models iron in the cloud droplets at Puy de Dôme is mostly present as Fe(II): the ratio Fe(II)/Fe$_{\rm TOT}$ is 0.77 on average. No clear relationship is found between Fe and carboxylic acids. Same samples are irradiated between 300 and 450 mn in a Teflon-made reactor: H$_2$O$_2$, TOC, Fe$^{2+}$, Fe$^{3+}$ and pH are continuously monitored during the irradiation. We observed no tendency for the Fe(II)/Fe$_{\rm TOT}$ ratio to increase or decrease and the DOC is not strongly varying during the irradiation. In consequence of the stability of the Fe(II)/Fe(III) ratio, we can assume that cloud water contains compounds that undergo the action of oxidants, and possibly, organic substances play an important role in this processus