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A Light Driven Burst of Hydroxyl Radicals in Newly Formed Cloud Droplets from Reaction Between Iron(II) and Organic Peroxides


One of the most uncertain aspects of the climate system are aerosol particles and their interaction with clouds. Aerosol aging in the aqueous phase often involves reactive oxygen species such as hydroxyl radicals (OH), hydrogen peroxide (H2O2), superoxide (O2•-), and organic peroxides. In clouds, chemistry driven by hydroxyl radicals is a well-known contributor to production of secondary organic aerosols. OH in cloud and fog droplets originate from various sources, and models currently assume the main source of hydroxyl radicals in clouds is uptake from the gas phase. Work done previously in the group suggested a new, potentially substantial source of OH in cloud droplets when exposed to near UV light.

In this study, we are able to produce similar behavior to field sample observations from mixtures of Fe(II) and peracetic acid (PAA) in the presence of light. We investigate this reaction to see if we can explain the ‘OH burst’ which occurs within moments of sample preparation. This is a previously unrecognized reaction and we observe a concentration-saturation effect to the amount of OH produced in the burst.

We observe the OH burst from the reaction of Fe(II) and PAA in 320 nm light and in dark conditions. The reaction in the presence of near UV light immediately produces almost double the OH as the reaction in the dark demonstrating a clear light dependence. This is a possible photo-Fenton like reaction cycling Fe(II) amplifying the OH production. At higher concentrations of Fe(II) and PAA molar yield of OH decreases in both the light and dark conditions, producing less OH per �M of reactant compared to lower concentrations of either Fe(II) or PAA. Inhibition from iron-organic complexes or saturation effects are possibly limiting OH yield at high concentrations of initial reactants.

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