Atmospheric water: transformation of ozone into OH-radicals by sensitized photoreactions or black carbon

In droplets of clouds and fog the transformation of ozone (O3) to secondary oxidants, such as hydroxyl radicals (OH ), is an important process. As demonstrated by experiments on model solutions, chromophoric compounds, represented by ubiquitous iron(III)–oxalato complexes, act as primary photoreactants. Upon photolysis these compounds produce peroxide anion (O2 −). O2 − reacts with aqueous ozone. This transforms aqueous O3 into OH . As estimates show, the quick equilibration with the gas-phase maintains aqueous O3 in droplets of clouds at about 1 nM. But already in the presence of ubiquitous traces of dissolved copper at 1 nM, O2 − rather reduces copper(II) to copper(I) than reacts with ozone. Therefore, it is rather an indirect loop in which Cu(I) acts as a further chain carrier to convert O3 to OH . Ubiquitous aqueous formate and formaldehyde then convert a fraction of non-selective OH to highly selective HO2 and O2 −, i.e. to O2(-I). Either directly or indirectly this O2(-I) reacts with further O3 to produce further OH . However, ubiquitous acetate and some other types of solutes, such as bisulfite, scavenge OH without recycling O2(-I). Experimental calibrations on model solutions showed that acetate limits the kinetic length of the O3 converting chain reaction just in proportion to its concentration relative to that of formate and formaldehyde. As demonstrated by further experiments, a similar aqueous O2(-I)–O3–OH radical-type chain reaction can also be initiated when some aqueous O3 reacts on black carbon particles. Black carbon was thereby studied as a surrogate for soot. Within cloud droplets the sum of all these O3 promoted aqueous-phase radical-type chain reactions increases the flux of aqueous-phase OH and its oxidation processes.