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

In droplets of clouds and fog the transformation of ozone (O-3) 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 (O-2(.-)). O-2(.-) reacts with aqueous ozone. This transforms aqueous O-3 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, O-2(.-) 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 O-3 to OH.. Ubiquitous aqueous formate and formaldehyde then convert a fraction of non-selective OH. to highly selective HO2. and O-2(.-), i.e. to O-2(-I). Either directly or indirectly this O-2(-I) reacts with further O-3 to produce further OH.. However, ubiquitous acetate and some other types of solutes, such as bisulfite, scavenge OH. without recycling O-2(-I). Experimental calibrations on model solutions showed that acetate limits the kinetic length of the O-3 converting chain reaction just in proportion to its concentration relative to that of formate and formaldehyde. As demonstrated by further experiments, a similar aqueous O-2(-I)-O-3-OH. radical-type chain reaction can also be initiated when some aqueous O-3 reacts on black carbon particles. Black carbon was thereby studied as a surrogate for soot. Within cloud droplets the sum of all these O-3 promoted aqueous-phase radical-type chain reactions increases the flux of aqueous-phase OH. and its oxidation processes. (C) 2000 Elsevier Science Ltd. All rights reserved.