Response to Reviewers Comments On dithiothreitol (DTT) as a measure of oxidative potential for ambient particles: evidence for the importance of soluble transition metals Atmospheric Chemistry and Physics

1. It should be pointed out that substances present in the test mixture act as electron transfer agents, or catalysts of the reaction which are recycled, i.e, reduced by DTT or superoxide and reoxidized by oxygen species in a cyclic manner (see Li et al 2009a reference). We do mention that the reaction is catalytic in redox-active species, on pg 11326 starting on line 10: “Oxidation of DTT by metals is a catalytic process, as it is for quinones (Kumagai et al., 2002). For example, in our experiments 1.0 μM of Cu(II) oxidizes 13.2 μM of DTT in 15 min. Cu(II) likely accepts an electron from DTT, forming Cu(I), which rapidly donates an electron to dissolved oxygen to form superoxide and re-form Cu(II).”