A detailed and extended chemical mechanism describing tropospheric aqueous phase chemistry (147 species and 438 reactions) is presented here as Chemical Aqueous Phase Radical Mechanism (CAPRAM) 2.4 (MODAC mechanism). The mechanism based on the former version 2.3 [Herrmann et al., 2000] contains extended organic and transition metal chemistry and is formulated more explicitly based on a critical review of the literature. The aqueous chemistry has been coupled to the gas phase mechanism Regional Atmospheric Chemistry Modeling (RACM) [Stockwell et al., 1997], and phase exchange accounted for using the resistance model of Schwartz [1986]. A method for estimating mass accommodation coefficients (α) is described, which accounts for functional groups contained in a particular compound. A condensed version has also been developed to allow the use of CAPRAM 2.4 (MODAC mechanism) in higher‐scale models. Here the reproducibility of the concentration levels of selected target species (i.e., NOx, S(IV), H2O2, NO3, OH, O3, and H+) within the limits of ± 5% was used as a goal for eliminating insignificant reactions from the complete CAPRAM 2.4 (MODAC mechanism). This has been done using a range of initial conditions chosen to represent different atmospheric scenarios, and this produces a robust and concise set of reactions. The most interesting results are obtained using atmospheric conditions typical for an urban scenario, and the effects introduced by updating the aqueous phase chemistry are highlighted, in particular, with regard to radicals, redox cycling of transition metal ions and organic compounds. Finally, the reduced scheme has been incorporated into a one‐dimensional (1‐D) marine cloud model to demonstrate the applicability of this mechanism.

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Laser Ñash photolysis of chloroacetone was used to measure the rate constants and activation energies for the reactions of the atom with a number of oxygen-containing compounds and inorganic anions in aqueous Cls olution. For the organic compounds there is a strong correlation at 25 ¡C between andCHO, CH 3 CO 2 H, and respectively. For and for and HCO 2 H HCO 2 ~, C H 3 CO 2 ~, k(Cl~] RH) A k(~OH ] RH), CH 3 COCH 3 Possible reasons for these di †erences are discussed in terms of CH 3 COCH 2 Cl, k(Cl~] RH) @ k(~OH ] RH). preferential attack by at OÈH groups in the neutral molecules, rather than H-abstraction from CÈH as with Clã nd electron transfer for the reactions of with the anions. For the inorganic anions X \ OCN~, ~OH, ClS CN~, ranges from 1.0 ] 108 to NO 3 ~, SO 4 2~, ClO 3 ~, HCO 3 ~, N 3 ~, NO 2 ~, HSO 3 , k(Cl~] X) (NO 3 ~) 5.3 ] 109 dm3 mol~1 s~1 (SCN~) but there is no strong correlation between k and the reduction potential of X. Comparison of the reactivity of with reported rate constants for the reactions of indicates that, in Cl~Cl 2 ~many cases, these rate constants are largely accounted for by the fraction of present in equilibrium with ClT he implications of these results for atmospheric chemistry are discussed. Cl 2 ~~.

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