Kinetic data for the radicals H. and. OH in aqueous solution, and the corresponding radical anions,. O − and e aq − , have been critically reviewed. Reactions of the radicals in aqueous solution have been studied by pulse radiolysis, flash photolysis and other methods. Rate constants for over 3,500 reactions are tabulated, including reactions with molecules, ions and other radicals derived from inorganic and organic solutes.

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.

The thiocyanate dosimeter (lo-, rnol dm-3 SCN-in 0,-saturated water) has been standardised against the super-Fricke dosimeter mot dm-3 Fe" in 0,-saturated 0.4 mot dm-3 H2S04) using the hexacyanoferrate(1i) dosimeter [5 x rnol dm-3 Fe(CN)64-in 0,-saturated water] as a secondary standard. On the basis that G(Fe"') = 1.67 x = 220.4 m2 mol-' at 304 nm and 25°C in the super-Fricke dosimeter, we obtain G&[Fe(CN)63-] = (3.47 2 0.06) x m2 J-' at 420 nm and Gc(SCN),'-= (2.59 &-0.05) x m2 J-' at 475 nm. These values remain unchanged when the solutions are saturated with air instead of 0, and are doubled in N20-saturated solution.rnol J-' and Aqueous solutions of thiocyanate are commonly used as dosimeters for pulse radiolysis because (SCN),' -formed in reaction (1) has a strong absorption band' with Amax at ca. 475 nm.

Rateconstants measured over the temperature range 2Cb2OO"C are reported for the following reactions: (a) reaction of the hydrated electron with oxygen, the proton, hydrogen peroxide, nitrate, nitrite, nitrobenzene and methyl viologen ; (b) reaction of the hydroxyl radical with another hydroxyl radical and ferrocyanide; (c) reaction of the hydrogen atom with permanganate and oxygen. To evaluate methods of estimating rate constants at high temperatures these rate constants and others in the literature have been fitted to the following equation: kobs = kdiff/(l + kdiff/kreact)where kobs is the measured rate constant for the bimolecular reaction in solution, kdiff is the encounter rate constant of the two reacting species, and k,,,,, is the rate constant that would be measured if diffusion of the species was not rate influencing. With the exception of reactions of the hydrated electron with nitrate and nitrite ions and nitrous oxide, good fits have been obtained to the above equation, and the results demonstrate that few, if any, of the reactions which are pertinent to water radiolysis are truly diffusion controlled at elevated temperatures.

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The photodecomposition of alkaline aqueous solutions of hypochlorite ion has been investigated using steady state and flash photolysis techniques. At 365 nm the primary products are O(3P), 0-, Cl and C1-formed by the following processesc10-7 with & = 0.28&0.03 and #b = 0.08f0.02. At 313 nm and 253.7 nm O ( l D ) is also produced and the quantum yields are & = 0.075f0.015, $b = 0.127f0.014, c $ ~ = 0.020f0.015 at 313 nm, and & = 0.074-+0.019, 4b = 0.278f0.016, $c = 0.133f0.017 at 253.7 nm.The relative rates of reaction of O(3P) with 02, C10and ClO; have been found to be in the ratios 1 : 2.8 : 15.9. O(3P) reacts with ClOand ClOi by abstraction of 0 as well as by addition. A complete reaction mechanism for the photodecomposition of (210is proposed.

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 ~~.

The rates of reaction of OH with benzene, chlorobenzene, nitrobenzene, benzoate ion and benzoic acid have been measured in aqueous solution up to 200 "C using pulse radiolysis to generate OH. The temperature dependence of the observed rate constant, kobs , is essentially the same for each compound and kobs changes by less than three-fold between 20°C and 200°C. The kinetic data are consistent with a mechanism whereby OH reversibly forms a n-complex with the aromatic compound, irrespective of the substituent on the ring, which then transforms to a a-bonded hydroxycyclohexadienyl radical. The values of kobs were determined from the rate of formation of this radical. There is no evidence for dissociation of the a-bonded radical nor for H atom abstraction from the ring which have been reported for the gas phase. The apparent mechanistic differences between the two phases may be due to the different timescales over which the kinetics measurements were made.