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 uptake kinetics of ozone (O3) and methyl hydroperoxide (CH3OOH, MHP) by aqueous solutions were studied as a function of temperature using the droplet train technique combined with mass spectrometry detection. The uptake of ozone by pure water was found to be too small to be directly measured. Using NaI as a scavenger increased the uptake coefficient γ from below the detection limit to a range from 0.0037 to 0.0116 for I- activities in the range from 0.3615 to 2.889 at 282 K. From these experiments, we estimated the second-order rate constant for the reaction O3 + I- → products to be in the range 3.2 × 108 to 2.4 × 109 M-1 s-1 for temperature between 275 and 293 K. The activation parameters for this reaction were also estimated. For methyl hydroperoxide, the uptake rate on pure water was fast enough to be directly measured. According to the physicochemical properties of this hydroperoxide, the uptake was mainly due to the diffusion and accommodation processes. It was therefore possible to measure its mass accommodation coefficient α as a function of temperature. The observed values are in the range 0.92 × 10-2 to 2.08 × 10-2 for temperature between 281 and 261 K. The activation parameters for the accommodation were also determined.

The uptake of N2O5 by pure water and NaCl solution was studied as a function of temperature in the range from 262 to 278 K with the droplet train technique, where a highly controlled beam of droplets was exposed to N2O5 in a low-pressure flow tube reactor, and the formation of nitrate in the liquid phase was determined by ion chromatography. The uptake coefficients, y, for N2O5 on pure water are observed to decrease from 0.03 to 0.0 13 with increasing temperature. This behavior corresponds to the expected negative temperature dependence of mass accommodation leading to an enthalpy m o b , = (-9.6 f 1.6) kcal mol-' and to an entropy m o b s = (-43 f 6) cal mol-' K-l for the phase transfer, corresponding to a continuous nucleation process with a critical cluster size N* of about 2.4. A significantly lower yield of nitrate than with pure water is observed in the experiments on NaCl solution (1 mol/L), indicating that nitrogen compounds (such as ClN02) are formed after the uptake of N2O5 by subsequent reactions with NaCl and escape from the droplets. After correction for the known yield for the formation of CIN02, the results exhibit a slight systematic tendency for the uptake coefficient on NaCl solution to be greater than on pure water, indicating that the uptake of N2O5 by these aqueous media might be reaction-controlled. This assumption leads to a lower limit of 800 mol L-I atm-I s-l12 for the product Hk112 from a simple steady state model (where H is the Henry's law constant for N2O5 and k is the first-order hydrolysis rate constant).

The uptake kinetics of N2O5 were studied with the droplet train technique as a function of temperature between 262 and 278 K on different aqueous solutions. No pronounced temperature dependence was observed, and the average uptake coefficient in this temperature range is 0.018 ± 0.003. When interacting with salt solutions (i.e., NaCl, NaBr, or NaI), N2O5 contributes to the formation of ClNO2 and BrNO2. The multiphase chemistry of these nitryl compounds was further investigated using the wetted-wall technique as a function of temperature between 275 and 293 K on different aqueous solutions. The uptake coefficients are reported for both species, and no distinct temperature dependence was observed. Their uptake rate was efficiently enhanced by the presence, in the aqueous phase, of halogenides ions. When reacting with Br- or I-, both nitryl compounds deliver to the gas phase the molecular form of the halogen, i.e., Br2 or I2. A reaction scheme potentially explaining these observations is presented and its importance for the sea-salt aerosol chemistry is discussed.

figures also show the calculated concentrations of 02F, 02F2, and F. Temperature DependenceTable III summarizes the room-temperature rate constants described above (center column). The last column is our estimate of the temperature dependence of these and several other rate constants. These estimates will be useful for estimating synthesis or decomposition rates for the oxygen fluorides.