Chemical aqueous phase radical mechanism

Herrmann H., B. Ervens, H.-W. Jacobi, R. Wolke, P. Nowacki and R. Zellner CAPRAM2.3 A chemical aqueous phase radical mechanism for tropospheric chemistry, J. Atmos. Chem. 36 (2000) 231 -284. 

Fig. 13 Fe(in) speciation in cloud water and wet particles when included in the chemical aqueous phase radical mechanism (CAPRAM) to simulate aqueous troposphcaic chemistry...

Sander, R. Cmtzen, P.J., 2000 Comment on A Chemical Aqueous Phase Radical Mechanism for Tropospheric Chemistry by Herrmann et al. , in Chemosphere, 41 631—632. 

Assessment of Aqueous-Phase Oxidation Pathway. The results of two modeling studies (7. 13) employing chemical reaction schemes that included aqueous-phase formic acid formation are summarized in Table III. The reaction mechanism used by Adewuyi et al. (7) included the aqueous-phase oxidation of formaldehyde by hydrogen peroxide and hydroxy radicals the mechanism of Chameides (13) included only oxidation by hydroxy radicals. Neither model included reactions for the formation of acetic acid. By comparing Table III with Table I, it can be seen that the concentrations of formic acid... 

One of the most important parameters in the S-E theory is the rate coefficient for radical entry. When a water-soluble initiator such as potassium persulfate (KPS) is used in emulsion polymerization, the initiating free radicals are generated entirely in the aqueous phase. Since the polymerization proceeds exclusively inside the polymer particles, the free radical activity must be transferred from the aqueous phase into the interiors of the polymer particles, which are the major loci of polymerization. Radical entry is defined as the transfer of free radical activity from the aqueous phase into the interiors of the polymer particles, whatever the mechanism is. It is beheved that the radical entry event consists of several chemical and physical steps. In order for an initiator-derived radical to enter a particle, it must first become hydrophobic by the addition of several monomer units in the aqueous phase. The hydrophobic ohgomer radical produced in this way arrives at the surface of a polymer particle by molecular diffusion. It can then diffuse (enter) into the polymer particle, or its radical activity can be transferred into the polymer particle via a propagation reaction at its penetrated active site with monomer in the particle surface layer, while it stays adsorbed on the particle surface. A number of entry models have been proposed (1) the surfactant displacement model (2) the colhsional model (3) the diffusion-controlled model (4) the colloidal entry model, and (5) the propagation-controlled model. The dependence of each entry model on particle diameter is shown in Table 1 [12]. 

The reactivity of NMP towards OH radicals was studied in the aqueous phase, under tropospheric conditions. The kinetic results show that the OH oxidation of NMP is fast compared to that of other WSOC, and thus should induce modifications of the composition of water droplets, due to the reaction products formed. A new experimental technique was developed to study the aqueous phase OH oxidation of NMP. A mass spectrometer was coupled to an aqueous phase simulation chamber, thus providing an on-line analysis of the solution. The mass spectrometer was equipped with an electrospray ionisation (ESI) unit and a triple quadrupole, which allowed ESI-MS, ESI-MS, and ESI-MS-MS analysis. The results proved that this experimental technique is highly promising, as it allowed us to detect the formation of 66 reaction products, of which 24 were positively identified. Based on the results obtained, a chemical mechanism has been suggested for the OH oxidation of NMP in the aqueous phase. The developed equipment can be used to study other molecules and other reactions of atmospheric interest. 

The notion that chemical reactions in the aqueous phase may be important to atmospheric chemistry dates back at least 30 yr, when Junge and Ryan (1958) called attention to the great potential of cloud water for the oxidation of dissolved S02 by heavy-metal catalysis. At that time the process appeared to be the only viable oxidation mechanism for atmospheric S02. Later, when the concept of OH radical reactions gained ground, the gas-phase oxidation of S02 by OH was recognized to be equally important. The recent revival of interest in aqueous phase reactions is connected with efforts to achieve a better understanding of the origins of rainwater acidity. An oxidation of N02 to nitric acid also takes place in cloud water. Contrary to previous ideas, however, this process was recently shown to have little influence on atmospheric reactions of N02. 

Ultrasonic strong digestion is based on mechanical and, especially chemical effects. The chemical effects result from the reactivity of the chemical agents (oxidants or reduc-tants) promoted by the radicals generated by sonolysis of the solvents in the liquid phase (particularly in aqueous solutions). In fact, the radicals act as promoters of the chemical reactions involved in matrix decomposition. 

This mechanism is rather unconventional in assuming the existence of OH-radicals in solution and hindrance of their chemical reaction with the electrode surface. This contradicts the generally recognized instability of OH- radicals in aqueous solutions. In spite of some early efforts to calculate their free energy in aqueous solution and to consider them as bulk phase inter-... 

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