OH radical concentrations

From the prevailing NO and HONO levels occurring during this period of the irradiation, the HONO photolysis rate (11,14), and the rate constant for the OH + NO reaction (15), we estimate that steady state OH levels of "2 x 1Q7 molecule cm" were present. From this OH radical concentration and assuming an UDMH + OH rate constant similar to those observed for N2Hi and MMH (, j ) we calculate a UDMH decay rate which is in reasonable agreement with what is observed. Thus, the HONO level measured during the initial period is entirely consistent with our assumed mechanism. 

Oxidation rate constant k, for gas-phase second order rate constants, kOH for reaction with OH radical, kNC,3 with N03 radical and k(), with 03, or as indicated data at other temperatures see original reference kOH = (7.49 0.39) x 10 11 cm3 molecule-1 s-1 at (298 2) K with a calculated tropospheric lifetime ranging from 1.9 to 2.4 h using a global tropospheric 12-h daytime average OH radical concentration of 2.0 x 10s molecule cm-3 (relative rate method, Phousongphouang Arey 2002)... 

Air calculated tropospheric lifetime ranging from 1.9 to 2.4 h for dimethylnaphthalenes using a global tropospheric 12-h daytime average OH radical concentration of 2.0 x 106 molecule cm-3 for the reaction with OH radical (Phousongphouang Arey 2002). 

Photolytic. Grosjean (1997) reported a rate constant of 1.87 x lO " cm /molecule-sec at 298 K for the reaction of 2-ethoxyethanol and OH radicals in the atmosphere. Based on an atmospheric OH radical concentration of 1.0 x 10 molecule/cm , the reported half-life of methanol is 0.35 d (Grosjean, 1997). Stemmier et al. (1996) reported a rate constant of 1.66 x 10 " cm /molecule-sec for the OH radical-initiated oxidation of 2-ethoxyethanol in synthetic air at 297 K and 750 mmHg. Major reaction products identified by GC/MS (with their yields) were ethyl formate, 34% ethylene glycol monoformate, 36% ethylene glycol monoacetate, 7.8% and ethoxyacetaldehyde, 24%. 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

Using the kinetics for the OH + NO reaction discussed in this chapter, estimate the steady-state concentration of HONO that would exist at noon at the earth s surface if the OH radical concentration is 5 X 106 radicals cm"3, the NO concentration is 1 ppb, and the photolysis rate constant for HONO is 1.4 X 10"3 s"1. 

Hiibler, G D. Perner, U. Platt, A. Toennissen, and D. H. Ehhalt, Ground-Level OH Radical Concentration New Measurements by Optical Absorption, . /. Geophys. Res., 89, 1309-1319 (1984). 

This comparison is only theoretical. In reality a high production of OH° can lead to a low reaction rate because the radicals recombine and are not useful for the oxidation process. Also not considerd are the effects of different inorganic and/or organic compounds in the water. Various models to calculate the actual OH-radical concentration can be found in the literature, some are described in Chapter B 5, Further information concerning the parameters which influence the concentration of hydroxyl radicals is given in Section B 4.4, as well as a short overview about the application of ozone in AOPs in Section B 6.2. 

The concentration of ozone is relatively easy to measure, however, the OH-radical concentration must be calculated. It is for this OH° concentration that a diversity of models has been developed. The concentration is influenced by the water matrix, with its initiators and... 

In the following first example the liquid ozone concentration and the OH-radical concentration are calculated with semi-empirical formula from the mass balance for ozone (Laplanche et al., 1993). For ozonation in a bubble column, with or without hydrogen peroxide addition, they developed a computer program to predict the removal of micropollutants. The main influencing parameters, i. e. pH, TOC, U V absorbance at 254 nm (SAC254), inorganic carbon, alkalinity and concentration of the micropollutant M are taken into consideration. 

Wang et al. (1058) have recently measured OH radical concentrations in a simulated smog chamber by the laser induced fluorescence of OH. The OH concentrations in the chamber range from 0.5 to 1.5 x I07 molec cm"3. In view of the difficulties involved in the absolute determination of OH radicals at such low levels, the uncertainty must be larger than 50%. Table VIII-1A summarizes the ambient concentrations of reactive species and their rate constants with hydrocarbons and NO in polluted air. 

Based on direct spectroscopic measurements of OH radical concentrations at close to ground level, peak daytime OH radical concentrations are typically (3-10) x 106 molecule cm-3 (see, for example, Brauers et al., 1996 Mather et al., 1997 Mount et al., 1997). A diur-nally, seasonally, and annually averaged global tropospheric OH radical concentration has been derived from the emissions, atmosphere concentrations, and OH radical reaction rate constant for methyl chloroform (CH3CC13), resulting in a 24-hr average OH radical concentration of 9.7 x 10s molecule cm 3 (Prinn et al., 1995). 

As mentioned above, at gas-phase conjugated oxidation with hydrogen peroxide in a flow system with stationary mode H02 radical concentration is several orders of magnitude higher than "OH radical concentration. Based on this fact and literary data on olefm epoxidation... 

Figure 5.12 shows temperature dependencies of final and intermediate product accumulation or consumption for N2 hydroperoxide oxidation. Fixed nitrogen concentration in the reaction mixture reaches the maximum at 873 K and then decreases with a temperature increase to 923 K. Flence, hydrogen peroxide concentration decreases abruptly, which is associated with its consumption in the target reaction of nitrogen fixation HO and OH radical concentrations sharply increase with temperature. 

Needless to say, such division of the process is conditional all stages follow one another without abrupt interfaces, but physicochemical characteristics clearly distinguish them at any time. Elementary HO -dependeni reactions are the first to be sensitive to H202 deficiency, which affects the selectivity of the substrate oxidation. Naturally, the increase of OH radical concentration in the reaction mixture is damaging for the substrate (i.e. it intensifies the degradation reactions). 

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