Nitrate photolysis rate

NOx as it is in the gas phase, in spite of the fact that the nature of nitrate at the ice surface is not well established. Recent spectroscopic experiments indicate that nitrate exists at the ice surface in solvated form with a local environment similar to that in concentrated solution [302]. Still, this does not rule out a reduced solvent cage compared to dilute nitrate solution that would allow NO2 to escape, more likely due to recombination being suppressed, as has been suggested based on quantum yield measurements for frozen nitrate solutions [192]. Since in snow the nitrate anion is often co-located with other ions, e.g., halogenide ions, in a brine solution, ion specific effects may lead to enhancement of nitrate ions at the aqueous brine-air interface. Such effects have been shown to lead to enhanced nitrate photolysis rates in aqueous solution [303, 304]. In some contrast to the case of H2O2, the particular enviromnent in snow or ice makes photolysis of nitrate more efficient than in solution. Such effects would help to explain the significant cycling of NOx mediated by nitrate photolysis in polar snow [276, 298, 305-308]. 

During SOAPEX-2, measurements of the free-radicals OH, HO2, HO2+XRO2, NO3, IO and OIO were supported by measurements of temperature, wind speed and direction, photolysis rates (j D) and j(N02)), water vapor, O3, HCHO, CO, CH4, NO, NO2, peroxyacetyl nitrate (PAN), a wide range of NMHCs, organic halogens, H2O2, CH3OOH and condensation nuclei (CN). 

Luke, W.T., Dickerson, R.R., and Nunnermacker, L.J. Direct measurements of the photolysis rate coefficients and Henry s law constants of several alkyl nitrates, J. Geophys. Res., 94(D12) 14905-14921, 1989. 

In addition to degradation by hydroxyl and nitrate radicals, all three cresol molecules absorb small amounts of W light with wavelengths above 290 nm (Sadtler Index 1960a, 1960b, 1966). Therefore, direct photolysis is also possible however, the photolysis rate is probably slow compared to the reaction with atmospheric radicals. 

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants (kp) for ethyl and n-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0. 

FIGURE 4.22 Experimental values of the photolysis rate constant, kp, for (a) ethyl nitrate and (b) n-propyl nitrate as a function of zenith angle compared to calculated values shown by the solid lines. Different symbols represent different measurement days (adapted from Luke et al., 1989). 

Toole DA, Kieber DJ, Kiene RP, White EM, Bisgrove J, del Valle DA, Slezak D (2004) High dimethylsulfide photolysis rates in nitrate-rich Antarctic waters. Geophys Res Lett 31 Article no. LI 1307... 

Hoffmann and co-workers [178,179] have studied the photolysis of nitrate and nitrite in both water and water ice over broad temperature ranges. In the case of nitrite formation from nitrate photolysis, the photochemical reaction reaches a steady state condition that can be described by the following rate expression ... 

The pH trend of nitrophenol formation is interesting also because with this datum, it is possible to discuss the role of peroxynitrous acid in a transformation process induced by nitrate photolysis, thus giving a first answer to the observation made by Mack and Bolton [24], HOONO forms upon irradiation of nitrate (reactions 9 and 10), although its formation at k > 280 nm is uncertain. In the presence of HOONO phenol undergoes various transformation reactions, among which nitration [57]. The pH trend of nitrophenol formation in the presence of HOONO is of the kind Rate a [H+], as discussed in... 

The best conditions for photolysis in natural aqueous bodies involve low turbidity, low concentrations of dissolved organic compounds, bright sunny days, and quiescent waters or slow-moving streams. For example, the rate of photoproduction of OH radicals from nitrate photolysis (see below) is lower at higher latitudes. For example, at a latitude 0 N (like in Lake Victoria, in Tanzania) this rate is more than twice that for latitude 60 N (like in lake Tyrifjor-den, in Norway). Likewise, at this last latitude the rate can vary by a factor of 40 from mid-summer to mid-winter. In deep-water bodies (i.e., lakes) the photoprocesses are generally much slower. Some... 

Bromine nitrate possesses a similar spectrum (Figure 4.51) to that of chlorine nitrate. It is, however, shifted toward slightly longer wavelengths such that the photolysis rate of BrON02 is faster than that of CIONO2. Like chlorine nitrate, the absorption cross sections are temperature dependent. They have been measured in the laboratory by Spencer and Rowland (1978) and more recently by Burkholder et al. (1995). Nickolaisen and Sander (1996) have shown that the quantum yields should be approximately 0.71 for the... 

The objectives of the present study were (1) to determine the steady-state concentration of OH in river water, (2) to determine the rate constants for the reaction between BPA and OH that was produced by nitrate photolysis, and (3) to estimate the lifetime of BPA in the surface river water calculated from the steady-state concentration of OH and the rate constant for the reaction between BPA and OH in river water. 

Nitrate also reacts with OH, while it was used as the source of OH to determine rate constants for reactions between OH and BPA. Using lXlO Mof sodium nitrate solution containing 2 X M of benzene without BPA, most of OH (>98%) initiated by nitrate photolysis was scavenged by benzene. Hence, it was assumed that the nitrate-mediated OH is consumed by benzene and BPA in the experiment solutions (IXlO Mof sodium nitrate, 2XlO Mof benzene, and 0-477 X M of BPA). There was no production of phenol in the reaction for OH with BPA. It suggested that the photoproduction of phenol is only originated from the reaction of benzene with OH. 

The photochemical formation rate (0.70-3.25 X 10 Ms ), the total consumption rate constant (1.66-3.89 X 10 s ) and the steady-state concentration of OH (3.3-8.4 X 10 M) in river water samples collected in Higashi-Hiroshima, Japan were measured in this study. The measured values were similar to previous values reported for river, rain and dew water samples. In the investigation of production mechanisms of OH, it was found that OH production from nitrite photolysis was greater than that from nitrate photolysis in Kurose river water. The summation of consumption rate constants of OH for major anions occurring in river water was less than 25% of the total consumption rate constant. Based on the reaction rate constant of OH for DOM, it is estimated that DOM accounts for 12-56% of the total consumption rate constant of OH in river water. 

One of the objectives of LACTOZ has been to provide absorption cross-sections and quantum yields so that photolysis rates can be calculated under atmospheric conditions. Emphasis has been on (a) refinement of data for the simple, well-characterised molecules (O3, HCHO) involved in HOx radical production, and (b) determination of data for photolysis of organic compounds formed as products in the degradation of alkanes, alkenes and selected multifunctional VOCs, mainly carbonyl compounds and nitrates, which produce HOx indirectly. 

Under atmospheric conditions, the nitro-oxyalkyl peroxy radicals will probably form mainly the corresponding nitro-oxy alkoxy radicals. Thermal decomposition, yielding carbonyl compounds and NO2, and reaction with O2 giving carbonyl nitrates, appear to be the dominant reactions under most atmospheric conditions. The extent to which carbonyl nitrates can act as temporary reservoirs for NOx will largely depend on their photolysis rates or reactions with OH radicals. 

Nitric acid HNO3 (HONO2) is mainly formed by the chain termination reaction OH -b NO2, and ubiquitously exists in the troposphere. The photolysis rate of HNO3 in the troposphere is not very large, and the formation of ammonium nitrate aerosols... 

Atrazine is successively transformed to 2,4,6-trihydroxy-l,3,5-triazine (Pelizzetti et al. 1990) by dealkylation of the alkylamine side chains and hydrolytic displacement of the ring chlorine and amino groups (Figure 1.3). A comparison has been made between direct photolysis and nitrate-mediated hydroxyl radical reactions (Torrents et al. 1997) the rates of the latter were much greater under the conditions of this experiment, and the major difference in the products was the absence of ring hydroxylation with loss of chloride. 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Chemical/Physical. In the gas phase, cycloate reacts with hydroxyl and NO3 radicals but not with ozone. With hydroxy radicals, cleavage of the cyclohexyl ring was suggested leading to the formation of a compound tentatively identified as C2H5(Cff0)NC(0)SC2H5. The calculated photolysis lifetimes of cycloate in the troposphere with hydroxyl and NO3 radicals are 5.2 h and 1.4 d, respectively. The relative reaction rate constants for the reaction of cycloate with OH and nitrate radials are 3.54 x lO " and 3.29 x 10 cm /molecule-sec, respectively (Kwok et al., 1992). 

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