Troposphere nitrates

In addition to reactions with HO, tropospheric organic compounds may be oxidized by ozone (via ozonation of non-aromatic carbon/carbon double bonds, Atkinson 1990) and in some cases by reaction with nitrate radical, described below. Table I gives representative trace-gas removal rates for these three processes. In spite of these competing reactions, HO largely serves as... 

Reactions in the troposphere are mediated by reactions involving hydroxyl radicals produced photochemically during daylight, by nitrate radicals that are significant during the night (Platt et al. 1984), by ozone and, in some circumstances by 0( P). 

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. 

Considerable attention has been directed to the formation of nitroarenes that may be formed by several mechanisms (a) initial reaction with hydroxyl radicals followed by reactions with nitrate radicals or NO2 and (b) direct reaction with nitrate radicals. The first is important for arenes in the troposphere, whereas the second is a thermal reaction that occurs during combustion of arenes. The kinetics of formation of nitroarenes by gas-phase reaction with N2O5 has been examined for naphthalene (Pitts et al. 1985a) and methylnaphthalenes (Zielinska et al. 1989) biphenyl (Atkinson et al. 1987b,c) acephenanthrylene (Zielinska et al. 1988) and for adsorbed pyrene (Pitts et al. 1985b). Both... 

Seefeld S, JA Kerr (1997) Kinetics of reactions of propionylperoxy radicals with NO and NOj peroxypropio-nyl nitrate formation under laboratory conditions related to the troposphere. Environ Sci Technol 31 2949-2953. 

Reactions that simnlate tropospheric conditions have been carried ont in Teflon bags with volumes of ca. 6 m htted with sampling ports for introduction of reactants and snbstrates, and removal of samples for analysis. Substrates can be added in the gas phase or as aerosols that form a surface him. The primary reactants are the hydroxyl and nitrate radicals, and ozone. These mnst be prepared before use by reactions (a) to (c). 

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). 

There are many different types of surfaces available for reactions in the atmosphere. In the stratosphere, these include ice crystals, some containing nitric acid, liquid sulfuric acid-water mixtures, and ternary solutions of nitric and sulfuric acids and water. In the troposphere, liquid particles containing sulfate, nitrate, organics, trace metals, and carbon are common. Sea... 

Alfassi, Z. B S. Padmaja, P. Neta, and R. E. Huie, Rate Constants for Reactions of NO, Radicals with Organic Compounds in Water and Acetonitrile, J. Phys. Chem., 97, 3780-3782 (1993). Allen, H. C., J. M. Laux, R. Vogt, B. J. Finlayson-Pitts, and J. C. Hemminger, Water-Induced Reorganization of Ultrathin Nitrate Films on NaCI—Implications for the Tropospheric Chemistry of Sea Salt Particles, J. Phys. Chem., 100, 6371-6375 (1996). Allen, H. C., D. E. Gragson, and G. L. Richmond, Molecular Structure and Adsorption of Dimethyl Sulfoxide at the Surface of Aqueous Solutions, J. Phys. Chem. B, 103, 660-666 (1999). Anthony, S. E R. T. Tisdale, R. S. Disselkamp, and M. A. Tolbert, FTIR Studies of Low Temperature Sulfuric Acid Aerosols, Geophys. Res. Lett., 22, 1105-1108 (1995). 

While these reactions are much slower than the corresponding OH reactions, the nighttime peak concentrations of NO, under some conditions are much larger than those of OH during the day, 400 ppt vs 0.4 ppt. Even given the differences in concentration, however, as seen from the lifetimes in Table 6.1, the nitrate radical reaction is still relatively slow. While the removal of the alkanes by NO, is thus not expected to be very significant under most tropospheric conditions, reaction (20) can contribute to HNO, formation and the removal of NOx from the atmosphere. 

In summary, alkyl radicals, R, are all converted to alkylperoxy radicals, R02, in the troposphere. R02 reacts with NO, except in very remote regions where the concentration of NO is of the order of 40 ppt or less. The reaction converts NO to N02 and hence acts as a source of O-, via the N02 photolysis. For the larger alkylperoxy radicals, a substantial portion of the R02 + NO reaction also gives the stable alkyl nitrate,... 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

Seefeld, S and J. A. Kerr, Kinetics of the Reactions of Propi-onylperoxy Radicals with NO and N02 Peroxypropionyl Nitrate Formation under Laboratory Conditions Related to the Troposphere, Enriron. ScL Technol., 31, 2949-2953 (1997). 

Oxides of nitrogen play a central role in essentially all facets of atmospheric chemistry. As we have seen, N02 is key to the formation of tropospheric ozone, contributing to acid deposition (some are toxic to humans and plants), and forming other atmospheric oxidants such as the nitrate radical. In addition, in the stratosphere their chemistry and that of halogens interact closely to control the chain length of ozone-destroying reactions. 

It was not until the late 1970s that the importance of the nitrate radical was recognized when it was first reported by Noxon and co-workers (1978) in terms of its total column abundance, i.e., the concentration integrated through a column extending through the atmosphere from the earth s surface (see Chapter ll.A.4a). N03 was subsequently confirmed to be in the troposphere by Noxon et al. (1980) and by Platt and coworkers (1980, 1984) in polluted atmospheres and rural continental air. 

Allen, H. C., J. M. Laux, R. Vogt, B. J. Finlayson-Pitts, and J. C. Hemminger, Water-Induced Reorganization of Ultrathin Nitrate Films on NaCI Implications for the Tropospheric Chemistry of Sea Salt Particles, . /. Phys. Chem., 100, 6371-6375 (1996). 

The use of the sun or moon as the light source allows one to measure the total column abundance, i.e., the concentration integrated through a column in the atmosphere. This approach has been used for a number of years (e.g., see Noxon (1975) for NOz measurements) and provided the first measurements of the nitrate radical in the atmosphere (Noxon et al., 1978). As discussed later in this chapter, such measurements made as a function of solar zenith angle also provide information on the vertical distributions of absorbing species. Cloud-free conditions are usually used for such measurements as discussed by Erie et al. (1995), the presence of tropospheric clouds can dramatically increase the effective path length (by an order of... 

A number of alkyl nitrates have been observed in the troposphere, including methyl nitrate and ethyl nitrate, as well as all of the isomers of the higher alkyl nitrates up to C5 (e.g., see Buhr et al., 1990 Ridley et al., 1990a O Brien et al., 1995 and Flocke et al., 1998). Although the specific isomers were not identified, the Cf)-C8 alkyl nitrates have also been measured (O Brien et al., 1995 Flocke et al., 1998). A summary of the measurements through about 1998 is found in Flocke et al. (1998). 

Gaffney, J. S., N. A. Marley, and E. W. Prestbo, Measurements of Peroxyacetyl Nitrate at a Remote Site in the Southwestern United States Tropospheric Implications, Environ. Sci. Technol., 27, 1905-1910(1993). 

Penkett, S. A, N. J. Blake, P. Lightman, A. R. W. Marsh, P. Anwyl, and G. Butcher, The Seasonal Variation of Nonmethane Hydrocarbons in the Free Troposphere over the North Atlantic Ocean Possible Evidence for Extensive Reaction of Hydrocarbons with the Nitrate Radical, J. Geophys. Res., 98, 2865-2885 (1993). 

The family of photo-oxidants includes tropospheric ozone, O3 (the bad ozone), ketones, aldehydes and nitrated oxidants, such as peroxy-acetylnitrate (PAN) and peroxybenzoylnitrate (PBN). The modeling of photo-oxidants is more complicated than that of acid deposition (NRC 1991). Here, the primary precursor is NOx, which as mentioned before, is emitted as a result of fossil fuel combustion. A part of NOx is the N02 molecule, which splits (photodissociates)... 

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