Nitrate radical atmospheric lifetime

Extensive research has been conducted into the atmospheric chemistry of organic chemicals because of air quality concerns. Recently, Atkinson and coworkers (1984, 1985, 1987, 1988, 1989, 1990, 1991), Altshuller (1980, 1991) and Sabljic and Glisten (1990) have reviewed the photochemistry of many organic chemicals of environmental interest for their gas phase reactions with hydroxyl radicals (OH), ozone (03) and nitrate radicals (N03) and have provided detailed information on reaction rate constants and experimental conditions, which allowed the estimation of atmospheric lifetimes. Klopffer (1991) has estimated the atmospheric lifetimes for the reaction with OH radicals to range from 1 hour to 130 years, based on these reaction rate constants and an assumed constant concentration of OH... 

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. 

The kinetics of the reactions of many xenobiotics with hydroxyl and nitrate radicals have been examined under simulated atmospheric conditions and include (1) aliphatic and aromatic hydrocarbons (Tuazon et al. 1986) and substituted monocyclic aromatic compounds (Atkinson et al. 1987c) (2) terpenes (Atkinson et al. 1985a) (3) amines (Atkinson et al. 1987a) (4) heterocyclic compounds (Atkinson et al. 1985b) and (5) chlorinated aromatic hydrocarbons (Kwok et al. 1995). For PCBs (Anderson and Hites 1996), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 days for 3-chlorobiphenyl to 34 days for 2,2, 3,5, 6-pentachlorbiphenyl. It was estimated that loss by hydroxylation in the atmosphere was a primary process for removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a... 

The nitrate radical (NO3) is formed in the atmosphere primarily by the reaction of nitrogen dioxide (NO2) with ozone (O3). At the outset of this project the potential importance of the role of NO3 as an oxidant in the troposphere had just been recognised. In order to assess the latter accurate physico-chemical models, describing the behaviour of NO3 in the troposphere, are needed. These require a detailed understanding of the elementary photochemical or chemical reactions and physical processes such as deposition or transport, which determine the tropospheric lifetime of NO3. 

The atmospheric lifetime of 2,4-dimethyl-2-pentanol with respect to reaction with OH radicals is estimated to be 10 h using [OH] = 2.5 x 10 molecule cm and the rate coefficient presented in this section. The product study conducted by Atkinson and Aschmann (1995) using GC-FK) and GC-MS showed that the reaction of OH with 2,4-dimethyl-2-pentanol gives the following products (molar yields) Acetone (92 15%), 2-methylpropanal (21 22%), 4-methyl-2-pentanone (5 1%), and 4-hydroxy-4-methyl-2-pentanone (12 2%). In addition, the yield of a tertiary nitrate was estimated to be 4 1%. 

The atmospheric lifetime of 1-butyl nitrate is around 7 days with respect to the reaction with OH radicals using [OH] = 1 x 10 molecule cm". By analogy with other alkyl nitrates, the reaction of OH with 1-butyl nitrate is expected to occur via H-atom abstraction from C—H bonds, probably involving the secondary H-atoms. Photolysis is... 

The atmospheric lifetime of 2-pentyl nitrate with respect to reaction with OH radicals is calculated to be around 7 days using [OH] = i x 10 molecule cm and k = 1.7 X 10 cm molecule" s Loss via photolysis is also an important atmospheric fate for 2-pentyl nitrate with a photodissociation lifetime of about 4.5 days with an overhead Sun (see table IX-M-1). 

The atmospheric lifetime of 2-methyl-l-butyl nittate with respect to reaction with OH radicals, using (OH+2-methyl-l-nitrate) = 2.3 x 10 cm molecule" s" and [OH] = 1 X 10 molecule cm", is calculated to be around 5 days. Photolysis is expected to contribute significantly to the atmospheric loss of 2-methyl-l-butyl nitrate... 

Combining the rate coefficients for the OH reactions presented above and an average concentration of OH radicals of [OH] = 1 x 10 molecule cm , provides an estimate for the atmospheric lifetime of the higher alkyl nitrates of 3 to 5 days. Photolysis is also expected to be an important atmospheric loss of higher alkyl nitrates see section IX-I-3. 

The atmospheric lifetime of peroxyacetyl nitrate with respect to the removal by reaction with OH radicals is estimated to be more than 1 year. The wet and dry depositions are minor removal processes (Roberts, 1990). The thermal decomposition remains the most important loss process up to around 7 km, above which photolysis takes... 

NO , is removed from the atmosphere by several mechanisms including via the formation of low-volatility organic nitrates in reaction (3b) which are incorporated into aerosol and then undergo wet and dry deposition. In addition, nitrogen dioxide reacts with OH radicals with a rate coefficient of 1.19 x 10 cm molecule" s in 1 atmosphere of air at 298 K (Atkinson et al., 2004) and gives HNO3 which is removed by wet and dry deposition. In the presence of [OH] = 10 molecule cm the lifetime of NO2 with respect to reaction with OH is approximately 1 day. NO , has a relatively short atmospheric lifetime and is not transported directly from polluted to remote areas. 

The reactions of amines with the OH radial are rapid, with lifetimes during daylight hours being on the order of hours. Ammonia, however, being highly soluble and reactive with atmospheric acids, will be removed preferentially by those routes rather than through reaction with the OH radical. Particulate-nitrate salts will undergo dry and/or wet deposition to surfaces. 

The very simple ultraviolet absorption spectrum of PAN, first observed by Stephens for samples obtained from ethyl nitrate photolysis with gas chromatographic separation, was confirmed in the early 1980s by using high purity samples.Interestingly, PAN has no strong structural features in its ultraviolet spectrum and does not absorb above 290 nm. The PAN molecule is not readily photolyzed in the atmosphere at altitudes below 5 to 7 km. " Photolytic lifetimes for PAN are calculated to be on the order of 20 X 10 sec The reaction of PAN with hydroxyl radical... 

Because of their short lifetimes at room temperature, the peroxy nitrates have been assumed not to act as key storage modes for peroxy radicals and NO2 in the lower atmosphere. At middle latitudes in the wintertime these may have hfetimes that approach days. Further, like the PANs these might be reformed to actively transport NO2 and peroxy radicals over long distances, depending upon the NO, hydroperoxy radical, and NO2 concentrations. With the possible exception of very cold air masses, these compounds are typically not present in significant concentrations in the troposphere because of rapid thermal decomposition to form NO2 and RO2. At room temperature they would be lost in samphng lines or during analysis. 

Peroxy acyl nitrates dissociate quite quickly at 298 K, to regenerate peroxyacyl radicals. For example, PAN has a lifetime of about 50 min. The lifetime increases rapidly at the lower temperatures experienced at higher altitudes and is several months at the temperatures ( 250 K) of the upper troposphere. This long lifetime provides a mechanism for the transport of NOjc from polluted areas to less polluted areas, by transfer of peroxyacyl nitrates from the boundary layer to the free troposphere subsequent subsidence can return them to the boundary layer where they dissociate at the higher temperatures encountered there. The atmospheric reactions of the nitrates are discussed in detail in chapters VIII and IX. 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

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