Troposphere nitrites

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. 

Alkyl nitrites (RONO) absorb light strongly in the actinic region, dissociating to form RO + NO. Because of this rapid photolysis, other reactions such as that with OH cannot compete, and alkyl nitrites have not been generally observed in the troposphere at significant concentrations. 

Alkyl halides (RX X = Cl, I) are an important source of halogens in the atmosphere. The major tropospheric sinks of these compounds are photolysis (RBr, RI) and reaction with OH radicals. In the case of alkyl iodides (RI) relative kinetic studies of their OH reactions in photoreactors are complicated by fast reactions with the 0( P) atoms generated by the photochemical OH radical sources. Figure 1 below shows a In-ln plot of the kinetic data from an experiment performed in a large photoreactor to determine the OH rate coefficient for the reaction OH + CH3CH2CH2I relative to OH + ethene using the photolysis of methyl nitrite (CH3ONO) as the OH radical source. A recent example of the implementation of the relative kinetic technique for the determination of OH radical rate coefficients in a photoreactor can be found in Olariu et al. (2000). 

The concept that free radicals are important intermediates in photochemical and other redox interactions of oxygen, organic compounds and heavy metals in natural waters has received considerable support recently ((1-3) and references therein this volume). Some of the major primary radicals expected are hydroxyl (OH), superoxide (02 ). and various organic moieties (R, RO, ROO). Of these, OH is of interest because of its extremely high reactivity, significant formation rate from a known source (nitrite photolysis, among others) and the analogy of its known key role in tropospheric chemistry. 

Of similar photochemical importance as H2O2 is nitrate, which absorbs light above 290 nm. The ubiquitous presence of nitrate in environmental ices is well documented for cirrus ice particles [296, 297] as well as permanent and perennial snow packs [298-301]. hi aqueous solution, photolysis of nitrate ion leads to either OH and NO2 or 0( P) and nitrite ion, with typically significantly higher quantum yields for the first pathway [197, 200]. In the upper troposphere, it is currently thought that uptake of HNO3 to ice makes it ineffective as a photolytic source of... 

Alkyl nitrites will be removed from the troposphere largely by photolysis. Hence, their reactions with OH radicals may have only a small contribution to the overall loss of these species from the atmosphere. See table IX-M-1. 

Figure IX-H-1. Absorption spectra of some simple aUcyl nitrites and (CH3)2NNO. The absorption regions of all of these compounds overlap well wavelengths present in the tropospheric actinic flux (overhead Sun) that is shown by the heavy gray curve.

The near ultraviolet absorption bands of nitrous acid, like all nitrites, involves an n n transition, and the absorption bands overlap well the envelope of actinic flux present in the troposphere see figure IX-H-2. The photodecomposition of HONO is efficient in generating OH radicals in the troposphere. Two primary processes have been proposed to rationalize the observed photodecomposition of HONO ... 

The absorption regions of the alkyl nitrites overlaps well the distribution of solar flux within the lower troposphere see figure IX-H-1. The characteristic band structure that appears in each spectrum, like that in HONO, originates from the excitation to the different vibrational energy levels (an N-O stretching mode, V2) in the Si excited state. Like HONO, the alkyl nitrites (RONO) photodissociate readily to form free radicals RONO+hv RO+NO, that can drive the chemistry that results in ozone generation within the troposphere. The alkyl nitrites (RONO) are often used as convenient sources of alkoxy radicals (RO) in laboratory experiments. 

Figure IX-H-4. Photolysis frequencies for methyl nitrite versus solar zenith angle as calculated for a clear day in the lower troposphere with a vertical ozone column of 350 DU. The total quantum yield of photodecomposition was taken as either 1.0 or 0.78, independent of wavelength. 

Figure IX-H-5. Comparison of the approximate absorption spectra for some simple alkyl nitrites and the wavelength distribution of the solar flux for an overhead Sun within the lower troposphere. The estimates shown were made by the current authors using relative spectral data of Heicklen (1987) as measured in hexane solution together with estimates of the cross section of Zabernick and Heicklen (1985a, b, c) that were made at one wavelength (366 nm).

The major product of ethyl nitrite photodecomposition in the troposphere is expected to be acetaldehyde, formed from the ethoxy radical reaction, C2H5O -I- O2 CH3CHO + HO2, as well as some ill-defined amount of CH2O as well as CH3CHO formed in process (II). 

In the calculation given here, we have assumed that the quantum yield of photodecomposition of ethyl nitrite in the lower atmosphere is 0.31, independent of wavelength. Using this assumption and the approximate cross sections given in table IX-H-3, we have calculated the approximate y-values as a function of solar zenith angle for ethyl nitrite on a cloudless day in the lower troposphere and an overhead ozone column of 350 DU. The data, given in figure IX-H-7, point to a photochemical lifetime of ethyl nitrite of only about 29 min. with an overhead Sun. 

Like HONO and the organic nitrites, CH3ONO, and C2H5ONO, the spectrum of o-propyl nitrite overlaps well that of the solar radiation within the troposphere ... 

The excellent overlap of the wavelength regions for absorption and those for solar flux leads to rapid photodecomposition of iio-propyl nitrite within the troposphere. 

The absorption of -C3H70N0 is very similar to that of the other nitrites. See figure IX-H-5. The tabular listing of the cross sections at wavelengths of interest in tropospheric chemistry is given in table IX-H-3 where they can be compared with those of some other simple alkyl nitrites. 

IX-H-5.3. Photolysis Frequencies for n-Propyl Nitrite Photodecomposition within the Lower Troposphere... 

There is little information on the extent of processes (II) or (III), but they appear to be unimportant at 366 nm they may become important at the shorter wavelengths of absorbed light that are available in the upper troposphere and stratosphere. As with the other alkyl nitrites, it is likely that process (TV) is unimportant at all wavelengths. 

We have used the cross sections of tert-butyl nitrite given in table IX-H-4, an assumed total quantum yield of photodecomposition of unity, independent of wavelength, and the appropriate actinic flux data (cloudless skies, vertical ozone column of 350 DU) to estimate the -values for tert-butyl nitrite photodecomposition within the lower troposphere. These data are summarized in figme IX-H-16. The data of figure IX-H-16 give a photochemical hfetime for tert-butyl nitrite of about 4.3 min. with an overhead Sun. 

IX-H-10. Summary of the Mechanisms of Tropospheric Photodecomposition of the Alkyl Nitrites... 

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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