Photolysis of HONO

The most common sources of OH used for relative rate studies include the photolysis of HONO (e.g., see Cox, 1975) or alternatively methyl nitrite (CH3ONO) in air in the presence of NO (Atkinson et al., 1981) ... 

FIGURE 7.8 Calculated rates of formation of OH radical from photolysis of HONO, 03, and HCHO at Long Beach, California, on December 10, 1987 (adapted from Winer and Biermann, 1994). 

As discussed earlier, one source of OH is the photolysis of HONO formed on surfaces by reaction (14),... 

The fraction of 0( D) atoms that form OH is dependent on pressure and the concentration of H2O typically in the marine boundary layer (MBL) about 10% of the 0( D) generate OH. Reactions (2.7 and 2.8) are the primary source of OH in the troposphere, but there are a number of other reactions and photolysis routes capable of forming OH directly or indirectly. As these compounds are often products of OH radical initiated oxidation they are often termed secondary sources of OH and include the photolysis of HONO, HCHO, H2O2 and acetone and the reaction of 0( D) with methane (see Figure 9). Table 2 illustrates the average contribution of various formation routes with altitude in a standard atmosphere. 

In the aromatic chamber experiments aerosol particles are formed and chemistry may take place on the surface of these particles. For example, NO2 could partition to the condensed phase and be reduced to HONO by the surface bound species on the secondary organic aerosol formed in the aromatic chamber experiments. Such heterogeneous processes would provide a sink for NOx and a source of radicals via the photolysis of HONO). 

In the course of the HONO photolysis, the OH radical is produced. Its importance for the production of the photochemical smog was mentioned earlier (during the NO oxidation reactions). The photolysis of HONO may thus be considered as the governing factor for the formation of the whole... 

Molecules that contain both unsaturation and abstractable hydrogen atoms, such as alkylated olefins or alkylbenzenes, almost always give products derived from both pathways, although, as mentioned, addition is usually much more important. Thus, for example, in a study in which -OH radicals were generated by photolysis of HONO, benzene was converted to phenol toluene, mostly to cresols with some benzaldehyde and ethylbenzene to ethylphenols, acetophenone, and benzaldehyde (Hoshino et al., 1978). The cresols were dominated by o-cresol, which made up... 

This widely discussed process only shifts (because of much faster photolysis of HONO compared with that of O3) the photo-steady states and species reservoir distribution. 

The possible mechanisms for the proposed processes (HI) and (IV) remains highly uncertain. It is possible that these processes may be related, as formation of OH could occur from the secondary dissociation of an excited HONO product of process (III) or the subsequent photolysis of HONO formed in that primary process. 

The relatively high concentrations of HONO observed during the initial period can be attributed to high OH and NO levels, with HONO being in photostationary state due to its rapid photolysis (11). 

It should be noted that only a portion of the O( D) formed generates OH via reaction (2a) the remainder is deactivated to ground-state 0(3P), reaction (2b), which then re-forms O,. For example, at 50% RH and 300 K at the earth s surface, about 10% of the O( D) formed generates OH. As a result, as discussed later in this chapter, the relative importance of (2a) decreases at higher altitudes due to the decrease in water vapor. This is also an important source in polluted areas, where, however, there are additional sources as well. These include the photolysis of gaseous nitrous acid (HONO) and hydrogen peroxide (H202) ... 

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. 

As expected based on our knowledge of gas-phase chemistry, in addition to the Fenton type chemistry involving iron, photolysis of Os, H202, HONO, and HNO-, are all potential OH sources in clouds and fogs. In addition, the photolysis of nitrite, nitrate, and HOJ in aqueous solutions can also form OH. In short, there are many potential sources of OH in clouds and fogs. 

HOBr also serves to couple bromine and chlorine chemistry in an indirect manner. Thus, photolysis of HOBr generates increased OH concentrations, which then cause a faster recycling of HC1 back into chlorine atoms (Lary et al., 1996 Randeniya et al., 1996a,b Tie and Brasseur, 1996). Lary et al. (1996) estimated that the lifetime of HC1 can be reduced by as much as a factor of three through this effect and suggest that the unexplained rapid rise in OH reported by Salawitch et al. (1994) at dawn may be due to the photolysis of HOBr formed overnight rather than of a nitrogen species such as HONO. 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

In an earlier long-path FTIR study of the photolysis of the HONO—NO—C2H4—02 system conducted in this laboratory [109], the reaction of HO radicals with C2H4 was shown to yield two HCHO molecules up to at least 80% of the time. These results suggest the occurrence of the following series of elementary reactions. 

Some molecules in this group (HONO, NC j 0, HONC ) have been extensively studied because the photofragments OH and NO can be probed by tunable lasers. These molecules are important minor constituents in the earth atmosphere and their photochemistry plays a major role in air pollution. Atmospheric pollutants N0X (NO, NO2, NO3) are formed from combustion of fuel and subsequent chemical reactions in the atmosphere. Photolysis of alkyl oxides produces NO and NO2 that can be probed by LIF the internal energy distribution provides an important clue to the mechanism of photodissociation. 

A preferential population of one spin-orbit state has been found for OH in the photodissociation of HONO, where the higher state is more populated than predicted by statistics (Vasudev, Zare, and Dixon 1984 Shan, Vorsa, Wategaonkar, and Vasudev 1989). In NO(2n) generated via photolysis of CH3ONO (Lahmani, Lardeux, and Solgadi 1986 Briihlmann, Dubs, and Huber 1987), (CHa NNO (Dubs, Briihlmann, and Huber 1986 Lavi and Rosenwaks 1988), or (CHa CONO (Schwartz-Lavi, Bar, and Rosenwaks 1986) the lower state is slightly more populated than the upper state. The reason for this preference is not yet clear. 

The direct photolysis of compounds such as HONO, 03, HCHO, and N02 in the tropospheric gas phase is a very important source of reactive species, which are then involved in the transformation of organic compounds. Additionally, some organic molecules including organic pollutants undergo photolysis as a significant or even the main process of removal from the atmosphere. It is for instance the case for nitronaphthalenes, the atmospheric lifetime of which can be as low as a couple of hours because of direct photolysis [11, 12]. 

Figure 5. Comparison of the four dominant atmospheric OH radical sources (photolysis of O3, HONO, HCHO, and ozonolysis of VOC s) for 20" and 21" of July 1998. The difference in the OH production in the early morning hours can be seen easily. After 12 00 on 20 July and 8 00 on 21" July, respectively, the modelled J(HCHO) values were used, because no measmements were available for this time period. The dotted line shows the calculated P(OH) from nitrous acid photolysis during daytime. The data were obtained diuing the BERLIOZ campaign at the Pabstflium site, northwest from Berlin (Platt et al., 2002).

The formation of HONO is balanced during daytime hours (when OH radicals are present at appreciable concentrations) by its rapid ( 10-15 min lifetime at solar noon) photolysis ... 

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