Nitrous Acid HONO

Nitrous acid (HONO) is formed not only by the homogeneous reaction of OH -I- NO, but by the heterogeneous reactions of NO2 with ground stuface, which is further enhanced by photo-irradiation (see Sect. 6.4.2), it exists in the polluted air 

Photolysis Quantum Yields The broad bands without rotational structure in the spectrum shown in Fig. 4.12 implies that the dissociation lifetime of the excited states is short. Indeed, the HONO molecules absorbing the radiation in this wavelength region is known to dissociate in the pathway, 

Wave- length (nm) 10 o (cm molecule ) Wave- length (nm) 10 o (cm molecule ) Wave- length (nm) 10 o (cm molecule ) Wave- length (nm) 10 o (cm molecule ) Wave- length (nm) 10 o (cm molecule ) 

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Scheme 2.1 Simplified mechanism whereby organic nitrates (RONO2) effect vasorelaxation. Thiols (R -SH) interact with organic nitrates to give nitrite (NO2-), which is converted successively to nitrous acid (HONO) and NO. NO then reacts with a thiol (R11 —SH) to give a nitrosothiol...

Of special interest here is the radical generation during the early part of the irradiation, before the oxidant concentration has developed photodissociation of nitrous acid, HONO, and aldehydes is very important. The concentration of nitrous acid in the atmosphere due to the reaction... 

Becker, K.H., Kleffmann, J., Kurtenbach, R., and Wiesen, P. Solubility of nitrous acid (HONO) in sulfuric acid solutions, J. Phys. Chem., 100(36) 14984-14990, 1996. 

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) ... 

Related to the uptake and reaction of N02 into liquid water and at the interface is a so-called heterogeneous dark reaction of gaseous N02 with water vapor to form nitrous acid, HONO. Potential formation processes and reactions of HONO in the atmosphere have been reviewed by Lammel and Cape (1996). This is a fascinating reaction in that, despite decades of research, the mechanism is still not understood. It occurs on a variety of surfaces, including water and acid surfaces (e.g., Kleffmann et al., 1998) and, as discussed in this chapter, on soot as well. 

In principle, N02 can abstract a hydrogen atom from organics to form nitrous acid, HONO. For example, Pryor and Lightsey (1981) suggest that N02 at low concentrations in solution abstracts from the weak allylic C-H bond ... 

Wingen, L. M., W. S. Barney, M. J. Lakin, T. Brauers, and B. J. Finlayson-Pitts, A Unique Method for Laboratory Quantification of Gaseous Nitrous Acid (HONO) Using the Reaction HONO + HC1 > C1NO + H20, J. Phys. Chem., submitted for publication (1999). 

Historically, the major acids believed to contribute to acid deposition in the troposphere have been sulfuric and nitric acids, formed by the oxidation in air of S02 and oxides of nitrogen, respectively. However, there is an increasing recognition that organic acids may contribute significantly to the total acid burden and indeed may represent the major acidic species even in polluted urban environments. In addition, since nitrous acid (HONO) is formed whenever N02 and water are present (see Chapter 7.B.3), its contribution to the total acidity, particularly to indoor air environments, has become of interest and concern. 

In the absence of such sources of NO, indoor and outdoor concentrations are quite similar (e.g., Weschler et al., 1994), since removal of NO and N02 indoors, e.g., on surfaces, is relatively slow. However, as discussed shortly, although the surface reaction of N02 is relatively slow, it is still of interest since it generates nitrous acid (HONO). Different surfaces found inside homes have been found to have different removal rates for N02. Figure 15.4, for example, shows measured rates of removal of N02 by a number of common household materials (Spicer et al., 1989). Large variations in removal rate (and hence the formation of products such as NO and HONO see later) are evident, varying from negligible for plastic storm windows to quite large for wallboard. 

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). 

Prepared (I) by reaction of silver hypnnitritc Ag N 0 and hydrogen chloride in anhydrous ether, an evaporation of the resulting solution. (2) by reaction of hydroxvlaminc IINOII plus nitrous acid HONO. 

Some of the most important reactions of amines are brought about by nitrous acid (HONO). The character of the products depends very much on whether the amine is primary, secondary, or tertiary. In fact, nitrous acid is a useful reagent to determine whether a particular amine is primary, secondary, or tertiary. With primary amines nitrous acid results in evolution of nitrogen gas with secondary amines insoluble yellow liquids or solid 7V-nitroso compounds, R2N—N=0, separate tertiary alkanamines dissolve in and react with nitrous acid solutions without evolution of nitrogen, usually to give complex products ... 

As a result of its unique structure and properties, the NO molecule exhibits a wide variety of reactions with various chemical species. In particular, it readily releases an electron in the antibonding tt orbital to form the stable nitrosyl cation NO+, increasing the bond order from 2.5 to 3.0, so that the bond distance decreases by 9 pm. The NO species in an aqueous solution of nitrous acid, HONO, is NO+ ... 

Propose a mechanism for the formation of the diazonium salt referred to in the chapter. The first step is the formation of nitrous acid HONO. 

When primary amines are treated with nitrous acid (HONO), or more usually with a nitrite salt or an alkyl nitrite in acid solution, an unstable diazonium salt is formed. You met diazonium salts in Chapter 22 undergoing coupling reactions to give azo compounds, but they can do other things as well. First, a reminder of the mechanism of formation of these diazonium salts. The very first stage is the formation of the reactive species NO+. 

Nitrous acid, HONO, is considered to be an important species [2] in the photochemistry of the troposphere since it is a source of hydroxyl radical [97]. 

REACTION WITH NITROUS ACID, HONO 1. Primary Amines (Diazonium Ion Formation)... 

Inorganic Reactions. Photooxidation of propylene in the presence of oxides of nitrogen involves numerous inorganic reactions. The role of the NO2 photolysis in initiating O- and Og-reactions has already been discussed. Another inorganic compound of photochemical interest is nitrous acid, HONO, since it provides another source for OH radicals. A reaction scheme for the formation and subsequent photodissociation... 

Each class of amine yields a different kind of product in its reaction with nitrous acid, HONO. This unstable reagent is generated in the presence of the amine by the action of mineral acid on sodium nitrite. 

The most significant soot reaction identified to date is the conversion of nitrogen dioxide (NO2) to nitrous acid (HONO) and smaller amounts of nitric oxide (NO) (Ammann et al. 1998 Gerecke et al. 1998 Kalberer et al. 1999 Longfellow et al. 1999 Al-Abadleh and Grassian 2000 Alcala-Jomod et al. 2000 Stadler and Rossi 2000 Saathoff et al. 2001), as follows ... 

Jefferson DA (2000) The surface activity of ultrafine particles. Phil Trans Roy Soc Lond A 358 2683-2692 Jose-Yacamai M (1998) The role of nanosized particles. A frontier in modem materials science, from nanoelectronics to environmental problems. Metall Mater Trans A 29 713-725 Kalberer M, Ammann M, Arens F, Gaggeler HW, Baltensperger U (1999) Heterogeneous formation of nitrous acid (HONO) on soot aerosol particles. J Geophys Res 104 13825-13832 Kamm S, Mohler O, Naumann KH, Saathoff H, Schurath U (1999) The heterogeneous reaction of ozone with soot aerosol. Atmos Environ 33 4651-4661... 

Broske, R., J. Kleffmann and P. Wiesen Heterogeneous conversion of NOj on secondary organic aerosol surfaces A possible source of nitrous acid (HONO) in the atmosphere Atmos. Chem. Phys. 3, (2003) 469-... 

Nitrous acid. HONO, which is formed by a heterogeneous reaction involving N02 and H20 (Calvert et al. 1994), is a reservoir for both HO, and NO,. HONO dissociates by photolysis to regenerate OH and NO ... 

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