Formation of HONO

As already discussed, a major source of HONO is believed to be heterogeneous reactions of N02, includ- 

Zhu and co-workers (1993) observed the formation of HONO in an environmental chamber during the decay of peroxynitric acid, H02N02, and suggested that H02N02, formed in the H02 + N02 reaction, 

FIGURE 7.9 Concentration-time profiles of H02N02, HONO, and HN03 after 30-s irradiation of a mixture containing Br2 (20 ppm), HCHO (3.9 ppm), and N02 (6.6 ppm) in 700 Torr of air (adapted from Zhu et al., 1993). 

It is noteworthy that measurements of OH, H02, NO, N02, and CIO in the lower stratosphere also suggest there is some as yet unrecognized source of HONO in that region. The measurements can be adequately modeled assuming that reaction (28) is the HONO source, with particles providing the surface (Salawitch et al., 1994). 

In addition to what appears to be a heterogeneous chemical source for HONO, it has also been shown to be emitted directly from combustion systems. For example, it has been measured in the exhaust of noncatalyst-equipped automobiles (Pitts et al., 1984b), from natural gas combustion in a kitchen stove, and in the emissions from kerosene and propane space heaters (e.g., Pitts et al., 1985, 1989 Brauer et al., 1990 Febo and Perrino, 1991,1995 Spicer et al., 1993 Vecera and Dasgupta, 1994). 

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Notholt et al. (1992) and Andres-Hernandez et al. (1996) measured HONO, NO, N02, and aerosol surface areas at both urban and nonurban locations. They observed that at Ispra, Italy, HONO concentrations tended to correlate with N02, NO, and aerosol surface areas. Such studies support the formation of HONO from heterogeneous reactions of N02 at the surfaces of aerosol particles, fogs, buildings, and the ground. 

Figure 7.9, for example, shows the decay of H02N02 and the formation of HONO and HN03 in their chamber. The peroxynitric acid was generated by reaction (27), where the H02 was formed by the bromine atom initiated oxidation of formaldehyde in air. Zhu et al. 

The belief generally has been that the smaller the S/V ratio (i.e., the larger the smog chamber), the less important such surface reactions will be, and hence the more representative of the ambient atmosphere the results. While there is doubtless some justification for this approach, it must also be kept in mind that there are a variety of surfaces present in real atmospheres as well. These include not only the surfaces of the earth, buildings, and so on but also the surfaces of particulate matter suspended in air (Chapter 9). If the heterogeneous formation of HONO occurs not only on chamber surfaces but also on those found in urban atmospheres as well, then it is important to include it in extrapolating the chamber results to ambient air. In this case, the effects on the kinetics due to the different types and available amounts of surfaces in air compared to chambers must, of course, be taken into account. 

While chamber contamination and the presence of unknown surface reactions are probably the most important problems in extrapolating smog chamber data to atmospheric conditions, other minor problems exist as well. These include the need to measure carefully and frequently a number of chamber-specific parameters such as the decay rate of 03 on the chamber walls and the initial formation of HONO. Such chamber-specific parameters raise the question again of how best to modify these parameters to describe ambient air. 

The first step is the formation of HONO, we are told, and it is clear that the amine must be a nucleophile as amines can only play that role. HONO must be the electrophile so we had better combine them to form an N-N bond as that is present in the product. 

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 ... 

Finally, the effects of oxygen we observed cannot be fully explained. However, expecting the chemistry related to the charge separation to be affected by oxygen is reasonable because donor-acceptor complexes between aromatic hydrocarbons and 02 are known (20). The formation of HONO in the nitration ultimately leads to the 1V6 stoichiometry reported by other workers. As described by Eberson and Radner (7), HONO decomposes to water, NO, and N02. However, in our case, NO must be oxidized by oxygen or other oxidants present in the system back to N02. Alternatively, HONO could be oxidized its reaction with oxygen in solution very rapid (21). 

Studies of the formation of HONO from secondary nitramines, R2N(N02) (R = -CH2-), illustrate an advance made possible by Fast Thermolysis/FTTR methods [I8]. HONO has been considered to be an important intermediate in the thermal decomposition of nitramines [19], but, because of its reactivity, was proposed based on indirect evidence [20,21] until this Fast Thermolysis/FTIR technique was applied. Cis- and trans-HONO are both present in the IR spectrum of the gas from RDX (see the PQR pair at 700-900 cm in Figure 2), but as shown in Figure 3, HONO is transient under the conditions of the experiment. The initial concentration most closely reflects its relationship to the composition of the parent molecule. Figure 4 shows the quantity of HONO as a percentage of the initial gas products for various nitramines [18] versus the H/NO2 ratio in the parent molecule. The general trend suggests that HONO arises from adventitious bimolecular encounters of H and N02 radicals in the condensed phase [18], rather than concerted decomposition of the 4- and 5-center unimolecular intermediates shown below that may contribute in the gas phase [22]. 

Tendency to Form HQNO(g). Closely related to NO2 is the formation of HONO. At least one additional process, that of H participation, is required before HONO(g) is detected. Kinetic modelling indicates that HONO formation plays a key role in the N-N bond fission process [32]. The formation of HONO has been used in many previous studies of nitramines to rationalize products, but it was not detected directly before Fast Thermolysis/FTIR Spectroscopy was applied. HONO is a reactive and, thus, transient molecule which is not observed without rapid heating and near real-time product detection. However, both the cis and trans-HONO isomers can now be routinely observed from nitramines [30]. 

The uptake and reaction of NO2 on soot is very important as a process to release nitrous acid (HONO) into the gas phase. The formation of HONO by the heterogeneous surface reaction and its enhancement by light irradiation has been found for the first time by Akimoto et al. (1987) relevant to the unknown radical source in a smog chamber (see column on p.278). The uptake of NO2 and photocatalytic reaction on soot has been interested in as a model reaction of such heterogeneous process to elucidate the characteristics of HONO formation in the atmosphere. 

Recently, Monge et al. (2010) found that the formation of HONO by the reaction of NO2 on soot is accelerated by the irradiation of light in the range of 300-420 nm,... 

Irradiation of the UDMH + Oq Reaction Products. One experiment was conducted in which the UDMH + O3 reaction products (with UDMH in slight excess) were irradiated by sunlight. The results are shown in Table I and Figure 1. It can be seen that rapid consumption of UDMH, the nitrosamine, and HONO occurred, with N-nitrodimethylamine (also dimethyInitramine) and additional formaldehyde being formed. The formation of nitramine upon irradiation of the nitrosamine is consistent with results of previous studies in our laboratories (9,10), and probably occurs as shown ... 

Dark Decay of UDMH in the Presence of NO, When 1.3 ppm of UDMH in air was reacted in the dark with an approximately equal amount of NO, 0.25 ppm of UDMH was consumed and formation of -0.16 ppm HONO and -0.07 ppm N2O was observed after -3 hours. Throughout the reaction, a broad infrared absorption at -988 cm" corresponding to an unidentified product(s), progressively grew in intensity. The residual infrared spectrum of the unknown product(s) is shown in Figure 2a. It is possible that a very small amount (50.03 ppm) of N-nitrosodimethylamine could also have been formed but the interference by the absorptions of the unknown product(s) made nitrosamine (as well as nitramine) detection difficult. No significant increase in NH3 levels was observed, in contrast to the UDMH dark decay in the absence of NO. Approximately 70% of the UDMH remained at the end of the 3-hour reaction period this corresponds to a half-life of -9 hours which is essentially the same decay rate as that observed in the absence of NO. 

The formation of nitrosamines in aprotic solvents has applicability to many practical lipophilic systems including foods (particularly bacon), cigarette smoke, cosmetics, and some drugs. The very rapid kinetics of nitrosation reactions in lipid solution indicates that the lipid phase of emulsions or analogous multiphase systems can act as "catalyst" to facilitate nitrosation reactions that may be far slower in purely aqueous media (41, 53, 54). This is apparently true in some cosmetic emulsion systems and may have important applicability to nitrosation reactions in vivo, particularly in the GI tract. In these multiphase systems, the pH of the aqueous phase may be poor for nitrosation in aqueous media (e.g., neutral or alkaline pH) because of the very small concentration of HONO or that can exist at these pH ranges. 

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

The formation of [M — HONO]+ has been demonstrated by deuterium labelling to be associated with hydrogen transfer from the 2-position to the nitro group101. This leads to a stabilized radical cation which, following the expulsion of HONO, produces a very stable daughter ion see Scheme 31. 

In addition, the formation of myosmine indicates that the diazo- hydroxide intermediate undergoes cyclization followed by loss of HONO. 

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. 

The production of NO has also been observed in this heterogeneous N02-H20 reaction (Sakamaki et al., 1983 Pitts et al., 1984a Svensson et al., 1987). In addition, recent studies show the formation of N20 at longer times, both in the absence of S02 (e.g., Wiesen et al., 1995) and in its presence (e.g., Eriksson and Johansson, 1991 Pires et al., 1996 Pires and Rossi, 1995, 1997). While the mechanism of formation of N20 is not clear, it is thought to involve secondary reactions of HONO (e.g., Kleffmann et al., 1994 see later). Indeed, this heterogeneous hydrolysis of N02 to HONO occurs in exhaust from combustion systems and is responsible for the artifact formation of N20 reported in such samples (e.g., Muzio and Kramlich, 1988 Muzio et al, 1989). 

HONO also undergoes deposition at surfaces in competition with its formation by the N02 heterogeneous reaction with water. For example, the mass accommodation coefficient for HONO on water has been reported to be in the range of 4 X 10 3 to 0.15 over temperatures from 278 to 297 K (e.g., Kirchner et al., 1990 Bongartz et al., 1994 Mertes and Wahner, 1995). Thus aqueous particles and surfaces having adsorbed water can also act as a sink for gaseous HONO. This is consistent with the observations of Harrison et al. (1996) on the direction of HONO fluxes from the surface at various concentrations of N02 at N02 concentrations below 10 ppb in rural areas, surfaces were observed to be a net sink of HONO (e.g., see Harrison et al., 1996 and Harrison and Peak, 1997). 

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). 

FIGURE 7.10 Observed formation of N20 during the decay of HONO in a laboratory system (adapted from Kleffmann et al., 1994). 

Gerecke, A., A. Thielmann, L. Gutzwiller, and M. J. Rossi, The Chemical Kinetics of HONO Formation Resulting from Heterogeneous Interaction of N02 with Flame Soot, Geophys. Res. Lett., 25, 2453-2456 (1998). 

In addition, aerosol particles have indirect effects. The most important of these is their effect on cloud properties, since clouds obviously also have major effects on climate. In addition, since heterogeneous chemistry can occur on aerosol particles (see Chapter 5), it is possible that such chemistry can alter the concentrations of other contributors to the climate system, such as the greenhouse gases. One example is the formation of N20 from reactions of HONO on the surface of aerosol particles (see Chapter 7.C). 

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. 

As discussed in Sections B.3 and C of Chapter 7, this reaction has been shown to be too slow in aqueous solution to be significant in the atmosphere it is faster on surfaces and has been proposed as a source of HONO in smog chambers (e.g., see Sakamaki et al., 1983 Pitts et al., 1984 Leone et al., 1985 and Chapter 7.0. Since HONO is a major OH source in the early stages of irradiation in smog chambers, it is important to understand the mechanism of its formation and to quantify its rate of production under various experimental conditions. Thus, if this reaction only occurs at... 

With in situ spectroscopic techniques, critical data on the formation of such species as HONO, HN03, and N03, which are essential to understanding the chemistry of these systems, can also be obtained. Figure 16.8, for example, shows one portion of an FTIR spectrum obtained in a chamber run for a propene-NOx... 

Repeat I.E above i.e., add the possibility of HONO formation to the mechanism and examine the effects on all of the isopleths. Comment on the reason for differences, if any. 

The key reaction step for the formation of HCHO in this reaction scheme is the unimoiecular dissociation of the oxy-radical CH2(0H)CH20, i.e., reaction (34). In this experiment, products arising from as much as 20% of the C2H4 reacted could not be fully characterized due to experimental difficulties encountered in the product analysis due to the presence of large amounts of H20 impurity in the HONO samples employed [109], Notably, thermochemical estimates for the unimoiecular reaction (34) indicated the predominant occurrence of the competing bimolecular reaction with 02 under atmospheric conditions [104,108,113]. 

However, other studies indicate that the N02 rapidly saturates the surface, so the production of HONO becomes insignificant at longer timescales [12,117]. More studies are needed to clarify the mechanism of HONO formation. 

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