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kp for N02 (min"

1)

FIGURE 7.6 Enhancement in the rate of HONO formation from N02 and H20 (O) and in the rate of CH,ONO formation from CHjOH and N02 (x) as a function of light intensity expressed as kp for N02 (min"1). No enhancement corresponds to a factor of 1.0 (adapted from Akimoto and Takagi, f986, and Akimoto et al, 1987).

FIGURE 7.7 Infrared spectra of HONO formed in the reaction of 50 ppm N02 and 4700 ppm H20: (a) Hlf,ONO from the reaction with H260; (b) HlsONO from the reaction with H2sO (adapted from Sakamaki et al., 1983).

Wavenumber (cm1)

Wavenumber (cm1)

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, f997). 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 NzO reported in such samples (e.g., Muzio and Kramlich, 1988; Muzio et al., 1989).

A heterogeneous reaction of N02 has also been observed to occur with soot (e.g., Tabor et al., 1994; Chughtai et al., 1990, 1994; Kalberer et al., 1996; Rogaski et al., 1997; Gerecke et al., f998; Ammann et al., 1998). NO has been observed as a major product in most studies, but large yields of HONO have also been reported by Gerecke et al. (1998) and Ammann et al. (1998). The reaction to form HONO appears to be quite fast, with initial uptake coefficients for N02 in the range of ~10~'-10~4, depending on the conditions such as reaction time, NOz concentration, and type of soot. The initial rate of HONO production was fast, decreasing at longer reaction times (Ammann et al., 1998; Gerecke et ai, 1998).

The yields of HONO from the soot reaction have been observed to be greater than the 50% expected from the stoichiometry of reaction (14). For example, Gerecke et al. (1998) report HONO yields from 68 to 93%, depending on where in the flame the soot was collected and the fuel used to form the soot; this determines the nature of the soot surface, with the yield of HONO decreasing with distance from the flame base. As the HONO yield decreased, that of NO increased. In addition, Ammann et al. (1998) did not detect either gas-phase HNO, nor the amounts of particle-bound nitrogen that would be expected if the HNO-, remained on the surface of the soot. All of these observations led these groups to propose an additional route to HONO formation involving the reduction of N02 by a surface group:

HONO + surface oxidized site + OH

Gerecke et al. (1998) suggest that hydrogen at the surface may be involved in the reduction.

In short, the reduction of N02 on soot is quite fast and forms HONO in a process that may occur in parallel with reaction (14). This reaction may be particularly important in polluted urban areas as well as in the upper troposphere where soot from commercial aircraft is injected into the atmosphere.

There is also evidence from field studies for the generation of HONO, first detected unequivocally in the atmosphere by Perner and Piatt (1979), at surfaces, interpretation of such studies is complex due to the contributions of meteorology and the uncertainty in the nature of the available surfaces for reaction (14) (e.g., Lammel, 1996; Hjorth et al., 1996). However, in semirural areas, the surface appears to be a net source of HONO at concentrations of N02 above 10 ppb (Harrison and Kitto, 1994; Harrison et al., 1996).

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., f 996; and Harrison and Peak, 1997).

Notholt et al. (1992) and Andrés-Hernández 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.

Lammel and Perner (1988) measured gas-phase HONO using DOAS near Mainz (Germany) and, during the same period, collected aerosols using a low-pressure cascade impactor. Analysis of the aerosols for nitrite was carried out and the concentrations were found to be in great excess of that expected based on a Henry's law equilibrium. They suggested that the aerosols must be acting as a source of HONO. It is interesting that Clemens et al. (1997) also found that particles incubated in a reactor were a source of N20, perhaps from the reactions of HONO in or on the particles, as discussed earlier.

Another reaction that has been suggested as a source of HONO in the atmosphere is that of NO, N02, and water:

As for the N02 reaction (14), this reaction is slow if all reactants are in the gas phase, but a surface-catalyzed reaction is possible. Calvert and co-workers (1994) have reviewed possible mechanisms for HONO formation in the troposphere and, based on an analysis of ambient air data for NO, NOz, HzO, and particle concentrations, suggest that reaction (15) may be a good representation of a major HONO source. They hypothesized that the mechanism of the reaction may involve the initial formation of N203 from the reaction of NO with NOz and that N203 then reacts with water on surfaces to form HONO. However, a variety of laboratory studies on the formation of HONO from the reaction of N02 at surfaces find no effect of adding NO, indicating that reaction (f5) is unlikely to be important (e.g., Pitts et al., 1984a; Wiesen et al., 1995; Gerecke et al., 1998; Kleffmann et al., 1998).

Understanding the kinetics and mechanisms of the production of HONO from N02 reaction is very important in that it appears to be a major source of HONO in smog chambers, inside homes, from automobiles (see Chapter 15), and perhaps in the troposphere at large. Clearly, more studies are warranted, especially those that can elucidate the nature and concentrations of species at the interface itself.

c. Reaction with Alcohols

A reaction of NOz similar to reaction (14) occurs with alcohols. The reaction with methanol, for example, produces methyl nitrite:

The homogeneous gas-phase reaction is slow, depending on the square of the NOz concentration and with a third-order rate constant at room temperature of klh = (5.7 ± 0.6) x 10"37 cm6 molecule"2 s"1 (Niki et al., 1982; Koda et al., 1985).

However, on surfaces the reaction is much faster. For example, Takagi et al. (1986) studied this reaction in the presence of different surfaces, including quartz, Pyrex glass, stainless steel, and PFA (tetrafluoroethyl-ene-perfluoroalkyl vinyl ether copolymer) coatings, and found the reaction rate decreased in going from stainless steel to Pyrex glass to a PFA coating to quartz. The CH30H-N02 reaction (16) was found to have a reaction order with respect to N02 of 1.0-1.5, depending on the nature of the surface. As shown in Fig. 7.6, like the reaction with H20, the reaction can also be pho-toenhanced (Akimoto and Takagi, 1986).

This particular reaction may be important when sampling exhaust from methanol-fueled combustion sources. For example, significant concentrations (46-69 ppm) of methyl nitrite have been reported in the exhaust of a methanol-fueled bus (Hanst and Stephens, 1989). Such large concentrations would be of concern if the use of methanol became widespread, since methyl nitrite acts as a free radical source via its photolysis and hence contributes to the NO to N02 conversion and ultimately to 03 formation:

However, if one applies the heterogeneous kinetics reported by Takagi et al. (1986), one can calculate that methyl nitrite concentrations of the order measured in the exhaust sample could arise by reactions of un-burned methanol and NOz on the walls of the sampling bag prior to analysis (Finlayson-Pitts et al., 1992).

In short, while such heterogeneous reactions are not at all well understood, they may play very important roles in laboratory apparatus, in sampling systems, and perhaps in ambient air, where a variety of surfaces are available.

4. Other Reactions of NOz a. Organics

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

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