Reaction time (hr)

FIGURE 7.15 Measured loss of gaseous N205 in the presence of 8100 ppm H20 and formation of HN03 as a function of reaction time in a large chamber (250 m1) and model-predicted HN03 for the heterogeneous wall hydrolysis of N205 for the combinations of wall loss plus bimolecular gas phase (N205 + H20) reaction and wall loss plus bimolecular and termolecular (N2Os + 2H20) reaction (adapted from Wahner et al., 1998a).

2. Reactions of N205

It is interesting that while N2Os is believed to play a significant role in tropospheric chemistry, it has never been directly measured in the troposphere. However, using the measured concentrations of N02 and N03 and the equilibrium constant for the reaction in which it is formed,

peak concentrations of N205 up to about 10—15 ppb in a polluted atmosphere have been calculated (Atkinson et al., 1986). ft is interesting that even at a rural site in the Baltic Sea region, however, the calculated mean N205 concentration has been estimated to be ~1 ppb, based on measurements of N03 and NOz (Heintz et al., 1996). The major loss process in the troposphere appears to be hydrolysis, although other reactions may contribute to a small extent.

a. Hydrolysis

Gaseous N205 reacts with water to form HN03 both in the gas phase and on surfaces. The reaction of N2Os in the gas phase is sufficiently slow in most studies that only an upper limit to the rate constant of ~10~21 cm3 molecules_1 has been determined (e.g., see Atkinson et al., 1986; Hjorth et al., 1987; and Sverdrup et al., 1987). However, Mentel et al. (1996) and Wahner et al. (1998a) have used a large (250 m3) chamber to study both the gas-phase and surface hydrolysis of N2Os and report that the gas-phase reaction has both bimolecular and termolecular components:

N205(g) + HzO(g) 2HN03(g), (45) N2Q5(g) + 2H2Q(g) 2HNQ3(g) + H2Q(g), (46)

where k45 = 2.5 X 10~22 cm3 molecule 1 s_1 and k4h = 1.8 X 10-39 cm6 molecule-2 s-1. Figure 7.15, for example, shows the loss of gaseous N205 and formation of HN03 as a function of time compared to model predictions using three different assumptions. The lowest curve assumed that HN03 was only formed from the wall reaction of N2Os. The next curve assumed that in addition, the bimolecular, gas-phase reaction (45) occurred. The third curve, which matches the HN03 data quite well, assumes that in addition to the wall and bimolecular reactions, there is also a termolecular reaction (46). The implications of a possible termolecular contribution whose rate is proportional to [H20]2 are significant; for example, Wahner et al. (1998a) suggest that at 50% RH and 291 K, the lifetime of N205 with respect to hydrolysis decreases to about 1.5 h from 4.5 h when the termolecular reaction is included.

N205 hydrolysis is well known to be catalyzed by surfaces; i.e., the reaction occurs rapidly on the aqueous films found on many surfaces:

This is believed to be a major source of atmospheric HN03. For example, Ljungstrom and Hallquist (1996) report that the calculated rates of N03 formation and the measured rates of wet deposition of nitrate at a site in Sweden were very similar; they intepreted this as being due to the formation of N205, followed by its hydrolysis (although the direct uptake and reaction of N03 presumably could have contributed as well).

There have been many studies of reaction (47) using sulfuric acid or sulfate aerosols (e.g., ammonium sulfate) of various compositions and over a range of temperatures. The reaction probability (y47) for (47) falls in the range of 0.05-0. f 6 over H2S04 concentrations from 39 to 96% (w:w) and from 2f 3 to 298 K (e.g., see Lovejoy and Hanson, 1995; and DeMore et al., 1997). Hu and Abbatt (1997) have shown that y47 for hydrolysis on both sulfuric acid and ammonium sulfate particles decreases from ~0.05 to 0.02 as the relative humidity falls. However, Wahner et al. (1998b) have reported much smaller values on NaNO-, aerosols at low RH, y47 = 0.0018 at 48% RH and 0.0032 at 62% RH, which they attributed to increased importance of the reverse reaction of NO^" with NO^ to regenerate

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