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Source: Adapted from Spicer (1982).

" Based on 8-h daylight samples unless otherwise specified. h Generally below instrument detection limit. ' 24-h averages.

'' Only 8 days of data available. ' Tuazon et al. (1981). ' Ratio of peak concentrations was 0.3-0.8.

Source: Adapted from Spicer (1982).

" Based on 8-h daylight samples unless otherwise specified. h Generally below instrument detection limit. ' 24-h averages.

'' Only 8 days of data available. ' Tuazon et al. (1981). ' Ratio of peak concentrations was 0.3-0.8.

Measurements of the reaction probability for reaction (55) are somewhat controversial. The reaction is quite slow on single crystals, with a reaction probability < 4 x fO"4 (Laux et al., 1994). Small crystals or ground powders react much faster, with a reaction probability of about (f-2) x fCT2 (Leu et al., 1995; Fenter et al., 1996; Beichert and Finlayson-Pitts, 1996). Water adsorbed to defects, steps, or edges of the crystal plays a major role in determining the reactivity, and the reaction has been hypothesized to occur in water bound to certain sites on the surface (Beichert and Finlayson-Pitts, 1996). Thus, HN03 is taken up into a "solution" on the surface and acidifies it to the point that HC1 degasses. As HN03 continues to be taken up into this aqueous portion of the surface, nitrate is removed by precipitation of NaN03 and H+ in the form of gaseous HC1. This is consistent with the lack of an apparent surface saturation for the reactions of NaCl powders, in contrast to single crystals under high vacuum, which are known not to hold adsorbed water.

On the other hand, Davies and Cox (1998) have reported a dependence of the reaction probability on the gas-phase concentration of HN03 as well as water vapor. The reaction probabilities they report are also one to two orders of magnitude smaller than those of the other powder studies. They interpret their data in terms of a modified version of the Beichert and Fin-layson-Pitts (1996) mechanism, in which HN03 first physisorbs to the surface and then diffuses to a reactive site that holds water. A modified Langmuir reaction scheme has been proposed by Ghosal and Hemminger (1999), which explains the observed dependence on

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