Reactions of Gases at Particle Surfaces

There are some well-known examples of reactions of gases with solids at the interface that are potentially important in the atmosphere. For example, the reactions of 03 with adsorbed solid polycyclic aromatic hydrocarbons (PAH) are discussed in Chapter 10. Another example is the reaction of NaCl and NaBr in sea salt particles with gaseous oxides of nitrogen such as HN03 (see Chapter 6.J)  

The replacement of chloride in sea salt particles by nitrate as well as by sulfate has been observed in many measurements in coastal areas using bulk filter samples (e.g., see Section C.l and Chapter 6.J.2b) and, more recently, in single particles (e.g., Murphy et al., 1997 Gard et al., 1998). Even in these relatively simple cases, 

A further complication is the recent indication that even small amounts of strongly bound surface-adsorbed water may play a critical, indeed determining, role in the interaction of gases with surfaces traditionally thought to be solids. For example, in the NaCl-HN03 reaction, there is evidence that the reaction even in laboratory vacuum systems occurs on sites holding adsorbed water. As a result, the surface does not become saturated as one would expect for a solid surface, since the underlying reactant salt continues to dissolve in the surface water (Beichert and Finlayson-Pitts, 1996). 

Another example of reactions at interfaces that is only now being recognized, due to the lack of suitable experimental techniques in the past, is that of species such as SOz and NOz at liquid interfaces. As discussed in Chapters 7 and 8, there is increasing evidence that the reactions of such species at the air-water interface can be fast relative to that in the bulk and may have unique reaction mechanisms compared to those in the bulk or gas phases. Given the paucity of data on such processes at the present time, they are generally not included in present models of aerosol growth. How- 

TABLE 9.10 Typical Time Scales for Various Aerosol Fates  

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