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FIGURE 7.18 Ratio (K*/K)54r54, where K* and K are the equilibrium constants over the solution and over the solid at various temperatures (°C) (adapted from Mozurkewich, 1993).

Frequency (cm"1)

FIGURE 7.19 Infrared spectrum of air in Riverside, California, on August 12, 1977, showing the coexistence of the strong acid HNO, and the strong base NH3 (graciously provided by E. C. Tuazon).

Frequency (cm"1)

FIGURE 7.19 Infrared spectrum of air in Riverside, California, on August 12, 1977, showing the coexistence of the strong acid HNO, and the strong base NH3 (graciously provided by E. C. Tuazon).

It is interesting that because of this equilibrium, the strong acid HN03 and the strong base NH3 have been observed to coexist in air. Figure 7.19, for example, shows the infrared spectrum of air when both of these species could clearly be identified.

From such measurements, the product (-Puno^nii,) measured in air has been calculated and compared to the calculated value based on the known equilibrium constants. Figure 7.20, for example, shows one comparison between the calculated and measured values in Rubidoux, California, about 85 km east of Los Angeles. The agreement between the two is quite good on the first day in particular (Hildemann et al., 1984). That the agreement is not perfect is not surprising, given that the particles undoubtedly contain a variety of other

30 August 1982 Time (PDT) 31 August 1982

FIGURE 7.20 Experimentally measured concentration product [HN03][NH3] (in units of ppb2) and calculated value of K = [HN03][NH3] for two days at Rubidoux, California (adapted from Hildemann et al., 1984).

30 August 1982 Time (PDT) 31 August 1982

FIGURE 7.20 Experimentally measured concentration product [HN03][NH3] (in units of ppb2) and calculated value of K = [HN03][NH3] for two days at Rubidoux, California (adapted from Hildemann et al., 1984).

FIGURE 7.18 Ratio (K*/K)54r54, where K* and K are the equilibrium constants over the solution and over the solid at various temperatures (°C) (adapted from Mozurkewich, 1993).

species as well (see Chapter 9), and hence the thermodynamics should take these into account rather than treating it as a pure HN03-NH3-NH4N03 system.

The formation of ammonium nitrate has some interesting implications for visibility reduction. In the Los Angeles air basin, for example, the major NOx sources are at the western, upwind end of the air basin. Approximately 40 mi east in the vicinity of the city of Chino, there is a large agricultural area that has significant emissions of ammonia. The geographical distribution of the estimated emissions of NOx and NH3 in 1982 are shown in Fig. 7.2L (It should be noted that vehicles with three-way catalysts appear to be significant sources of NH3 as well, so that this distribution has likely changed with the introduction of such controls on vehicles; Fraser and Cass, 1998.) As a result, under typical meteorological conditions, air is carried inland during the day, with NOx being oxidized to HN03 as the air mass moves downwind. When it reaches the agricultural area, the HN03 reacts with gaseous NH3 to form ammonium nitrate. As discussed in Chapter 9, the particles formed by such gas-to-particle conversion processes are in a size range where they scatter light efficiently, giving the appearance of a very hazy or smoggy atmosphere even though other manifestations of smog such as ozone levels may not be highly elevated. A result of this reaction is that particle nitrate levels measured further downwind, e.g., at Ru-bidoux, tend to be elevated. Table 7.3, for example, shows the distribution of nitrogen for Rubidoux and at West Covina, a location upwind. The large increase in particulate nitrate after the air mass passes over the region of high ammonia emissions is clear (Spicer, 1982).

Similar "titrations" of gaseous HN03 with NH3 to form new NH4N04 particles and/or to enhance the growth of existing particles as the air mass passes over rural and agricultural areas have been observed in other locations as well, such as the Fraser River Valley east of Vancouver, in rural Ontario, Canada (e.g., Barthelmie and Pryor, 1998; Makar et al., 1998), and in Phoenix, Arizona (Watson et al., 1994).

Finally, in some areas with unique chemical compositions, HN03 may have the opportunity to react with other species as well. For example, it reacts relatively rapidly with NaCl, the major component of sea salt particles:

FIGURE 7.21 Estimated daily emissions of NH^ and NOr in the Los Angeles area in 1982 (adapted from Russell and Cass, 1986).
TABLE 7.3 Oxidized Nitrogen Distribution at Selected Locations" (Percentage of Total)

Location

West Covina, CA

Claremont, CA"*

Rubidoux, CA

Phoenix, AZ

St. Louis, MO

Quail Farm, NJ

Year:

0 0

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