Info

FIGURE 9.36 Average size distributions for sulfate in continental and marine aerosol. A M M-r is the fraction of the total mass of sulfate (Mt) found in each particle size range (adapted from Milford and Davidson, 1987).

Accurate and artifact-free determination of nitrate in particles is much more difficult than accurate determination of sulfate in particles because both positive and negative artifacts can occur. For example, gas-phase HN03 can be taken up by particles as the air mass being sampled is drawn over the collected particles, leading to a positive interference. On the other hand, since ammonium nitrate (which can dissociate to HN03 and NH3; see Chapter 7.E.2) is believed to be one of the major forms of particle nitrate, volatilization of nitrate can occur as the NH4N03 particles are being collected, leading to a negative interference (see, for example, Dunwoody, 1986; Milford and Davidson, f987; and Sturges and Harrison, 1988).

Because of this possibility of artifacts in determining either the concentration of particulate nitrate or its size distribution, caution must be exercised in interpreting field data. However, it appears to be generally true that nitrate is found in both the fine and the coarse particle modes, with the relative amounts in each being highly variable. Milford and Davidson (1987) show that aerodynamic mass median diameters (MMD) from 0.3 to 4.2 /tm have been reported for nitrate in particles in the literature, the larger ones generally being associated with marine regions. For example, the total nitrate size distribution measured in a relatively clean marine area (Fig. 9.35) shows that in this case, it is found primarily in the large-particle mode. This has been attributed to the reactions of NaCl and NaBr in sea salt particles with gaseous oxides of nitrogen such as HN03, N02, N2Os, and C10N02, all of which form NaN03 (see Chapter 7).

However, it is not only in marine areas that nitrate is found associated with larger particles. Table 9.14 for example, shows the composition of particles from the Amazon Basin where there was no significant sea salt contribution (Artaxo et al., 1988). Concentrations of

TABLE 9.f4 Average Elemental and Nitrate Concentrations of Particles Measured at Ducke Reserve in the Amazon Basin"

Concentration (ng m 3 sd)

TABLE 9.f4 Average Elemental and Nitrate Concentrations of Particles Measured at Ducke Reserve in the Amazon Basin"

Concentration (ng m 3 sd)

0 0

Post a comment