Similar chemistry leading to the formation of the products shown in Table 9.22 is discussed by Forstner et al. (1997b). Some of these products may accelerate the photodegradation of less reactive species in the condensed phase (e.g., McDow et al., 1996).

The yields of secondary organic aerosols from a series of aromatic hydrocarbon-NOx oxidations have been measured by Odum et al. (1997a, 1997b). They showed that the total secondary organic aerosol formed from a mixture of aromatic hydrocarbons can be approximated as the sum of the individual contributions. Based on their experiments, the yield of secondary organic aerosols expressed as the total organic particle mass concentrations formed, A M0 (in ¡xg m 3), divided by the mass concentration of aromatic precursor reacted, A (aromatic), is given by


The yield of secondary organic aerosol depends on the organic particle mass concentration because of the gas-particle partioning of the semivolatile organic products (see later). Thus, Odum et al. (1996) showed that the yield of secondary organic aerosol, Y, is given by

In Eq. (LL), M0 is the concentration of the condensed-phase organic (in fig m 3) available to absorb semivolatile organic products, ai is a constant that relates the concentration of the ith secondary organic aerosol component formed, C(, to the amount of parent precursor organic reacted i.e., C, (ng m 3) f000a,A(parent organic in ¡xg m 3), and Komi is the gas-particle partioning coefficient for the /th component. As discussed in more detail in Section D, K j is in effect an equilibrium constant between the condensed- and gas-phase concentrations.

Thus, if a particle secondary oxidation product does not get partitioned efficiently into the condensed phase (i.e., Kt)m • is small) or the available organic condensed phase for uptake of the semivolatile product is small, Eq. (LL) reduces to Y M()T.ajKomJ and the secondary organic aerosol yield is proportional to the amount of condensed phase available for uptake of the low-volatily gaseous products. On the other hand, if KomJ and M0 are large, Eq. (LL) becomes Y Ea;, independent of the amount of condensed phase available for product uptake.

Figure 9.53, for example, shows a plot of the yield of secondary organic aerosol from the VOC-NOx oxidation in air of some aromatic compounds as a function

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