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isoprene with respect to OH at 1 X 106 cm"3, 03 at 30 ppb, NO, at 10 ppt, and CI at 1 X 105 cm"3 are approximately 3, 30, 2, and 6 h, respectively. Because these compounds are so reactive, they can contribute significantly to the formation of 03 and other secondary pollutants (vide infra) and hence are important to include in both urban- and regional-scale models.

For example, Chameides and co-workers (1988) suggested that the emissions of biogenic VOC in the southeastern United States may be sufficiently large that without concurrent NOx controls, even 100% control of the anthropogenic emissions of VOC would not be sufficient to meet the air quality standard for 03. When the high reactivity of biogenics is taken into account (vide infra), their contribution to 03 formation is proportionately much larger than expected simply on the basis of concentrations (e.g., see Cardelino and Chameides, 1995). At some locations even in relatively polluted urban areas such as Los Angeles, the contribution may be more important than previously realized (e.g., see Harley and Cass, 1995). However, in the latter case, a majority of the emissions appears to occur in the mountains on the northern and eastern ends of the airshed where ozone is most sensitive to NOx; hence their contribution to ozone formation may be less than would be implied by their total mass (e.g., Benjamin et al., 1997).

The relative importance of biogenic emissions depends on the nature of the air mass into which they are emitted, which thus includes where in a particular air basin they are emitted as well. For example, model calculations suggest that if there are large NOx sources, the interaction with biogenic emissions can lead to significant 03 formation. (On the other hand, if biogenic emissions are small compared to anthropogenic sources, this is not the case (Roselle et al., 1991).)

As expected, then, inclusion of biogenic emissions in models can have a significant effect under some conditions on the predicted effects of VOC versus NOx control. For example, Pierce et al. (1998) show that when increased isoprene emissions are included in the RADM model, ozone formation in many regions of eastern North America is predicted to be more sensitive to reductions in NOx rather than in VOC.

The relative contributions of biogenics clearly will be location dependent. For example, studies based on radiocarbon (l4C) abundances suggested that the biogenic contribution to the total VOC concentration in Atlanta, Georgia, was only 9-17%, with a level of uncertainty that it could be as low as 0% (Klouda et al., 1996). On the other hand, modeling studies of the northeast United States suggest that based on reactivity (vide infra), biogenic isoprene emissions could contribute up to 30% of the total organic reactivity at midday (Mathur et al., 1994).

Urban vegetation can impact ozone levels not just in terms of their chemical reactions, but also indirectly.

For example, Taha (1996) suggests that increased urban vegetation with low organic emission rates may lead to a net decrease in 03 formation by lowering surface temperatures and biogenic emission rates as well as increasing dry deposition of pollutants.

In summary, at the present time there are large uncertainties in both the magnitude and speciation of emissions of organics in various areas, which introduces significant uncertainties into the model predictions. This is an area that clearly warrants continuing attention.

(4) Chemistry Clearly, the kinetics and mechanisms of reactions in the chemistry submodels play a key role in the predicted concentrations of 03 and other secondary pollutants. Since key issues involved in incorporating this complex chemistry faithfully into models have been discussed above and indeed are the focus of this book, we shall not consider this further.

In summary, the development and application of urban and regional grid-scale models are critical to the development and implementation of cost-effective control strategies. However, these models are very complex, not just in their chemistry but also in a variety of other critical parameters, such as meteorology, emissions, and deposition. Discussion of these issues is found in Seinfeld (1988), in the 1991 National Research Council report "Rethinking the Ozone Problem in Urban and Regional Air Pollution," in a series of papers in "Regional Photochemical Specialty Measurement and Modeling Studies" (/. Geophys. Res., 100 (Dll), 1995), and in the proceedings of a conference on regional photochemical measurements and modeling (/. Air Waste Manage. Assoc., 45, 253, 1995; Atmos. Environ., 30 (12), 1996).

e. Models Incorporating Particles

For a number of years, models included gas-phase chemistry only. However, increasing computational power and a larger data base on the physical and chemical properties of particles in the atmosphere and their interaction with gases have permitted interactions with aerosol particles to be included in more recent years. Table 16.4 summarizes some of the characteristics of several models that incorporate particles. These models treat the transport of the gases to the particles, their uptake, and ultimately equilibria between the gas and aqueous phases. Although they all include inorganics in the particles, they vary in whether they also include organics and elemental carbon in the condensed phase. The next generation of models will incorporate real-time chemistry as well, rather than assuming equilibrium as is commonly done for many species.

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