Handbook of Solvents - George Wypych - ChemTech - Ventech!

1194 Michelle Bergin, Armistead Russell
Although many types of reactions are involved, 4,13,16,17 the major processes for most
VOCs can be summarized as follows:
VOC + OH → RO 2 + products [17.4.10a]
RO 2 + NO → NO 2 + radicals [17.4.10b]
radicals → ...→ OH + products [17.4.10c]
products → ...→...+ CO 2
[17.4.10d]
The last two pseudo-reactions given comprise many steps, and the products often include
formaldehyde, carbon monoxide and organonitrates. The rate of ozone increase
caused by these processes depends on the amount of VOCs present, the type of VOCs present,
and the level of OH radicals and other species with which the VOCs can react. One of
the major determinants of a compound’s impact on ozone is the rate of the reaction of the
particular VOC with the hydroxyl radical via reaction [17.4.10a], above. The total amount
of ozone formed is largely determined by the amount of VOC and NO x available.
The dependence of O 3 production on the initial amounts of VOC and NO x is frequently
represented by means of an ozone isopleth diagram. An example of such a diagram is
shown in Figure 17.4.1. The diagram is a contour plot of ozone maxima obtained from a
large number of air quality model simulations using an atmospheric chemical mechanism.
Initial concentrations of VOC and NO x are varied; all other variables are held constant. Notice
that there is a “ridge” along a certain VOC-to-NO x ratio where the highest ozone concentrations
occur at given VOC levels. This is referred to as the “optimum” VOC-to-NO x
ratio. While the atmosphere is more complicated than this idealized system, important features
are very similar.
VOC-to-NO x ratios sufficiently low to retard ozone formation from an optimum ratio
(represented in the upper left quadrant of Figure 17.4.1) can occur in central cities and in
plumes immediately downwind of strong NO x sources. Rural environments tend to be characterized
by fairly high VOC-to-NO x ratios because of the relatively rapid removal of NO x
Figure 17.4.1. Ozone isopleth diagram showing the dependencies of ozone on varying levels of initial VOCs and
NO x. Concentrations are given in ppb. [Adapted from M.S. Bergin et al., Enc. of Env. Analysis and Remediation,
29, 3029, (1998)]