Natural VOCs can participate in the photochemical reactions that produce ozone, but they cannot, in principle, be directly controlled like those from anthropogenic sources. In regions where natural VOCs represent a significant fraction of the total reactive VOCs, NOx controls might be needed to reduce ozone substantially even if the oxidant chemistry is VOC-limited.
Moreover, the compounds that make up the general category of anthropogenic VOCs can be quite varied with widely different chemical characteristics and reactivities that lead to different rates of ozone formation. Thus, ton-for-ton, the reduction in the emissions of one VOC might lead to more or less reduction in ozone than the reduction of another VOC. The concept of ozone-forming potential, discussed in the next chapter, attempts to account for the differing chemical characteristics of VOCs as they relate to ozone photochemical production.