Armed with an understanding of ozone chemistry and the specific control regime of an urban area, policymakers can move beyond generic emission caps to develop more sophisticated and cost-effective strategies. A science-based approach allows for the targeting of pollutants that will yield the greatest benefit for the least cost. This section details two such targeted policy levers.

Lever 1: Target VOCs Based on Chemical Reactivity

A foundational insight from atmospheric chemistry is that not all VOCs contribute equally to ozone formation. Different organic compounds react at vastly different rates and produce ozone with varying efficiency. This property is quantified by a metric called Maximum Incremental Reactivity (MIR), which measures the amount of additional ozone formed (in grams) when one gram of a specific VOC is added to a simulated urban air mixture.

The MIR values for various VOCs show a dramatic range, enabling a powerful policy approach. Rather than treating all VOCs equally, regulations can prioritize the control of a smaller subset of high-reactivity compounds to achieve the greatest reduction in ozone for a given level of effort.

Source: Carter (1994).

As the table demonstrates, controlling one gram of propene has over 600 times the impact on ozone formation as controlling one gram of methane. By focusing regulatory and technological efforts on sources that emit highly reactive species like propene, isoprene, and certain aromatic compounds (e.g., xylene), cities can achieve their air quality goals more efficiently.

Lever 2: Promote Reformulated and Alternative Fuels

The concept of VOC reactivity can be directly applied to transportation policy, a major source of urban emissions. The chemical composition of gasoline and other fuels is a major determinant of the reactivity of both evaporative and exhaust emissions.

Atmospheric chemistry shows that oxygenated fuel components, such as methanol and ethanol, generally have lower incremental reactivities than the larger alkane compounds characteristic of conventional fuels. Therefore, policies that encourage or mandate the use of reformulated fuels can serve as an effective, large-scale strategy to lower the overall reactivity of the urban VOC mix. By changing the fuel itself, this approach reduces the ozone-forming potential of every vehicle on the road, complementing vehicle-specific emission control technologies.

These targeted levers demonstrate a more nuanced approach to controlling photochemical smog. However, the benefits of these strategies extend beyond ozone, as the same chemical processes are also responsible for forming another harmful secondary pollutant: fine particulate matter.