Warming, Cooling, and Urban Ozone Pollution

Part 4: Background: Urban Ozone Pollution

The rest of this module is based on "The Role of Biogenic Hydrocarbons in Urban Photochemical Smog: Atlanta as a Case Study," by W. L. Chameides, R. W. Lindsay, J. Richardson, and C. S. Kiang (Science 241, 1988, pp. 1473-1475).

Ozone is a good thing if it's in the high-altitude ozone layer -- it prevents skin cancer from exposure to the sun -- but it's a bad thing at ground level, where it is a dangerous component of smog. Ozone (O₃) is produced, especially in urban environments, by photooxidation of hydrocarbons in the presence of nitrogen oxides. In particular, high concentrations of automobiles produce large quantities of anthropogenic (i.e., man-made) hydrocarbons (AHCs) and nitrogen oxides (NO_x). Summer sunshine provides the photooxidation and often drives ozone levels above the 0.12 ppmv National Ambient Air Quality Standard .

The Clean Air Act of the 1970's mandated plans for ozone abatement based on reduction of AHC and/or missions. Implementation of the Act by EPA and related state agencies has focused primarily on reduction of AHC's. However, despite expensive and extensive efforts in this direction, O₃ levels in many cities have not gone down at all or not enough to comply with NAAQS.

One flaw in this strategy appears to be deliberate exclusion of biogenic or natural hydrocarbon (NHC) sources, especially trees. On balance, urban trees do more good for the environment than harm. [Click here for a brief statement of positive and negative effects -- use your Back button to return here.] However, NHCs such as isoprene are much more reactive with NO_x than AHCs are, and so can have a significant effect even in lower concentrations.

Atlanta is one of the cities that has made little progress in ozone abatement by reduction in AHC emissions. Atlanta is also 57% forested. In the next part of the module, we will use simulated data to try to determine the extent to which ozone levels in Atlanta are associated with NHC sources, and what the implications of that might be for smog reduction strategy.

Our approach is this: We suppose that, over a 10-year period, Atlanta has been implementing an aggressive AHC reduction plan, one that drives emissions down in proportion to the amount present. If NHCs are neglgible in ozone production, the effect of the AHC reduction should be to reduce ozone to zero. But if NHCs are not negligible, then the limiting value of the reduction should be the "background" or "natural" level of ozone. Note that this level is not directly measurable, so that is one of the things we hope to extract from the data.

For simplicity, our data consist of ozone concentration readings at a center-city location at 1:00 pm on July 1 each year over a decade. Because the data are simulated, they are somewhat smoother than real data would be -- and that makes detection of the limiting value much easier. Thus, this exercise is demonstrating a concept, not showing how real data would be used in practice.

Last modified: October 21, 1997