Can Random Environmental Fluctuations Increase Air Pollution Levels?

A new paper suggests this possibility. Motivated by the problem of ozone production in urban areas, an Arizona State team (Zonghua Liu, 480-965-2311, zhliu@chaos4.la.asu.edu) simulated a chemical reaction between tens of thousands of hydrocarbons and nitrogen oxides in a two-dimensional airflow.

In actuality, the production of ozone requires dozens of distinct chemical species and a chain of sunlight-triggered chemical reactions.

Yet in their single reaction between two species the researchers were able, for the first time, to consider how the reactants' size and mass affect their rate of chemical reaction in the complex 2D flow they examined.

Moreover, the simulated 2D flow has important similarities to real-life airflows in the troposphere, the approximately 15 kilometers above ground where air pollution can occur.

For example, the simulations explored a situation that can sometimes occur in real-life tropospheric flows: "Lagrangian chaos," in which the overall flow of air is regular (completely predictable) but the individual particles in the air move in a chaotic (unpredictable) fashion.

In efforts to mimic the repeating cycle of day and night, the researchers incorporated a repeating cycle that influenced the reaction.

Most strikingly, however, the chemical reaction rates were increased by up to two times when the researchers added the right amount of environmental "noise," random fluctuations, such as wind-speed variations, whose intensity depends on temperature.

As it turned out, the addition of noise caused the behavior of particles to become more spatially and temporally regular. This in turn created favorable conditions for increasing the chemical reaction rate.

This result suggests that perhaps environmental noise plays an important role distinct from other effects in generating air pollution. If borne out this would represent the latest natural example of stochastic resonance (Update 121), in which an ordinarily weaker process can be magnified by the right amount of random fluctuations ("noise") in the background.

The researchers' next step is to study chemical reactions in more realistic flows. (Zonghua Liu; Ying-Cheng Lai; and Juan M. Lopez, Chaos, June 2002.)