Ozone Research Unlocks Keys to Guide Local Air Policies

The ozone layer in the stratosphere protects humans by absorbing some of the ultraviolet light that otherwise would reach the Earth’s surface. 

But ozone that forms near Earth’s surface is a pollutant, a highly reactive molecule that can damage lung tissue. Areas with consistently elevated levels of ozone tend to have higher rates of asthma and other respiratory problems.

Local and state officials can enact policies to limit ozone formation and improve air quality. The most effective guidelines are based on information from scientists about how air quality is affected by weather, wind and human activity.

Associate Professor of Physics Paul Walter studies the complex factors that influence air quality in regions such as Houston. He and his students have released weather balloons equipped with ozonesondes, instruments that measure ozone as the balloon rises through the atmosphere. The ozonesondes transmit data to a receiving station on the ground until they pop, about 30 km above the surface of the earth. (The air pressure decreases as the balloon rises, allowing it to expand until it stretches too much and pops.)

These weather balloons are often part of larger field campaigns to examine how a region’s air quality is influenced by weather patterns as well as human-generated pollution. For instance, Walter was part of a campaign called TRACER-AQ that studied air quality in the greater Houston-Galveston area. The project was a collaboration with partners including the University of Houston and NASA. An aircraft equipped with a remote sensing method called LiDAR flew over the area at different times throughout the day to measure how pollution changed as the wind direction and temperature shifted. Meanwhile, teams on the ground – and on boats in Galveston Bay and the Gulf of Mexico – released balloons with ozonesondes. “All these measurements together can help us understand what leads to poor air quality on particular days,” Walter said.

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The campaign confirmed that, although Houston is well known for its petrochemical plants, shipping industry and crowded freeways, these are not the only factors influencing air quality. Wind direction matters: cold fronts from the north sweep across the continent, bringing more polluted air with them. Conversely, persistent winds blowing in from the Gulf can disperse pollution in Houston before it has a chance to build. And wind direction can change over the course of a day, as the land and the water of the Gulf heat at different rates. Pollution can be blown out over Galveston Bay and be pushed back over Houston hours later. Days with no wind at all are generally worse for air quality, Walter says, because pollution formed locally has no way to disperse.

In short, “So much of the air quality on a given day depends on the weather,” Walter says.

Surface-level ozone, in particular, is connected to the weather. This ozone is a secondary pollutant, meaning that it is not emitted directly from tailpipes or factories but is produced through photochemical reactions. Those reactions depend on the availability of UV light. Clear, cloudless days allow more UV light to reach the Earth’s surface and generally correlate with higher levels of ozone – which is why, Walter says, some of the most beautiful days have poor air quality.

Ozone formation also is influenced by the ratio between two primary pollutants: nitrogen oxides, or NOx, and volatile organic compounds, or VOCs. NOx are emitted from the burning of gasoline and coal; VOCs are carbon-containing compounds that are quick to evaporate, such as paints and varnishes; formaldehyde; and emissions from chemical plants (the common VOC isoprene also can be produced by trees). 

Research like Walter’s helps unpack the complex relationships among factors that engender ozone formation and poor air quality. Understanding which primary pollutant is dominant in an area, and under what conditions, helps government agencies develop the most effective policies to reduce ozone. For example, to reduce NOx, Travis County encourages residents to avoid driving and using gas-powered lawn equipment on Ozone Action Days, when ground-level ozone reaches high levels. Some Texas cities have offered reduced-fare or free bus rides on Ozone Action Days to encourage people to use transit.

“The best thing to do is not always obvious,” Walter says. “You can say, ‘Just have less pollution, and you’ll have better air quality,’ and in general that’s true, but it’s not that straightforward.” Research such as Walter’s helps leaders make more informed decisions.