When I rented a house near a small airport, I knew what I was in for – occasional low-flying aircraft drowning out phone calls or waking me from a nap. But when I bought a house in Houston two years ago, I didn’t realize I would be living on a flight path for industrial smog.

At a recent research conference on air quality, an impressive gathering of scientists laid out initial findings from the Texas Air Quality Study 2000 (TexAQS) about what causes Houston’s ozone smog. Their findings corroborate what many have long believed – the majority of Houston’s ozone problem is directly attributable to emissions from the major industrial complexes in the region.

It is too early to draw firm conclusions from the studies – data was gathered in the summer of 2000 – but some of the early indications being shared among air quality experts have important public policy implications.

It Comes from Industry
The scientific analyses link most ozone pollution events – and probably all of the most serious episodes – to plumes of pollution that spread downwind from the Houston Ship Channel and smaller industrial complexes. Most of the ozone in these plumes can be traced to the reaction of nitrogen oxides (NOx) with several volatile organic compounds (VOCs), specifically the alkenes, and more specifically ethylene, propylene, and 1,3-butadiene.

Although the basic chemistry of ozone has been known for decades (NOx + VOCs + sunlight = Ozone), the scientists found that the ozone production rates within Houston’s industrial plumes were among the highest ever observed. It’s as if someone swapped out gunpowder for kindling in my campfire.

The scientists also appear to have found higher actual concentrations of ethylene and propylene than estimates based on emissions inventories compiled by companies and government agencies predict. Studies are beginning to determine exactly where those emissions come from, but already it appears unlikely that they are due to upsets or accidental releases. These emissions appear to be routine and widespread.

It’s the Weather, Too
While the chemicals necessary to form hazardous levels of ozone are emitted routinely, ozone episodes only occur when certain weather conditions develop. Meteorologists are building a sophisticated description of the weather patterns that affect ozone concentrations. Sometimes the typical afternoon bay and sea breezes bring fresh air to the region, but on other occasions they bring highly concentrated pollution back into the city when the sun is high and bright – creating a burst of ozone.

This burst often begins near the ship channel and blows across southeast Houston, the Texas Medical Center, the Heights, and Aldine. Sometimes, when the wind is blowing steadily, the ozone peak isn’t achieved until the plume reaches Conroe. When the air is stagnant, the plume remains close to its origin and becomes quite concentrated. On particularly hazardous days the plume may slowly stretch out over 75 miles.

Although the north-northwest pattern is a typical “flight path” for ozone – passing right over my house in the Heights – on other days it can be observed stretching east into Beaumont, south along the coastal beaches to Clute, or any other direction.

Lots More to Study
While the alkenes, industrial NOx, sunlight, and weather are enough to explain the most serious and frequent ozone episodes, they are not a complete explanation. Ozone production, at lower rates, is also caused by other industrial air pollution (such as the emissions from the Parrish power plant in Fort Bend County) and pollution from “urban” sources such as cars, trucks, and construction equipment. The urban heat island effect also plays a significant but as yet unmeasured role in air chemistry.

Another notable finding is that trees are probably NOT a significant part of the ozone production problem. While it has been known for years that trees help clean the air, in other places this effect is partially offset by the ozone produced from VOCs emitted by trees. These new data suggest that VOC emissions from trees do not have much of an effect on ozone production in our region, and are not a cause of the hazardous ozone episodes that originate from industrial areas.

Next Steps
Some preliminary lessons: Maintain the focus on routine industrial emissions. Deal with other emissions sources as well. Quit complaining about emissions from trees. Thank your elected officials for finally committing the financial resources to this problem.

When complete, the TexAQS 2000 will be regarded as one of the most impressive scientific investigations into an urban pollution problem ever. Literally hundreds of scientists have focused their expertise and experience on Houston’s problems. The results are particularly exciting because the study included several types of ground-based monitors in coordination with three research aircraft gathering samples all over the region. Now those are some airplanes I’m happy to have flying over my house!