March 2002

Surprising news from ACD on Houston’s ozone problem

Some of the nation’s most intense pollution episodes plague the Houston metropolitan area. A mitigation plan proposed by the state of Texas aims to tackle the pollution by reducing nitrogen oxide (NOx) emissions up to 90% percent by 2007. But a group
of scientists in the Atmospheric Chemistry Division (ACD) and their colleagues at the NOAA Aeronomy Laboratory have helped focus attention on an important partner in air-quality crime that needs to be addressed as well: volatile organic compounds (VOCs).

 

Members of ACD’s TexAQS team, left to right: Steven Donnelly, Alan Fried, Andy Weinheimer, Sue Schaffler, Elliot Atlas, Kristen Johnson, Frank Flocke, Verity Stroud, and Bryan Wert. Not pictured: Rick Shetter and Eric Apel. (Photo by Carlye Calvin.)

New data on VOCs emerged from the field campaign of the Texas Air Quality Study, carried out in the late summer of 2000. In close collaboration with the Texas Natural Resource Conservation Commission (TNRCC), TexAQS involved 14 universities and 6 state and federal labs. The lead investigators were Peter Daum (Brookhaven National Laboratory), James Meagher and Fred Fehsenfeld (both of NOAA), James Price (TNRCC), and David Allen (University of Texas at Austin). A significant part of the air-chemistry measurements made aboard the now-retired NSF/NCAR Electra aircraft during TexAQS came from several ACD groups. Their work was presented in January at the American Meteorological Society’s annual meeting in Orlando.

Background levels of ozone in Houston’s air are typically on the order of 50 parts per billion (ppb), not out of line with other big cities. The real problem is intermittent yet severe events. Plumes of pollution generated and transported over the area can push ozone to unsafe levels for hours or days. During a three-week period in August and September 2000, ozone readings topped the Environmental Protection Agency’s one-hour standard of 120 ppb on 14 days and occasionally exceeded 200 ppb, a level considered "very unhealthy" by the EPA.

To help avoid loss of federal funds and other penalties, Texas developed a state plan for reducing ozone in the Houston area. It calls for a variety of strategies, including an area-wide reduction in highway speed limits from 65 to 55 miles per hour. The overall plan has drawn legal challenges from all quarters, from state legislators irked by the speed-limit drop to environmental groups who feel the plan doesn’t do enough to heal the region’s air.

Many parties have called for more research on the factors dirtying Houston’s air. TexAQS is part of the state’s response. The study’s goals include sifting out local versus regional processes; unraveling the area’s blend of automotive, industrial, and biogenic emissions; and studying the diurnal cycle of pollution.

A swan song mission pays off

The NSF/NCAR Electra was called into service for TexAQS with only a few months’ notice, after a severe thunderstorm in Galveston damaged a NOAA P-3 originally scheduled for the project. The Electra ended up joining four other aircraft for what turned out to be its last mission.

On board the Electra, scientists from ACD and NOAA carried out near–real time analyses of ambient air at altitudes close to 2,000 feet (600 meters). The data helped scientists on board to specify when a canister-sized air sample should be taken. The scientists brought back more than 500 of these air-filled canisters—each about the size of a loaf of bread—to the Mesa Lab for more in-depth analysis. These samples provided an essential base of data for modelers to use in simulating the mix of hydrocarbons in Houston’s air.

Early on, a puzzling connection emerged. Ozone typically forms through a reaction involving NOx and hydrocarbons in the presence of sunlight. This process is hard to document in action, so chemists often measure byproducts and then work their way backward through the chain of events. Among the byproducts is a set of organic nitrates referred to as PANs (peroxyacetyl nitrates). ACD’s Andy Weinheimer and Frank Flocke, using an instrument that measures and separates out the different kinds of PANs, found that the highest ozone levels were colocated with high levels of PANs formed through hydrocarbons that came from industrial rather than biogenic sources. Thus, it appears that trees, and the PANs generated from their emissions, aren’t a major factor in Houston’s ozone problem.

In fact, says Andy, "the overall distribution of the PAN species in Houston didn’t seem that unusual [compared to other U.S. cities]. However, the mix of hydrocarbons is very much different from other large cities in the U.S."

Why, then, does Houston get such intense ozone episodes? Part of the problem is meteorological. The light winds and shallow marine air that predominate over the northwest Gulf of Mexico make it hard to fully disperse big plumes of pollution, such as those emitted by Houston’s petrochemical plants, before lots of ozone gets generated.

Although the evidence is not all in, it seems that those petrochemical plants may indeed be at the root of the matter. Houston produces as much as half of the nation’s emissions of alkenes, commonly used as base products in plastics manufacturing. Houston’s air shows large amounts of two alkenes, ethene and propene, which are used in making polyethylene and polypropylene, respectively. According to Andy, the presence of so much ethene and propene should have shifted the mix of PANs toward a more unusual configuration, rather than the garden-variety mix actually observed.

The conundrum was solved using Sasha Madronich’s "master mechanism" air-chemistry model. Andy and Frank simulated the events for 1 September, when one of the biggest ozone spikes occurred. They added and subtracted various types of hydrocarbons and found that two similar-sounding groups—alkanes and alkenes—had produced contrasting effects. While these effects seemed to cancel each other out in an overall analysis of the observed PAN mixing ratios, a closer look revealed that the alkenes (especially ethene and propene) were closely associated with the intense ozone pollution.

Which compound rules the roost?

One atmospheric byproduct of ethene and propene, formaldehyde, is an especially useful marker, and NCAR was well positioned to follow it. Bryan Wert, Al Fried, and Bill Potter put ACD’s newly upgraded fast-formaldehyde sensor through its paces above Houston. The early data were gathered at 10-second intervals, and beginning on

1 September, the levels were measured every second. The levels of ozone and formaldehyde in plumes matched up almost perfectly, especially near petrochemical plants, says Bryan. He adds, "The modeling work done by Michael Trainer and his colleagues at NOAA shows that we can explain virtually all of the formaldehyde and much of the ozone just by the photochemical oxidation of ethene and propene."

Reducing ethene and propene may be a tough job, Bryan notes. The amounts of these two compounds measured by researchers in TexAQS case studies were up to 100 times greater than the official data from EPA inventories. "The petrochemical plants don’t really know where the excess is coming from," he adds.

Ethene and propene can’t produce ozone without nitrogen oxide, but Houston’s cars and factories produce vast quantities of NOx, and it seems to be especially efficient at producing ozone in Houston. A single NOx molecule in Houston’s air can lead to 10 to 20 ozone molecules, according to TexAQS results.

Thus, cutting back on NOx may not necessarily impede ozone production as much as previously assumed—a question that lies at the heart of current lawsuits challenging the state’s mitigation plan. Models run by NOAA and ACD may help clarify the matter and suggest an effective strategy to reduce ozone extremes in the Houston area.

•Bob Henson

Edited by David Hosansky, hosansky@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised: Wed Mar 13 17:08:40 MST 2001