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Environment

Smog, Policy, and Chemistry

EPA guidance on exempting volatile organic compounds from regulation gets scrutiny

by CHERYL HOGUE, C&EN WASHINGTON
January 24, 2005 | A version of this story appeared in Volume 83, Issue 4

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Credit: PHOTODISC
Credit: PHOTODISC

A primary goal of the Clean Air Act is to reduce the formation of ground-level ozone, the main ingredient of smog. Under that law, the Environmental Protection Agency regulates organic chemicals that evaporate quickly into the atmosphere and help form ozone. Many of these volatile organic compounds (VOCs) are key ingredients--such as solvents--in paints, coatings, inks, and other commercial products.

Just because a chemical meets the definition of a VOC doesn't mean that the substance will always remain regulated. Manufacturers can petition the agency to exempt a VOC from regulations if they have data showing the chemical has little effect on ozone formation. Though this petition process may take years to complete, companies have the potential to rack up sales if EPA relaxes regulation of their product and if that chemical can substitute for a more tightly controlled VOC.

EPA's policy for exempting VOCs dates to 1977, and the agency says it will review and possibly update that guidance in the coming years. But an environmental group is fearful that such changes could lead to more ozone pollution as a result of increasing deregulation of VOCs that it believes should stay controlled.

The issue came to light recently when EPA, at the request of Lyondell Chemical, exempted tert-butyl acetate (TBAc) from two types of controls on VOCs. Those regulations generally limit emissions of VOCs and restrict the content of these chemicals in products such as paint and coatings. Lyondell expects to sell TBAc to paint and coating makers who want to replace heavily regulated VOCs, including xylene and toluene, in their product formulations. EPA's deregulation was effective Dec. 29, 2004.

The Natural Resources Defense Council (NRDC) is attacking EPA for granting the regulatory exemption for TBAc, which the environmental group claims should remain tightly controlled.

"The TBAc exemption likely is a strategic step in the Bush Administration's larger campaign to undermine clean air protections," the group said in a statement. "EPA clearly hopes the public will overlook the glaring flaws in its TBAc exemption, leaving the agency free to use that exemption as a precedent for future giveaways to companies like Lyondell."

NRDC's concern is how EPA is applying its exemption policy.

That policy is based on a compound's propensity to help form ozone--a property called reactivity. Under that guidance, EPA puts VOCs into one of two categories. Either a substance is reactive, meaning it will remain under tight regulation, or it is "negligibly reactive" and possibly eligible for exemption, explains Terry J. Keating, senior environmental scientist in EPA's Office of Air & Radiation.

WHEN EPA issued its policy 28 years ago, the agency determined that some VOCs, including ethane, have a negligible ability to enhance ozone formation. Laboratory studies showed that these chemicals do not raise ozone levels beyond the regulatory ceiling for ozone. The agency selected ethane's reactivity as its benchmark for making future exemptions. If a VOC has a reactivity less than or equal to ethane, it is categorized as negligibly reactive.

"Reactivity, at face value, is an appealing concept" for separating VOCs for ozone-control purposes, says Jeremy (Jake) M. Hales. "But the more you look at it, the more convoluted it gets," says Hales, management coordinator for NARSTO, a government/industry/academic group formerly called North American Research Strategy for Tropospheric Ozone that now studies many types of air pollution.

Hales says determining the reactivity of VOCs makes sense in areas where there are emissions of nitrogen oxides. NOx are required to form ground-level ozone.

IN DETERMINING whether a VOC has negligible reactivity and thus is exempt from at least some regulations to control ozone, EPA has generally compared a compound's reactivity with the hydroxyl radical, or KOH, to that of ethane.

But KOH determinations have a drawback, says Robert Wendoll, director of environmental affairs at Dunn-Edwards Corp., a Los Angeles-based manufacturer of paints. Some compounds react quickly--in other words, they have a high degree of reactivity with hydroxyl--yet they do not yield as much ozone as other chemicals with a lower KOH, he tells C&EN.

In 1994, William P. L. Carter, research chemist at the University of California, Riverside, developed an alternative method that takes into account reactions in the atmosphere that can affect the yield of ozone formed in the presence of a particular VOC. This technique calculates "incremental reactivity"--the change in the amount of atmospheric ozone at a particular geographic location relative to the amount of VOC emitted, he explains.

EPA in 1994 used incremental reactivity calculations to exempt volatile methyl siloxanes from regulation as VOCs. These compounds "proved to be less reactive than ethane on a per-mole basis," according to the agency. Since then, EPA has exempted methyl acetate, acetone, and TBAc based on incremental reactivity measurements using grams, not moles, as units.

In its petition to EPA to delist TBAc, Lyondell relied on studies by Carter which found that TBAc formed 0.4 times the ozone of an equal mass of ethane. EPA reports that "calculations based on Carter's results show that a mole of TBAc forms 1.5 times the ozone formed by a mole of ethane." The difference in these two results, the agency adds, is because "a molecule of TBAc is almost four times heavier than a molecule of ethane." Carter tells C&EN that he recommended that the agency evaluate this data on TBAc on a per-gram basis to make its regulatory determination.

Carter says the numbers, which have an uncertainty factor of about 30%, show that TBAc is very close to ethane in its reactivity. In contrast, NRDC interprets the figures as demonstrating that, on a molar basis, "TBAc is 50% more reactive than ethane."

Using a mass instead of a molar comparison flies in the face of basic chemistry, says Linda Greer, senior scientist at NRDC.

"They wanted to deregulate this substance," Greer tells C&EN. "They were not really examining the science."

But, Keating explains, VOCs are substituted for each other on a mass basis, so the agency made its decision on TBAc using a mass-by-mass comparison.

Wendoll says that if EPA's policy goal is to promote one VOC with a lower reactivity that could substitute for a compound with a similar mass that has a greater reactivity, a mass basis might be more appropriate for exemptions.

TBAc falls into this category. Gail Kelly, business development manager for Lyondell Chemical, tells C&EN: "Most solvents are more photochemically reactive than TBAc. TBAc is expected to be used in solvent-based industrial coatings, inks, and adhesives to replace other solvents including toluene, xylene, ketones, and other acetates, depending on the chemistry used in a particular application."

TBAc has roughly the same mass as xylene, Wendoll notes. Replacing xylene with TBAc would provide "net air quality benefits in terms of its reduction in ozone-forming potential," he says. Wendoll adds that his company is unlikely to use TBAc because Dunn-Edwards strives to reduce the odor of its products, which are used in homes and public places. TBAc has an "unusual odor," Wendoll says. However, the compound may find use in coatings for original equipment manufacturing or in industrial maintenance coatings, he adds.

Aside from the TBAc exemption, NRDC is worried about how the agency may revise its policy.

Greer says she remains open-minded, noting that scientific understanding has progressed since the policy was issued. But, she says, she's cynical about the outcome since the agency's recent ruling on TBAc "is so flawed."

"They're starting out with no credibility on this issue for us," Greer says. "I'm worried this is driven more by political motivation than by scientific discourse."

Keating responds, "We've learned a lot since 1977 about ozone chemistry," which should be reflected in agency policy.

However, formulating changes to the current policy is proving tough, he tells C&EN. "It's complicated from the science side, and there's a bunch of complicated policy questions," he says.

FOR THE PAST several years, a panel organized under the auspices of NARSTO has focused on defining and obtaining the kind of scientific data regulators need for determining which VOCs they should exempt from ozone-control regulations, Keating says. The Reactivity Research Working Group includes representatives from EPA, the California Air Resources Board, Environment Canada, industry, and academe.

Wendoll, a member of the working group, says one of the main questions the panel is probing is whether ethane should be the chemical EPA uses as a benchmark for reactivity comparisons.

Picking a benchmark reactivity involves careful balancing, Carter says. If the benchmark is too reactive, policy decisions will lead to formation of more ozone as more reactive VOCs are deregulated, he explains. If the benchmark has too low a reactivity, businesses may not substitute high-reactivity VOCs now in use with substances that have significantly less reactivity but do not quite meet the benchmark.

Wendoll says the working group is also probing whether KOH or incremental reactivity is the best metric for comparison and whether VOCs should be compared with ethane on a molar or mass basis.

Keating adds that regulators are wrestling with other issues as well. The most basic, Keating says, is whether they should continue to exempt any VOCs from regulations designed to curb ozone formation. Another is whether VOCs should be ranked on a scale rather than fall into two categories of "reactive" or "negligibly reactive," he says. A scale for VOCs might be similar to the one for greenhouse gases that ranks substances on their atmospheric heat-trapping potential.

In addition, regulators are trying to look at an integrated approach to control all types of pollution concerns about VOCs, not just formation of ground-level ozone, Keating says. These chemicals can also pose acute or chronic health risks, cause depletion of stratospheric ozone, be greenhouse gases, or form fine particles.

Keating is unable to estimate when EPA might officially get the VOC exemption policy reassessment under way, but he adds that the agency has been pondering the issue for years. The goal of the effort, he says, will be to make EPA "more efficient and effective" in protecting air quality.


ATMOSPHERIC CHEMISTRY
How VOCs Contribute To Formation
Of Ground-Level Ozone

RO2 represents a peroxyl radical that is formed from the organic compound by the replacement of a hydrogen with an oxygen molecule. For example, propane reacts with OH to generate propyl peroxy radical and water, Sillman says.

This initiation reaction is followed by the conversion of NO, also present in the air as a pollutant, to NO2 through reaction with the HO2 or RO2 radicals. This also regenerates OH.

For example, the propyl peroxy radical reacts with nitric oxide to generate the secondary VOC propionaldehyde.

Next, NO2undergoes photolysis by sunlight and releases atomic oxygen, which combines with diatomic oxygen to create ozone. This reaction requires that an unreactive molecule, represented by M, be present.

Ground-level ozone pollution is linked to lung irritation and damage. Even at low levels, ozone can aggravate asthma, reduce lung capacity, and increase susceptibility to respiratory illnesses like pneumonia and bronchitis, according to the Environmental Protection Agency.


 

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