The regulation of nitrogen oxides, or NOx, has been increasing for years and, according to rules already on the U.S. EPA's books, will continue to do so for some time. In the March issue of National Geographic, senior editor Tim Appenzeller's article, "The Coal Paradox," highlights this agency-driven curbing of NOx. "With much government prodding, coal-burning utilities have cut pollutants such as sulfur dioxide and nitrogen oxides ...," Appenzeller writes. The operative phrase there is, "with much government prodding," emphasizing the EPA's drive to regulate the emissions of NOx.

Cutting NOx and sulfur dioxide emissions from power plants and other high-emission sources sounds like a responsible thing to do, considering their purported contributions to the formation of acid rain and other unfortunate environmental consequences as a result of their emission. Atmospheric chemistries are complicated phenomena though, and the EPA has placed certain bets on NOx reductions from stationary and mobile sources to improve urban air quality. But not everyone agrees with that wager.

A collaborative analysis called the Weekend Ozone Effect, conducted by the National Renewable Energy Laboratory (NREL), the Coordinating Research Council (CRC) and the California Air Resources Board, and funded by the CRC and U.S. DOE's Office of FreedomCAR and Vehicle Technologies, is uniquely compelling. Its persuasion is rooted in the ambient, real-world emissions data the researchers collected, as opposed to data from predictive modeling studies. NREL principal scientist Dr. Douglas Lawson, who's been an integral part of the Weekend Ozone Effect study, which began more than five years ago in southern California, says the data that's been collected points to a need for a new approach to a not-so-new problem.

Eastern Texas has been battling with deteriorating air quality in places like Houston, Dallas and San Antonio for years. To help these metropolitan regions meet the EPA's ozone attainment status, the Texas Commission on Environmental Quality (TCEQ) implemented Texas Low-Emission Diesel (TxLED) in a 110-county zone in east Texas. Until a recent reprieve for biodiesel was issued, TCEQ had outlawed biodiesel blends not certified by the state. To certify a blend of biodiesel for sale or use in the area, testing had to show the fuel had been formulated to lower NOx, making it comparable with TxLED standards. "This is a NOx-control strategy," says the TCEQ's Morris Brown. "In Texas, in the three areas in non-attainment status, our modeling shows NOx as the primary driver [of ozone formation]."

In early February, Texas gave biodiesel a free, one-year bypass of the TxLED regs, in part due to having only one approved formulation, but also because the renewable fuel simply isn't used widely in the state. "Biodiesel is not a significant contributor to the NOx inventory here yet," Brown tells Biodiesel Magazine.
Despite the TxLED timeout in Texas, industry observers say the state has placed the same wager on NOx reductions to improve its urban skies that the EPA has anted. The wager entails is regulating NOx to control ozone in hopes of stymieing its contribution to smog, increased respiratory problems and the high cost of medical care to treat those unfortunate side effects of dirty air in polluted American cities.

Grasping the effect
All of this might be a little difficult to sort through without some background information on what has been observed prior to and during this study.
The Weekend Ozone Effect looks at ground level ozone (O3) formation as a significant health threat and contributor to the pollutant-filled smog hovering over most metropolitan areas of the United States.

"The two main precursors to ozone formation are hydrocarbons (VOCs) and nitrogen oxides," Lawson says. "There's been a debate for years and years as to which of those two pollutants, or both, should be controlled to reduce ozone." Some industries have asserted that the best way to control ozone is to reduce hydrocarbons without reducing NOx. Others have pushed the idea that the best ozone control is gained by reducing NOx, not hydrocarbons. "And the (environmental) agencies have said you've got to reduce both," Lawson tells Biodiesel Magazine.

The Weekend Ozone Effect really quantifies what has been experienced for some time now, especially in the L.A. Basin region, which experiences higher ozone levels on weekends, even though significantly less NOx is emitted on weekends compared to weekdays. Less NOx is emitted on weekends because fewer diesel trucks and buses are on the roads Saturdays and Sundays, with even fewer diesels operating on Sundays relative to Saturdays.

"It's really quite complex because there are literally hundreds of chemical reactions that are taking place because of all the different types of hydrocarbons being emitted," Lawson explains. In urban settings, Lawson says it really boils down to one basic equation: O3 + NO = NO2 + O2. According to him, this is the single most important equation in understanding the Weekend Ozone Effect.

Approximately 95 percent of the NOx emitted from any combustion process is NO-whether from a diesel engine or a power plant-while NO2 makes up the remaining 5 percent, Lawson says. "That NO molecule combines with ozone in the atmosphere very rapidly, and forms NO2 (nitrogen dioxide) and O2 (oxygen)," he says. "So if you have more fresh NO emissions emitted in the morning, it means that any ozone that's around in the atmosphere-or that's being formed-is very quickly quenched, or gobbled up, to remove the ozone and form those two products, NO2 and O2."

Because of this "natural" reduction of NOx on the weekends due to much lower diesel traffic on the roads Saturdays and especially on Sundays, there's less NO around to titrate the ozone, thus, the ground-level ozone begins forming approximately an hour earlier on Sundays relative to Wednesdays. "It's like getting an extra hour of sunlight for the photochemistry to take place," Lawson tells Biodiesel Magazine. Because ozone starts forming an hour earlier, more of it can form during the day.

A more detailed look
Lawson is openly sharing data that shows NOx emissions in the L.A. Basin on Sundays are approximately 50 percent less relative to the emissions of NOx on Wednesdays. Conversely, ozone formation in the L.A. Basin on Sundays is approximately 50 percent higher compared to Wednesdays, and, again, it starts forming an hour earlier on Sundays, relative to Wednesdays.

In that important equation-O3 + NO = NO2 + O2-there are no hydrocarbons or VOCs on either side of the equation, Lawson points out. However, the role of hydrocarbons in the formation of ground-level ozone is important. "Hydrocarbons are responsible for the rapidity with which the reactions take place," Lawson explains. Hydrocarbon reductions also take place on the weekends, but not to the extent that NOx reductions are seen.

To reiterate, one main factor of the Weekend Ozone Effect is that, with less NO around to titrate the ozone on the weekends, ozone formation occurs earlier. The second factor governing higher ozone formation on the weekends is the hydrocarbon-to-NOx ratio. In other words, the larger the hydrocarbon-to-NOx ratio is-the largest of which is seen on the weekends-the faster ozone forms.

Lawson says there's not a lot of data on ambient hydrocarbons, but carbon monoxide is a great "surrogate." Although hydrocarbons have many sources, one source for both carbon monoxide and hydrocarbons is gas-powered automobiles. "If you've got carbon monoxide around that's coming from cars, then intuitively you know the hydrocarbons from cars are lower as the carbon monoxide drops," Lawson says.

But Lawson notes that the L.A. area has had probably the most aggressive hydrocarbon regulations in the country, going so far as micromanaging things like the use of barbeque fluid, for instance.

Getting answers, assessments
With such a stranglehold on hydrocarbons in the L.A. Basin, and with "natural" reductions of NOx on the weekends due to less diesel traffic on the roads-and remembering that both hydrocarbons and NOx are precursors to the formation of ozone-why then are ozone levels as much as 50 percent higher on Sundays compared to Wednesdays?
According to Lawson, the Weekend Ozone Effect provides an answer to that question: Reducing only hydrocarbon emissions-and not limiting NOx-is what's needed to effectively regulate metro-area ozone formation. NO from NOx emissions is needed to quench ozone, so if ample amounts of NO are present while hydrocarbon levels drop to minimize or invert the hydrocarbon-to-NOx ratio (remember the larger the hydrocarbon-to-NOx ratio, the faster ozone forms), then more of the ozone gets titrated while the rate at which the formation of ozone occurs is slowed down. In other words, lowering hydrocarbons while not regulating NOx would help reduce ground-level ozone in urban areas and regions downwind of those locations.

Sure, the L.A. Basin has aggressive hydrocarbon controls in place, but the biggest source of hydrocarbon emissions is from a very small portion of the existing gasoline automotive fleet on the roads-those out-of-tune cars with ineffective or no emissions control systems, including critical sensors and catalytic converters. Lawson says these represent about 5 percent of all the cars on the road but account for about 75 percent of all mobile-source hydrocarbon emissions. It's these dirty "beaters" still in circulation that are holding hydrocarbon levels up compared to NOx reductions on the weekends relative to weekdays. There are less gas-powered cars operating on Sundays compared to Wednesdays, but that decrease in gas-powered traffic on Sundays is far less than the drop of NOx-emitting diesels.

For biodiesel advocates like Anne Tazewell, the alternative fuels program manager at North Carolina State University, it's critical that this discrepancy in NOx's role in ozone formation is reconciled. "I'd like to see the 'weekend effect' research more widely understood. Especially as it relates to the NOx disbenefit attributed to biodiesel," she says. "Air quality planning puts a premium on NOx reduction that, frankly, could be undeserved. Consequently, the disbenefit associated with using biodiesel creates a challenge to folks that are tracking, and held accountable for, the emissions benefits of alternative fuels use."

National trend, outlook
One might naturally respond to the findings in this study by arguing, "The Weekend Ozone Effect is unique to southern California" and that different measures are needed for unique areas around the country. That's not true according to new research that has analyzed ambient emissions data in ozone problem areas in 23 states.

"We did the work first in southern California because that's the mecca for ozone in the United States," Lawson tells Biodiesel Magazine. "Sunday ozone is much higher on average in L.A. than it is on Wednesdays. In other parts of the country, it isn't as extreme."

Weekend ozone increases in other parts of the country might only be 5 percent, or 2 percent, but because it's not as extreme as L.A.'s 50 percent increase in ozone, Lawson says people were convinced that their particular region of the country didn't suffer from the effect. Whether ozone levels remain the same or rise slightly on the weekends in various metropolitan regions of the United States, urban areas all across the country experience NOx reductions of 50 percent on Sundays relative to Wednesdays, Lawson says. If NOx reductions lowered ozone formation, then significant reductions of weekend ozone should have been observed. However, that wasn't the case anywhere in the United States, including Dallas/Ft. Worth, Houston, Atlanta and the Northeast.

Aside from reducing ozone levels, EPA supports NOx reductions nationwide because the agency also believes particulate matter (2.5 microns and smaller), of which particulate nitrate is a component, will be reduced. However, the Weekend Ozone Effect research shows that the presence of these nitrates is hardly changed at all on weekends, despite drastic cuts in NOx emissions.

All of these findings are of great immediate and near-term concern, but what's even more compelling is what the data says about the future, a decade or more out.
"The thing that's most intriguing about our study is that changes taking place today on the weekends, in terms of emissions reductions that naturally happen every weekend relative to weekdays, are similar in magnitude to the changes that are mandated by the EPA nationwide over the next 10 or 15 years," Lawson says. "That's probably the biggest red warning flag of all of this work. The regulations that are in place suggest that in the next 10 or 15 years we'll have about a 50 percent reduction in NOx emissions nationwide and maybe a 35 percent reduction in hydrocarbons." Thus, there would be more ground-level ozone in the future, not less.

Again, proponents of this study believe that the quantification and projection of this data is indicative of the future-provided EPA regulations stay their present course.
Whether or not the EPA's NOx-limiting decisions, which continue to burden consumers who ultimately pay for these regulations, will ultimately dissuade the agency from changing its course is hard to say. "The issue of NOx control for ozone attainment is an issue we, and others, have worked on for some time," the EPA's John Millet tells Biodiesel Magazine. "We are aware, as the National Renewable Energy Lab's work states, that 'disbenefits' can exist. However, our conclusion is that despite localized disbenefits, NOx control is a major tool in reaching ozone attainment, and we analyzed that issue carefully in the [on-road] diesel rulemaking." Millet also says the EPA will continue to look at all available data in the context of drafting the nation's renewable fuels standard rulings.

Are these atmospheric changes observed between weekends and weekdays truly indicative of what's to come in the future? More specifically, although the percentage of reductions in NOx on weekends is comparable to the full-force NOx reductions the EPA expects to achieve within a decade, is there something different about these drastic short-term reductions that take place in a matter of days and what's to be gradually implemented over the course of several years?

"We don't know what's going to happen over the long term," Lawson admits. "But over the short term it's very obvious what's happening, and without any modeling whatsoever, our studies provide a very good snapshot of what's likely to occur."