Why not?

DIESEL DAMAGE: Diesel trucks rumble north on Freeway 99 near Olive Avenue. Regulators now estimate that diesel trucks account for one-sixth of the region's nitrogen-oxide emissions. (Mark Crosse / The Fresno Bee)

Blame regulators with more pressing business.

Blame the complexities of diesel engines and diesel fuel, which created pollution control hurdles that gasoline engines never faced.

But also blame a diesel industry that went to great lengths to escape controls -- at one point even designing engines that emitted less pollution during government-required tests than in normal use.

"For many, many years the diesel industry was very good at keeping the finger pointed at other pollution sources," said Richard Kassel, senior attorney for the Natural Resources Defense Council and leader of the advocacy group's decade-long "Dump Dirty Diesels" campaign.

"The oil industry fought cleaning up the fuel, the engine manufacturers fought cleaning up the engines, and the trucking companies fought any legislation that had a cost attached to it," Kassel said.

As a result, diesel emissions remain a major pollution source.

In the San Joaquin Valley, truck traffic has nearly doubled in the past 20 years, as increases in population and commerce statewide have led ever greater numbers of trucks to crisscross California's midsection.

Regulators now estimate that on-road diesels account for about one-sixth of the region's emissions of nitrogen oxides (NOx), one of the two main building blocks of both summertime ozone and wintertime particles.

But diesels were low on the regulatory agenda for two decades, largely because until the mid-1980s, regulators tried to control ozone mainly by reducing emissions of its other major component, hydrocarbons, a type of reactive organic gas. Diesels produce few hydrocarbons. They produce NOx in spades.

They also produce more lung-damaging small particles than gasoline engines. Some are emitted directly from their exhausts. Others are produced indirectly on cold winter days, when NOx reacts with ammonia from the San Joaquin Valley's factory-farm dairies to create dangerous bits of ammonium nitrate measuring less than 1/25,000 inch in diameter.

Last year, the federal Environmental Protection Agency and state Air Resources Board finally set emissions standards that someday will make diesels as clean as modern gasoline vehicles.

But even though today's diesel engines are much cleaner than those from the 1960s and 1970s, they still have a long way to go before they are as clean as the average new gasoline engine.

"The average person today, when they think of air pollution, they're more likely to think of diesels than gasoline cars and trucks," said Howard Fox, an attorney for another advocacy group, the Earthjustice Legal Defense Fund.

Diesels key to cleaner Valley air

For the San Joaquin Valley, diesel emissions pose a serious threat. But they also offer an opportunity. With many other pollution sources already cleaned up, diesels are among the few where big improvements still can be made. And regulators are counting on that.

"Control of heavy-duty diesel engines is one of the main things that needs to happen for us to meet air quality standards," said Tom Jordan, senior air quality planner at the San Joaquin Valley Air Pollution Control District.

Jordan said his agency's sense of urgency is prompted by the Valley's soaring truck traffic, which in turn is driven at least partly by the region's emerging role as a West Coast distribution hub for major retailers such as The Gap, Wal-Mart and Ikea.

It's easy to understand why diesels are the engine of choice for companies that move a lot of freight. They're economical, consuming about one-third less fuel than gasoline engines for the same amount of power. And they're durable, able to run 500,000 miles or more with regular maintenance.

But diesels also are naturally dirty. Their low hydrocarbon emissions are offset by NOx emissions that typically are much higher without treatment than gasoline engines.

And the smoke that escapes from their stacks is a potentially deadly brew of carbon soot and toxic chemicals. It contributes to the haze obscuring the Sierra, triggers lung spasms in asthmatics and is considered likely to cause cancer.

That smoke contains an encyclopedia full of dangerous chemicals: benzene and acrolein; acetaldehyde and formaldehyde; lead, cadmium and indium; polychlorinated dioxins and dibenzofurans. More than 30 studies have found that workers exposed to large amounts of diesel exhaust run a heightened risk of lung cancer.

Nevertheless, diesels' smaller numbers and their economic importance kept them safe from regulators through the 1960s and 1970s, even as gasoline-powered cars and light trucks were coming under ever-stricter control.

Neither California nor the federal government began placing limits on visible diesel smoke until the 1970s. They didn't address NOx and particles until the mid-1980s.

They've made some progress. Compared with diesels from the 1980s, current models produce about one-third as much NOx and one-sixth as many particles. But that's scant improvement compared with the twenty-fold reductions of gasoline vehicle emissions since controls began. Diesels won't reach that level of control until 2007 or later.

Technical difficulties are part of the explanation. So is the industry's response to those difficulties.

Diesel exhaust is cooler than gasoline exhaust, and its chemical composition is different. For that reason, treating diesel exhaust with devices such as catalytic converters is more difficult.

Instead, manufacturers have tried to meet the standards by fine-tuning their engines, adding turbochargers and aftercoolers, altering ignition timing and fuel injection rates. Those steps lowered engine temperatures and reduced NOx levels. But they also caused engines to burn more fuel and emit more particles.

In the late 1980s, seven engine manufacturers -- Caterpillar, Cummins, Detroit Diesel, Mack, Renault, Navistar and Volvo, which together control 95% of the market -- found a way around that problem. They equipped their engines with special computer controls that could recognize when they were undergoing an EPA test and make adjustments allowing them to run cleaner than in normal operation.

In a series of 1998 court settlements, the manufacturers said their actions weren't illegal and insisted that EPA officials knew about and condoned them. EPA officials denied that.

By then, the government estimated, the manufacturers had sold 1.3 million of the high-polluting engines. The resulting excess NOx emissions equaled the output of 65 million cars. Largely as a result, the average big diesel truck's NOx rose in the 1990s, despite tightened standards.

"It was an absolutely incredible decade-long act of hubris and disregard for the consequences to the environment, across the whole industry," said Kassel, the leader of the "Dump Dirty Diesels" campaign. "In order to get a couple of points improvement in fuel economy, they did something that tripled NOx emissions."

Diesel manufacturers point out that the negotiated settlements didn't require them to agree that they tricked the government. They say the real problem was EPA test procedures that didn't keep up with the changing capabilities of modern electronically controlled engines.

"We were very much in compliance when EPA alleged we were not," said Christine Vujovich, a Cummins vice president. "As an industry, together, we agreed to disagree with EPA."

When the settlements were announced in 1998, Attorney General Janet Reno and EPA Administrator Carol Browner called them a major step in reducing air pollution. They noted that the manufacturers agreed to pay a record $83.4 million in fines and to spend almost $1 billion on other corrective actions.

There was just one problem: Except for requiring that the computerized controls be fixed when an engine was rebuilt, the agreements did nothing to reduce emissions from the 1.3 million high polluters that the engine manufacturers had produced for the past decade.

As of January, more than three years after the settlements, EPA records showed the manufacturers had provided the new controls for only about 2% of the engines that were subject to the update requirement.

"What they should have done instead is a combination of recalls and offsets to make up the emissions from other sources," Kassel said.

The manufacturers did agree to one other step that promises reduced emissions. Starting in October, they began making new engines to meet standards that weren't scheduled to go into effect until 2004. EPA officials estimate that change will remove 1.2 million tons of NOx from the air.

Later, outside of the settlements, standards taking effect between 2007 and 2010 will cut allowable NOx and particulate emissions to a fraction of their 1970s levels. The standards also will force diesel manufacturers to put catalysts, particulate traps and other treatment devices on the exhaust.

Cummins and other manufacturers filed lawsuits to block or delay those rules, arguing that the new standards could not be met without relying on unproven technology. They were joined by refiners that objected to a requirement that they strip diesel fuel of sulfur, which can damage treatment devices.

"The standards would be feasible if the technology to achieve them was feasible," Vujovich said, "but we don't see the technology as being feasible."

In May, a federal appeals court rejected all of the industry challenges.

Environmentalists cheered the court's decision, and the industry has filed no further appeal.

"It really resolves any residual legal uncertainty that exists over the standard," said Vickie Patton, an attorney for the advocacy group Environmental Defense and a former EPA staff lawyer.

Cleanup may take years

The new standards are eagerly awaited by San Joaquin Valley air planners and regulators. As they work to reduce ozone and particles to levels that don't violate clean air laws, the Valley's air pollution officials say diesel will be a major focus.

"It's one of the largest sources that can still have considerable controls placed on it," local air quality planner Jordan said. "We see that as one of the main keys to reaching attainment [with air standards] here in the Valley."

But it won't happen overnight.

Eventually, the diesel emission standards that go into effect between 2007 and 2010 are supposed to prevent 2.6 million tons per year of NOx from reaching the nation's air. But that level of reduction won't be achieved until 2030, when most of the high-polluting diesels currently on the road have been retired.

"Fleet turnover could take 30 or 40 years," Kassel said. "If the goal is clean air this decade, then fleet turnover isn't going to do it."

That's the downside of the long working life of the durable diesel. And it's the consequence of a generation during which diesel controls were less aggressive -- and more widely avoided -- than those for gasoline-powered vehicles.

"We are exactly at the point we were in 1975 when EPA realized we had to take lead out of gasoline to clean up cars. That's exactly where we are," Kassel said.

"At least the question is no longer, 'Can a truck be cleaner?' but 'How quickly can we do it?' That's a major paradigm shift."