Experts debate wisdom of NOx-limits for big-city ozone control schemes

Diesel Fuel News, July 21, 2003 by Jack Peckham

U.S. EPA and California Air Resources Board (CARB) regulations forcing expensive, complex technologies to slash nitrogen oxides (NOx) from diesel engines over the coming decade ironically could lead to higher ozone in big cities -- or at best, fail to make much difference (see Diesel Fuel News 9/30/02, p1).

In part, that's because NO emission (about 90% of diesel NOx is NO) is an effective quenching agent for ground-level ozone in polluted air basins such as metro Los Angeles, as air-quality experts point out in separate papers published in the July 2003 edition of EM, the magazine of the Air & Waste Management Association.

The "quenching" chemistry: NO combines with [O.sub.3], creating [NO.sub.2] and [O.sub.2].

Problem: Much of this ozone-quenching NO disappears on weekends, when diesel vehicles (nearly all commercial) stop working. The result is the "weekend effect," where ozone actually increases despite a very sharp decrease in NOx emissions -- a phenomenon known for decades, but only now starting to be understood for its regulatory implications.

Now, the fear is that the "Weekend effect" could spread to weekdays in coming years, if future NOx cuts outpace reductions in the other primary ozone precursor: hydrocarbon (HG) emissions.

What's more, alarming evidence shows the "weekend effect" in a growing number of major U.S. cities, including Chicago, Philadelphia, Atlanta, Houston, Dallas/Ft. Worth and Denver, as researcher Doug Lawson (principal scientist at National Renewable Energy Lab) points out in his article on the "weekend effect" for "EM" magazine.

This raises broader questions about EPA's national scheme to slash diesel NOx emissions over 90% in new diesel vehicles, and GARB's never-ending ratchet-down on new-vehicle emission standards, which are almost zero-emissions anyway, Lawson says.

The implication: Many cities are "hydrocarbon limited" or, in air-quality lingo, "volatile organic compound (VOG) sensitive." That means if such cities really want to cut ozone, then they need to slash HG/VOC (also know as "ROG" -- reactive organic gases) rather than NOx.

Cracking down on "high-emitter" gasoline cars seems to be the most cost-effective ozone strategy, Lawson argues.

Since a very small portion of the gasoline car fleet is responsible for most of this emission, a sharply focused control program could bring huge returns for a remarkably small investment, Lawson argues.

Counter-point: In the same edition of EM, GARB air-pollution research specialists Bart Croes, Leon Dolislager and Lawrence Larsen contend that the multi-party working group (including Coordinating Research Council, U.S. Department of Energy, NREL, and Sierra Research) that carried out intensive studies of the ozone "weekend effect" in metro Los Angeles (with GARB assistance) still can't be 100% sure that slashing NOx will mean higher weekday (or for that matter, weekend) ozone in future.

In a follow-up interview, Croes told us that GARB credits both VOG and NOx reductions over the last 20 years for lower ozone in metro L.A. While emission source "upwind" areas do indeed respond better to VOC controls, "downwind" areas seem to respond better to NOx controls, he said. On the other hand, NREL's Lawson counters that "downwind" (outskirts of L.A.) areas have ozone levels not much different on weekends versus weekdays, despite huge NOx reductions on weekends.

The point: If ozone goes up or even stays about the same when NOx emissions plummet, even in "downwind" areas, then why focus so much on slashing NOx?

Counter-point: As for NO quenching of ozone, "this seems to be a temporary effect," with uncertain ozone impacts later on, Groes contends. What's more, while air-pollution experts generally agree that source VOC controls can cut local ozone, "there's still debate on how effective VOC controls are," especially at certain altitudes or in certain areas such as San Joaquin Valley, he said.

CARB specialists also caution that current air modeling isn't yet perfect, as certain physicallchemical processes causing the "weekend effect" haven't been defined fully. Hence it's wrong to assume that the "weekend effect" is a surrogate for what will happen from future NOx-control programs, Croes and his colleagues contend.

Rather, the weekend effect might be a "complex perturbation that occurs on a weekly cycle" -- and it's theoretically possible that cutting NOx further might reduce rather than increase weekend ozone, the GARB researchers claim, citing a 20-year correlation between ozone reductions and NOx reductions in metro L.A.

But there's a problem with this correlation: HC/VOC (a.k.a. "ROG") emissions have been on a much steeper decline than NOx over the last 20 years -- about twice as much, GARB shows. Hence it's reasonable to consider that the huge cuts in HC/ROG -- a proven ozone precursor -- better explain the ozone drop, rather than modest NOx cuts, especially when NO is a proven ozone quencher in polluted areas.

Still, GARB researchers say they're not sure what's causing the "weekend effect." Other factors potentially explaining some of it could include sea-salt in the air. Maybe the 1996 introduction of reformulated gasoline -- which cut ROG emissions -- theoretically could argue for greater NOx reductions, GARB claims.