By volume, the single largest culprit is dust and dirt stirred up on paved roads, which are responsible for 450 tons per day of PM 10. But the tinier the particle, the more serious the health threat, so the AQMD and state board are likely to focus their regulations on particles measuring under 2.5 microns, which come predominantly from trucks and cars, rather than the larger particles blowing off dirty roads.
None of the sources of PM 10 presents an easy or cheap solution.
Controlling dust from roads and fields could require more thorough and frequent street sweeping, enclosures for manure and other waste piles, wind screens, planting of vegetation and slowing of traffic through dusty areas. Eliminating substantial amounts of particles from trucks, buses and other diesel-powered vehicles means switching to alternative fuels such as natural gas and methanol.
"In Los Angeles, the easy stuff has already been done," Watson said. "You can't just nail some power plant. You've already cranked down as far as you can on many of these sources."
Also, the state may have no choice but to focus on motor vehicles because the federal Environmental Protection Agency is leaning toward imposing a new standard for tinier particles. The agency is under court order to review its PM 10 standards by Jan. 31, 1997--eight days before local cleanup plans are due.
Trucking companies vehemently oppose standards that would end use of diesel--which is a cheap and efficient, but dirty, fuel.
Today's diesel trucks contain particulate-trapping filters mandated by state emission standards. Compared to trucks manufactured in 1970, 1994 trucks are 80% to 90% cleaner in particulates and 65% to 70% cleaner in nitrogen oxides.
"Diesel technology has already come a long ways," said John Fischer of Detroit Diesel, a major manufacturer of truck and bus engines.
In its ozone cleanup plan adopted last month, the state ARB proposed a standard that by 2002 would cut in half nitrogen oxides from new California-based heavy-duty trucks. The state plan also lists a future option of halving the limit again after 2004--which would require new catalytic technology or switching to an alternative fuel.
Stephanie Williams, a California Trucking Assn. vice president, said fuels such as natural gas might be an option for short hauls within the region, but not trips longer than 250 miles, in which heavy-duty trucks must climb mountain passes.
"Unfortunately," Fischer said, "with any alternative fuel, you lose one of the major advantages you have with diesel--its fuel efficiency."
Ammonia is also a mystery, although most fingers point to piles of cow manure at Chino's dairy farms. Dairy farmers, struggling with water quality regulations and dropping profits, worry that tough new PM 10 rules will target them.
Jack Broadbent, the AQMD's planning director, said the lack of understanding about the role of each type of particle poses a "significant impediment" as his agency plans its attack on the pollution.
"There needs to be a great deal of work in reducing the great number of uncertainties," he said. "We have a very open mind (about control techniques) at this point."
Yet air quality officials do not have much time to seek the answers before the 1997 deadline.
"The big question of concern," Younglove said, "is whether we can collect this data quick enough to meet federal deadlines."
In the face of politically influential opposition, White said the AQMD and state air board need political will if the Los Angeles region is to meet its mandate.
"If the promises are kept, I think 2006 is achievable," he said. "But it's a question of whether those commitments will be met and maintained."
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Where It Comes From
On an average day, more than 800 tons of PM 10--particles less than 10 microns in size--are emitted into the air of the four-county Los Angeles Basin. The following list shows the amounts from direct sources of particles; abundant gases such as nitrogen, ammonia and sulfur which indirectly form particles in the atmosphere are not included. Source: Amount per day Paved road dust: 449 tons Unpaved road dust: 137 tons Construction: 103 tons Industrial: 47 tons Trucks: 36 tons Cars: 24 tons Other mobile sources: 23 tons Farms: 22 tons Dust and fires: 11 tons ***
SHIFTING THE BLAME
The source of PM 10 varies from place to place, prompting a battle among cities and industries over what should be regulated.
In the Riverside County area of Rubidoux, which suffers the most severe PM 10 pollution, a large percentage comes from road dust and dairy farms.
Downtown Los Angeles, however, gets more from motor vehicles. The smallest and most dangerous particles, under 2.5 microns in size, nearly all come from trucks, cars and industries that burn fuel.