Are Secondary Particles the Culprits of Poor Air Quality?

Oct. 17, 2008
Studies point to the need to reassess air pollution.

When it comes to air pollution, we could be missing the point – both on a local and a global scale.

A new study published in  Environmental Science and Technology ( ),  led by the University of Colorado at Boulder, reveals that air quality regulations may not effectively target a large source of fine, organic particle pollutants that contribute to hazy skies and poor air quality over the Los Angeles region.

A much smaller percentage of organic haze than was previously thought is directly emitted by vehicles and industrial processes, says the study. Instead, 75% of fine, organic particle pollutants form when volatile organic compounds (VOC) are oxidized and condense onto existing particles in the air.

“Air quality regulations today effectively target most sources of ‘primary’, or directly emitted particles,” notes lead author Ken Docherty, a researcher with the university's Cooperative Institute for Research in Environmental Sciences. “Yet our study indicates that the ‘secondary,’ or chemically formed particles contribute more significantly to poor air quality, even in very polluted urban regions. Our study suggests that regulations need to focus much more attention on the gases — such as gasoline vapors — that form secondary organic particles and create visible haze.” 

Other examples of VOCs include vapors from paints, varnishes, cleaning supplies, automotive products and dry-cleaned clothing.

However, Docherty and his colleagues caution that it’s unclear which VOCs are most responsible for haze formation, and  little is known about how a particle’s chemical composition might exacerbate its impact on human health. 

But even when we get a better understanding of how secondary particles influence local air pollution, there’s still the question of what — if anything — happens to them on a global scale.

Scientists in the Department of Chemistry at the University of York in England may have found part of the answer. They have discovered a ‘chemical equator’ in cloudless skies in the Western Pacific. This about 50-km-wide band divides the Northern Hemisphere’s polluted air from the Southern Hemisphere’s largely uncontaminated atmosphere.

This showed for the first time that the chemical and meteorological boundaries between the two air masses aren’t necessarily the same.

The discovery will provide important clues to help scientists more accurately model simulations of the movement of pollutants in the atmosphere, and assess the impact of pollution on climate, according to the scientists. 

The study is part of the Aerosol and Chemical Transport in Tropical Convection (ACTIVE) project funded by the U.K.’s Natural Environment Research Council.

Scientists previously believed that the Intertropical Convergence Zone (ITCZ) formed the boundary between the polluted air of the Northern Hemisphere and the  Southern Hemisphere’s cleaner air. The ITCZ is a cloudy region circling the globe where the trade winds from each hemisphere meet. It’s characterized by rapid vertical uplift and heavy rainfall, and acts as a meteorological barrier to pollutant transport between the hemispheres.

But the new research, to be published in  the Journal of Geophysical Research — Atmospheres (“Observations of an Atmospheric Chemical Equator and its Implications for the Tropical Warm Pool Region” is available from Peter Weiss of the American Geophysical Union at (202) 777-7507, [email protected]), found huge differences in air quality on either side of the chemical equator, which was well to the north of the ITCZ. 

The study revealed that carbon monoxide, a tracer of combustion, increased from 40 parts per billion to the south, to 160 parts per billion in the north. The difference in pollutant levels was boosted by extensive forest fires to the north and very clean air south of the chemical equator being pulled north from the Southern Indian Ocean by a land-based cyclone in northern Australia.

The scientists discovered the chemical equator using sensors on a specially equipped airplane during flights north of Darwin. At the time, the ITCZ was well to the south over central Australia. 

“The shallow waters of the Western Pacific, known as the Tropical Warm Pool, have some of highest sea surface temperatures in the world, which result in the region’s weather being dominated by storm systems,” says Jacqueline Hamilton of the Department of Chemistry at York. The  chemical equator was to the south of the stormy region during the ACTIVE campaign. This means these powerful storms may act as pumps, lifting highly polluted air from the surface to high in the atmosphere where pollutants will remain longer and may have a global influence. To improve global simulations of pollutant transport, it’s vital to know when the chemical and meteorological boundary are in different locations.

Sean Ottewell is Chemical Processing's Editor-at-Large. You can e-mail him at [email protected].

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