Lead Isotopes Tag The Origins Of Particulate Air Pollutants

The Clean Air Act sets air quality standards that municipalities in the United States must meet. But some pollution travels from thousands of miles away. Now researchers have developed a method to more precisely identify the origins of small particulate pollutants (Environ. Sci. Technol., DOI: 10.1021/es101450t). With it, they determined that 29% of the San Francisco area's particulate pollution comes from eastern Asia.

Fine airborne particulate pollution—called PM2.5, because the particles measure less than 2.5 µm in diameter—can cause health problems such as asthma and lung damage, and exacerbate heart disease, according to the Environmental Protection Agency. Combustion as well as smelting and processing metals produces the particles. Major combustion sources include emissions from coal-fired power plants and automobile exhaust. These sources can leave a chemical signature of their origins in the particles that they emit.

One such signature is the abundance of various lead isotopes. The coal and metal ores mined in China and eastern Asia have a significantly higher proportion of  ²⁰⁸Pb, which forms from radioactive decay of thorium, than do coal and ores used in America .

China generates about 70% of its electricity with coal-fired power plants, creating large amounts of particulate pollution. Dust storms crossing China pick up these particles and carry them across the Pacific to the U.S.

Stephanie Ewing, then a postdoctoral scholar in isotope geochemistry at the University of California, Berkeley, and her colleagues wondered whether the ratios of lead isotopes in PM2.5 could quantify how much of the local pollution originated from Asia.

From December 2007 through May 2008, the researchers collected particulate pollution samples from two sites in the San Francisco Bay Area: an urban location, Chabot Observatory, as well as a coastal location, Mt. Tamalpais, where city pollution would be limited. They filtered out the PM2.5 from the samples and measured its lead isotope abundances with multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS).

At both sites, levels of ²⁰⁸Pb jumped at the same time between March and May. This isotope spike coincided with the spring, when Asian dust storms are most intense, so the researchers concluded that ²⁰⁸Pb isotopes are a marker for PM2.5 from eastern Asia. When they analyzed data from the entire six-month survey, Ewing and her team found that the median proportion of Asian lead in the PM2.5 was 29%.

Ewing, now an assistant professor of land resources and environmental science at Montana State University, thinks that the isotopic identification method also may help scientists understand the movements of pollutants throughout the atmosphere.

Julian Marshall, an assistant professor of environmental engineering at the University of Minnesota, Minneapolis, says that Ewing and her team have presented an "interesting new method and new results." He suggests that in addition to further dissecting the origins of PM2.5, high-resolution isotopic identification also could help test the accuracy of global atmospheric circulation models.