Dust carried on winds from Asia supplies calcium and phosphorus to Hawaiian flora and is a primary source of iron for North Pacific fisheries. But along with those nutrients, Asian dust also carries pollutants, according to new data from the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). The findings, which were presented in December at the American Geophysical Union 2001 Fall Meeting, provide new information regarding aerosol sources, transport, and sinks, and could help reduce uncertainties associated with aerosols in climate-change models.

(Top) An unusually intense Asian dust storm flows eastward on April 13, 2001 and (bottom) makes it way to the west coast of the United States on April 15, 2001. NASA/Goddard Space Flight Center, SeaWiFS Project, and ORBIMAGE

ACE-Asia is part of a series of experiments designed to investigate how aerosols affect the earth’s climate, both directly through light absorption and scattering, and indirectly by acting as cloud condensation nuclei (Environ. Sci. Technol. 2001, 35, 332A–340A). The intensive field-monitoring phase of ACE-Asia took place last spring, during which a team of scientists from 13 nations simultaneously, from aircraft, ships, satellites, and ground-based stations, measured the chemical and physical properties of aerosols coming out of Asia.

ACE-Asia probably is the largest surface-based aerosol study ever conducted in terms of geographical extent, number of samples collected, and number of analyses performed on each sample, according to Thomas Cahill, director of the DELTA Group (Detection and Evaluation of Long-Range Transport of Aerosols) at the University of California–Davis, who examined aerosol transport during ACE-Asia from China to as far away as Washington state and Oregon. “We covered 41% of the circumference of the earth simultaneously,” says Cahill.

The region off the coasts of China, Japan, and Korea, was chosen because of the known complexity of the aerosols in that area, says Barry Huebert of the University of Hawaii and lead scientist of ACE-Asia. Asian aerosols are much different than those in Europe and North America, because in Asia there is more coal and biomass burning, and less emissions controls. In spring, dust storms originating in the deserts of Mongolia and China make their way down to populated cities in China, Japan, and Korea, adding a large dust component to the aerosol mix. Sea salt also adds to the complexity of the region’s aerosols.

The timing of ACE-Asia couldn’t have been better, say those who were involved in the project. Thanks to a 50-year drought that occurred last spring in China and Korea, the region experienced one of the largest Chinese dust storms in years, says Young-Joon Kim, director of the Advanced Environmental Monitoring Research Center at the Kwanju Institute of Science and Technology in Korea, who monitored the vertical profile of aerosols during ACE-Asia using a multichannel lidar system. The unusually intense dust storm provided the ACE-Asia team with an interesting before-and-after picture, giving clues as to how dust particles interact with other aerosols, says Huebert.

Dust and sea salt can be associated with secondary species such as nitrate, sulfate, and carbon. Such chemical transformations affect the radiative properties of aerosols, and therefore must be understood in order to accurately model climate change. Analyzing Asian aerosol samples at the single-particle level helps scientists understand how aerosol particles interact with one another.

During ACE-Asia, Kim Prather and colleagues from the University of California–San Diego, investigated the temporal and spatial variability of chemical transformations on particles. They obtained real-time, continuous measurements of the size and chemical composition of single particles using aerosol time-of-flight mass spectrometry. “Very few of the particles were pure,” says Prather, indicating that dust carries a variety of pollutants along with the minerals. The group observed calcium-rich particles coated with secondary sulfate and nitrate. After the large dust event, there was more nitrate than sulfate, nitrite unexpectedly dominated, and iron and aluminum levels increased.

Additionally, Jim Anderson and colleagues from Arizona State University’s Environmental Fluid Dynamics Program observed mixing of anthropogenic particles with mineral dusts in aerosol samples collected during ACE-Asia. Anderson’s group used scanning electron microscopy to determine the chemical composition, size, shape, and degree of mixing. They frequently observed mineral particles covered with elemental carbon in the form of soot in the low altitude samples. Soot alters the optical properties of mineral dust, which ultimately could affect climate.

Scientists have only just begun interpreting the data from ACE-Asia, which could take several years, but the study has already confirmed the complexity of Asian aerosols. Kim’s lidar data show extensive layering of clean air, polluted air, and dust. As many as 14 different layers were observed in an altitude of 2 kilometers. “This kind of layering is going to be a tremendous challenge to model,” says Huebert.

Previous experiments in the ACE series, which is organized by the International Global Atmospheric Chemistry Program, were conducted in the pristine environment of Tasmania and in the Canary Islands to study Saharan dust merging with European pollution. “The Saharan dust tends to be much less associated with pollution than what comes out of Asia,” says Huebert. “The Asian dust is really unique.” —BRITT E. ERICKSON