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Spring 2001

ACE-Asia finds plenty to study

by Carol Rasmussen

Most of us want to get away from polluted, dusty air, but the scientists of the Aerosol Characterization Experiment–Asia reveled in it this spring. From land, sea, and air, a large group of international collaborators pointed all the instruments they could muster at the pollution and dust plumes blowing off the Asian continent into the North Pacific. In doing so, they got the first detailed measurements of the dust and pollution aerosols that are peculiar to the region.

The developing Asian countries "have got a unique fuel mix, but the dust is what makes it really interesting," says Barry Huebert (University of Hawaii), principal investigator for ACE-Asia and two earlier ACE experiments (see sidebar). That dust arises off the Mongolian and western Chinese deserts in great spring storms. The dust clouds can travel an amazing distance; haze from a storm this spring was noticed at least as far away as Colorado.

Barry Huebert adjusting the low-turbulence inlet on the NSF/NCAR C-130 (Photo by Carlye Calvin).

Huebert is quick to point out that Asian dust isn't the only globetrotting pollution. "In some parts of China you can see deposition of sulfate that came from North America. Every country has the same problem; we're all sending stuff downstream." But the dirty, dusty, glommed-together particles drifting out across the Pacific—dubbed multicomponent aerosols by scientist Irina Sokolik (University of Colorado)—have been studied much less than aerosols over the United States and Europe. The goal of ACE-Asia is to characterize these particles, understand how they evolve as they travel, and use these data as ground truth for chemical transport models.

"As scientists of many nationalities, our job is to provide our own policy-makers with responsible estimates of impacts," reads the ACE-Asia science and implementation plan. To do that, realistic regional models of Asian aerosols are necessary. Till ACE-Asia, the only data available have been individual observations of a few aerosol characteristics at scattered locations around the continent. ACE-Asia will offer the first chance to understand the full chemical and radiative effects across the region.

These effects are interdependent, variable, and difficult to model. For example, Asian dust contains calcium carbonate, which reacts in the atmosphere with sulfur (from coal burning and other sources) to form calcium sulfate. This reaction reduces the amount of sulfur dioxide (SO2), the main type of visibility-reducing sulfate aerosol in the submicron size. Thus, "it's quite likely that dust has a big influence on what happens to SO2," says Huebert. ACE-Asia's measurements of the composition and reaction rates of the dust will clarify the extent of that influence.

The radiative effects are also not yet understood. Sulfate aerosols are known to cool the atmosphere by their effects on clouds, whereas the black carbon and dust aerosols from Asia may cause warming, depending on a variety of factors such as their altitude. In many parts of the globe, the cooling and warming effects of these two types of aerosols are so similar in magnitude that, when they are modeled, the effect that prevails depends on what assumptions the modelers make about the size of the dust particles.

The Asian aerosols even have biological effects. The spread of plankton and other marine biota in the North Pacific is limited by the availability of soluble iron. Virtually the only source of iron in the region is Asian dust, although in its original state the iron in dust is not soluble. So both the amount of dust and the chemical reactions that make it soluble affect regional fisheries.

ACE-Asia participants brought a whole arsenal of atmospheric instruments (see sidebar) to record both clean air plumes and dust and pollution plumes over the ocean, with coordinated measurements from aircraft and shipboard. Besides the chemistry data, some flights studied the radiative effects of the plumes. Satellite data is providing information on how clouds change as they interact with the aerosols, among other large-scale questions. The experiment's operations center was Iwakuni Marine Corps Air Station in Iwakuni, Japan, about 20 miles from Hiroshima.

The timing for this experiment is good on two scales. Field operations director Richard Dirks (UCAR Joint Office for Science Support) says, "The season was chosen because of its predictable circulation. In the summer, with convective weather and thunderstorms, the circulation gets harder to predict." Beyond that, Huebert says, "China is right at the start of industrialization. Currently the motor vehicle population is small, but it's projected to rise rapidly. Most pollution [now] comes from biomass burning and coal burning. We're in a position to characterize aerosols at the start of a change. We'll attempt to use this [ACE-Asia data set] as a sort of baseline."

According to Huebert, one of the things that distinguishes the experiment is "a high level of cooperation from the Asian contributors, and also a high level of cooperation within each country. One of the challenges has been to make clear to all of our participants how much they have to gain by sharing their data with the entire science team. Obviously, the benefits of a large collaborative experiment like this are only going to be achieved if you can look at a picture larger than what any one group can obtain."

Just the facts

Who: More than 130 scientists from Australia, Canada, China, Chinese Taipei, England, France, India, Japan, Korea, the Netherlands, Russia, and the United States. Principal investigators include Huebert, Timothy Bates (NOAA), John Seinfeld (California Institute of Technology), and Gregory Carmichael (University of Iowa). The Asian coordinators are Kimitaka Kawamura (Hokkaido University, Japan), Young Joon Kim (Kwangju Institute of Science and Technology, South Korea), Neng-Huei Lin (National Central University, Chinese Taipei), and Shi Guangyu (Institute of Atmospheric Physics, Beijing, China). ACE-Asia is sponsored by the International Geosphere-Biosphere Programme and funded by many agencies, including NSF, NOAA, and the U.S. Office of Naval Research.

With what: Two U.S. aircraft (C-130, Twin Otter), one Australian aircraft (King Air), a U.S. research vessel (R.V. Ron Brown), and a Japanese research vessel (R.V. Mirai). Specially instrumented surface sites were located in China (Beijing, Hong Kong, Lin'an, Qingdao, Zhenbeitai), Japan (Hachijo), Korea (Kosan), and Taiwan (Lan Yu). Data were collected from lidar networks in China and Japan and from standard meteorological and air quality networks.

When: The last week in March through mid-May.

Where: The operations center was in Iwakuni, Japan. A surface facility "supersite" was located on Cheju Island (Korea). Data were collected from midlatitude eastern Asia out over the northwest Pacific Ocean.

On the Web:
International Global Atmospheric Chemistry Project
JOSS Support pages for ACE-ASIA

Catching the big ones

As in most fish stories, the big ones got away. But we're not talking two-foot-long trout here: these big ones were specks of dust, sea salt, and other airborne particles more than a micron (0.00004 inch) in size. Large-sized particles have been notoriously underrepresented in air samples collected from aircraft. But a new low-turbulence inlet aboard the NSF/NCAR C-130 aircraft is changing that.

The problem arises because the air that flows into an inlet must be drastically slowed down—from about 100 to 4 or 5 meters per second—before it reaches the collection instruments. In most inlets, the slowdown takes place in a funnel-shaped diffuser that inevitably creates turbulence. The swirling motion doesn't affect particles that are small enough to behave more or less like air molecules, but it slams larger particles into the diffuser walls, where they stick. As much as 90% of large particles can be lost that way. The scientific consequences are significant: in ACE-1, Huebert says, "Since we were unable to collect the big sea salt particles, we were unable to close the sulfur budget."

The new inlet has porous walls made of bonded stainless steel pellets. When the airflow hits the walls, as much as 80% of it simply passes through and is returned to the open air. The remaining 20%, however, is a laminar flow that retains the natural distribution of particle sizes.

Russell Seabaugh and his colleagues at the University of Denver developed the inlet with the help of Huebert and Jack Fox (NCAR Atmospheric Technology Division). This method of using boundary-layer suction to eliminate turbulence is well known in Seabaugh's field, aeronautical engineering.

The low-turbulence inlet was flight tested last July over the Caribbean, and it outperformed three other inlets in collecting dust and sea salt particles. From the test, it appears the new inlet will be able to measure particles up to at least 7 microns in size. Huebert, who's been wanting to catch the big ones for over a decade, is more than pleased. "[ACE-Asia] is the first experiment in which we really have some assurance that we're getting super-micron-sized particles."

Earlier ACEs and related studies

ACE-1: The first Aerosol Characterization Experiment took place in Tasmania in the fall of 1995, with more than 10 investigators from ten countries. ACE-1 studied background levels of aerosols in the remote marine atmosphere.

TARFOX: Despite its nonconforming name, the Tropospheric Aerosol Radiative Forcing Observational Experiment is usually considered part of the ACE series. In 1996, University of Washington scientist Peter Hobbs and colleagues measured the size and optical properties of aerosols over the Atlantic Ocean east of a 200-mile-long urban corridor centered on Washington, D.C.

ACE-2: In the summer of 1997, 200 European and U.S. scientists observed aerosols over the Canary Islands. Most of the aerosols came from Europe; the organizers also hoped to collect data on dust aerosols from Africa, but weather conditions were generally unfavorable.

INDOEX: Like ACE-Asia, the 1999 Indian Ocean Experiment was a large, multinational experiment to study aerosols. Its focus, however, was somewhat different. "INDOEX began with people who were interested in radiative transfer, and some chemistry was added," says Huebert. "ACE- Asia started with chemists who realized we also needed to know something about radiative transfer."

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Thu Jun 21 18:56:13 MDT 2001