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 ExperimentAsia 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 Pacificdubbed
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
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
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
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 downfrom about 100 to 4 or 5 meters per
secondbefore 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
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,
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
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
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
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.
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,
Prepared for the Web by Jacque Marshall
Last revised: Thu Jun 21 18:56:13 MDT 2001