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ACE-1: A beauty of a place, a beauty of a study

Background by Anatta, UCAR Communications
Update by Barry Huebert, University of Hawaii

Late last fall, dozens of researchers, two ships, and an instrumented aircraft traveled to Tasmania. They were attracted not by the island's rugged beauty, unique wildlife, and moderate climate, but by its distance from Northern Hemisphere sources of pollution. Their aim was to study the background or natural levels of various atmospheric aerosols in the remote marine atmosphere.

Hobart International Airport in Tasmania was the operations center for the Southern Hemisphere portion of ACE-1. The cluster of trailers in the left foreground housed temporary workstations, scientific offices, and laboratories for the project. Beyond them is the NCAR/NSF C-130 instrumented aircraft. (All photos by Carlye Calvin.)
The study was ACE-1, the first of the Aerosol Characterization Experiments, a series of international field programs to help scientists understand the chemical, physical, and optical properties of aerosols; how aerosols form and grow; and their effect on radiation and climate. ACE involved over 100 investigators from 57 institutions in Australia, France, Germany, Italy, Japan, the Netherlands, New Zealand, Sweden, the United Kingdom, and the United States. Major platforms included the instrument-laden NCAR/NSF C-130 aircraft, NOAA's Seattle-based research vessel Discoverer (known familiarly as the "Disco"), and the Australian research ship, the Southern Surveyor. NSF is the primary sponsor of the U.S. component.

NOAA's Seattle-based research ship, Discoverer, was one of two ships participating in the study. The Discoverer made measurements of seawater chemistry and diurnal variations in the atmosphere, and launched the balloons for the Lagrangian experiments.
The purpose of all this was to solve an atmospheric mystery. One of the uncertainties in climate models is whether and how much the increasing numbers of sulfates and other aerosols counteract the climate-warming effects of greenhouse gases. To know that, scientists need to find out the extent and character of aerosols in the atmosphere, and where they come from. "Existing theories suggest that it should be very hard to create new particles in the lower atmosphere, yet they keep showing up," says principal investigator Barry Huebert of the University of Hawaii, Honolulu. "We deployed state-of-the-art instruments to the remote marine atmosphere for the first time to seek the source of these new particles. It was the largest and most comprehensive experiment on natural background aerosols that we have ever done."

ACE-1 principal investigator Barry Huebert (right) and Davie Wylie troubleshoot a faulty vacuum pump. In an experiment with so much one-of-a-kind instrumentation, equipment maintenance and repair is a continuous process. Notes Huebert: "The need to keep roughly ten tons of high tech instruments in good repair while half a globe away from parts suppliers is why the international shipping and customs services of UCAR's JICP/PO were so critical to the success of ACE-1."
ACE did not begin or end in Tasmania. On its way there, the C-130, equipped with, among other things, NCAR's dual-wavelength airborne lidar, took the "scenic route." It headed first to Alaska, for a flight toward the North Pole and back, and then to Hawaii, for a flight through Kilauea's volcanic plume to study how its particles form and how much sunlight they reflect. A flight from Christchurch, New Zealand, toward the South Pole completed the aircraft's nearly pole-to-pole measurements. Additional data were gathered on the plane's return trip to the United States.

Here is Huebert's preliminary report on how the project went.

My impression is that ACE-1 was highly successful. We managed to get good flights in for virtually all of our major objectives, including the long transects down the Pacific, vertical profiles in many remote marine locations, studies of the Kilauea volcano plume, local and vertical column closure, aerosol production by clouds, Lagrangian aerosol evolution (in which we repeatedly sampled the same air mass as it moved), air-sea flux measurements, vertical profiles over the Disco and the Cape Grim measurement site (see photo), and intercomparisons with instruments on the other platforms.

For nearly 20 years, the Cape Grim Air Quality Monitoring Station, operated by the Australian Bureau of Meteorology, has been collecting measurements of a wide variety of atmospheric constituents. Some measurements are taken from the tower (which is primarily a microwave/telephone relay facility). During ACE-1, this dramatic promontory was also host to NCAR's Integrated Sounding System (the white trailer in the left foreground), which collected standard meteorological data. NOAA researchers occupied the trailer at the base of the tower, and scientists from other institutions conducted air chemistry experiments in the main building.
The C-130 was as solid as a rock. We had two down days to do an engine change, but in 295 flight hours no other flight was delayed or cancelled because of the aircraft! We had been worried that the noise and vibration of the aircraft would so fatigue the scientific party that their performance might suffer, but that turned out not to be much of a problem. Atmospheric chemistry packages are all pumps and noisemakers anyway, so this wasn't all that big a degradation from chemistry experiments on other aircraft. Three successive days of nine-hour flights, on the other hand, is enough to send anybody to bed for a day. It was important that we had duplicate scientific crews in addition to the double air crews, so fresh people were flying each day. We received excellent support from NCAR's Research Aviation Facility and UOP's Joint International Climate Projects/Planning Office and Office of Field Project Support.

During ACE-1, the C-130 was crammed with instruments from many institutions. Here, Nicola Blake of the University of California, Irvine, operates the university's whole air sampling system. A pump brings in air from outside the aircraft and circulates it through the system; Blake turns this series of valves to fill, in turn, each of 96 two-liter stainless steel canisters. At the end of each flight the filled canisters were sent back to the lab at Irvine, where the contents were analyzed for more than 50 different hydrocarbon and halocarbon gases. Sensitive measurements of these gases, which are emitted from a variety of natural and anthropogenic sources, are particularly useful in tracing air of different origin and photochemical age.
Our first Lagrangian try washed out when both the forecast and the balloon-data receiver were busted, but the next two worked great. We completed two three-flight Lagrangian experiments, the second one starting in clear, sunny air. For that one we had also done a pre-Lagrangian flight, in which we tried to characterize the source of the postfrontal air we would be studying the following two days. Disco then launched three balloons, all of which we followed for three flights. Interestingly, they maintained almost exactly the same orientation relative to one another, even though the wind speeded up and turned a 90-degree corner midway through the experiment. Cloudiness increased during both Lagrangians, and we have preliminary evidence that this changed the aerosol size spectra in sensible ways.

With the exception of the LIF ammonia instrument, which was being built and tested as we departed Jeffco [NCAR's aviation facility at Jefferson County Airport in Colorado], we rarely had an instrument down for a whole flight--pretty amazing! We had a number of close calls, when repair parts or expendables couldn't get through customs, but we managed to keep things working nonetheless. Midway through the Hobart deployment the ammonia system came on line, and we got very enlightening data from it the remainder of the experiment.

Among the most obvious results to emerge so far is the production of new aerosol nuclei in the outflow from cumulus clouds. Peter Hobbs [University of Washington] has noted this phenomenon before, but we were able to sample it with measurements of sulfuric acid and ammonia vapors (the substances that condense to make the new nuclei) and several complementary techniques for examining the ultrafine nuclei that were being produced. We traced thin, free-tropospheric layers of fine particles back to the air detraining from fields of cumulus clouds and were able to distinguish between productive and nonproductive conditions.

The Cape Grim and Disco experiments also went well, each with their highlights. So early indications are that the experiment was a huge success. Of course, the real proof will come when we have all the quality-controlled data in hand so we can do closure calculations and see how well the platforms intercompared. Much of that science will take several years to sort out. But the early indication is that we did exactly what we set out to do.

Working in Tasmania had its nonscientific benefits, as well, including ample opportunity for encounters with wildlife. A wallaby spotted at Cradle Mountain National Park has mist on its fur from the nearby rain forest.
For further information, see the ACE home page at http://home.ucar.edu/uop/jicppo/project/ace/ace.html.

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 09:13:34 MDT 2000