UCAR Communications


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November 2003

A little iodine may make a big difference

ACD tracks aerosol production on the Irish coast

Iodine gases emitted from seawater, seaweed, and phytoplankton may help trigger the formation of aerosols (airborne particles) that affect clouds and haze in some coastal areas. Thus, “the iodine may have a significant impact on the climate in these regions,” says Alex Guenther (Atmospheric Chemistry Division). A field study called BIOFLUX has been examining the extent of this process—whether it’s localized or widespread—and how the emissions might respond to future changes in climate.

Seaweed on the Irish coast. (Photo by Satu Piipponen.)

Alex and ACD’s Andrew Turnipseed and Jim Greenberg spent the last three weeks of September at the BIOFLUX field site on Ireland’s west coast at Mace Head and Mweenish. The study also included scientists from the National University of Ireland, Galway; the University of Helsinki (Finland); and the Institute of Spectroscopy and Applied Spectroscopy at the University of Dortmund (Germany).

BIOFLUX will validate recent laboratory and modeling studies that link iodine to new particle formation and will provide data for longer-term modeling on the role of new particles in the atmosphere. Aerosols are one of the biggest uncertainties in modeling clouds and global climate, and the ocean is a key source of these airborne particles.

What’s happening on the coast?

Sea spray injects many short-lived aerosols into the atmosphere. However, another process, called gas-to-particle conversion, yields a smaller number of aerosols that are lighter, smaller, and longer-lived. These form as gases, emitted from the oceans or marine life, undergo reactions that convert them to solids over a span of a few seconds to a few minutes. Made up of organic material and sulfur, these tiny particles—about 1 to 3 nanometers in width (a nanometer is one-billionth of a meter)—can agglomerate to much larger sizes.

Until recently, scientists believed that the main ingredient leading to these particles was dimethyl sulfide from marine organisms. But field studies in the last five years at the coastal Mace Head research site have detected “bursts” of particle formation that appear unrelated to dimethyl sulfide. These bursts are most common around midday and at low tide, when seaweed is most exposed to air and sunlight.

In the laboratory, scientists recently found that iodine from organic sources (such as seaweed) can lead to quick formation and growth of aerosols, given the right conditions and enough ultraviolet light. BIOFLUX was designed to test this hypothesis by relating emissions of organic iodine to the rate of aerosol formation near the coast.

Ballooning for chemistry

To measure large-scale conditions and the rate of particle growth at BIOFLUX, the ACD team used its high-flying Tethered Atmospheric Chemistry Observing System. TACOS, which has evolved gradually over the past decade, consists of a balloon hovering as high as 6,600 feet (2,000 meters) and a instrument-studded tethering line. The instruments measure basic meteorological variables as well as the presence of ozone, carbon dioxide, aerosols, and volatile organic compounds, including the iodine compounds being studied in BIOFLUX.

TACOS takes readings in Mace Head, Ireland. (Photo by Satu Piipponen.)

In one sampling mode, the TACOS balloon collects data every second as it ascends or descends at about 3 feet (1 meter) per second. “In the other mode, we just deploy the balloon to the maximum height and let it sample for a few hours,” says Alex. For those gases that can’t easily be measured while TACOS is in the air, the system uses battery-powered, computer-controlled air samplers to fill bags or solidabsorbent cartridges for later analysis.

In Ireland, TACOS flew mostly at heights of about 1,000 feet (300 meters). Four instruments along the tethering line profiled the boundary layer and the gas-particle reactions of interest to BIOFLUX. The ACD team is now analyzing the results, says Alex. “Our preliminary observations show that the particle bursts occur not only at the Mace Head research site but also all along the coast.”

As for the observing process itself, “everything went fine,” Alex says. Even the fickle Irish weather cooperated: “We wanted some nice sunny days at low tide, and to some degree we got that.” •Bob Henson

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