A Disappearing Act? Researchers Investigate Tropical Ozone


Researchers are investigating causes for ozone depletion in the tropical Western Pacific, where key processes driving global atmospheric circulation originate. Image by Peter Holm.
Massive storm clouds that form in tropical regions can generate heat and energy—convection—that drive air currents around the globe. This is especially true for the area known as the “Tropical Warm Pool” in the western Pacific Ocean and eastern Indian Ocean. In this region, convection drives air into the stratosphere as part of the global equator-to-pole transport cell known as the Brewer-Dobson Circulation.

A team of scientists from the United Kingdom (UK) are investigating the hypothesis that short-lived halogenated compounds emitted from the ocean surface can be transported high enough in the atmosphere by convective clouds in the Tropical Warm Pool that they enter the stratosphere and accelerate the rate of ozone destruction. This is important because, although man-made halogenated compounds are strictly controlled under the Montreal Protocol, the amount of bromine in the stratosphere can’t be accounted for by known sources.

To test their theory, the UK scientists spent the month of February at the ARM site on Manus Island, Papua New Guinea (PNG), as part of the Co-ordinated Airborne Studies in the Tropics, or CAST, field campaign. Funded by the UK’s Natural Environment Research Council and led by Professor Geraint Vaughan from the University of Manchester, they launched 39 ozonesondes to measure vertical profiles of ozone and meteorological variables from the ground up to 30 kilometers. Preliminary findings point to the uplift of ozone-poor air to the tropopause level during periods of deep convection to the east of of Manus.

The ozonesonde packages consisted of chemical sensors inside a lightweight foam box with a radiosonde transmitter strapped to the outside, plus a balloon to carry it. Out of 39 total launches, 35 reached full altitude for a complete measurement profile.
On February 22, they coordinated their launch with an overflight by a Gulfstream 5—one of three research aircraft based out of Guam on a six-week mission for a related study. The aircraft were able to make detailed measurements of atmospheric structure and composition from the ocean surface up to 20 kilometers all over the Western Pacific. The ozonesonde measurements made from the site on Manus serve as a bridge between the different aircraft observations.

“It wasn’t that easy to get flight permission to get into PNG airspace,” said Vaughan. “It was only with the help of ARM colleagues and their contacts in PNG that we were able to do this.”

The team also brought along specialized instruments—an ozone analyser, a cavity-ringdown absorption spectrometer, and a gas chromatograph—to gather continuous measurements of ozone, methane, carbon monoxide, carbon dioxide, water, and halogenated hydrocarbons at the ground station during the same period. To help interpret these chemical measurements, they plan to use routine meteorological data gathered by the ARM instrument suite on Manus.

This is one of the last field campaigns to be conducted at the ARM tropical sites. They are scheduled to cease operations this year as ARM reconfigures its operational strategy to support high-resolution models and process studies.