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Summer 2000

TOPSE studies springtime in the Rockies—and all the way to the Arctic

by Carol Rasmussen

TOPSE co-principal investigator Elliot Atlas. (Photos by James Hannigan.)

Participants

TOPSE investigators came from Dalhousie, Harvard, Rutgers, and York Universities; Georgia Institute of Technology; the Universities of California (Berkeley and Irvine), Maryland, New Hampshire, Rhode Island, and Virginia; NASA Goddard and Langley Research Centers; the Pacific Northwest National Laboratory; and NCAR. Environment Canada and Purdue University collaborated on PSE 2000.

So you think you have a long commute. This spring, a group of scientists from NCAR's Atmospheric Chemistry Division (ACD) spent every other week flying to, around, and back from Arctic Canada and Greenland. From 5 February through 23 May, the grouped logged tens of thousands of miles in the Tropospheric Ozone Production about the Spring Equinox experiment.

TOPSE was designed to study the evolution of the atmosphere's chemical composition from winter to spring in the mid- to high latitudes over North America, with an emphasis on the springtime peak in ozone in the mid- troposphere, according to Elliot Atlas of ACD. With Chris Cantrell and Brian Ridley of ACD, Atlas is a TOPSE principal investigator. Ozone sinks down from the stratosphere into the troposphere, and it's also formed there by a variety of photochemical reactions involving sunlight, carbon compounds, nitrogen species, and water vapor. In the dark polar winter, ozone production almost shuts down. When the sun returns in the spring, levels begin to increase, rising from 30–40 parts per billion to 50–60 ppb (still low compared to either the stratosphere or a heavily polluted city).

To capture the whole cycle from dark to light—even in the farthest north—the NSF/NCAR C-130 aircraft made seven biweekly runs from Jefferson County Airport near Boulder, at 40°N latitude, to Churchill, Manitoba, and usually on to Thule, Greenland. On each deployment, the plane continued north as far as possible to record the winter-to-spring transition. When the base was Thule, the plane flew as far as 85°N.

The NSF/NCAR C-130 weathered cold and blizzard winds on the runway in Churchill.

On each trip, the C-130 also zigzagged up and down from the lower stratosphere to the boundary layer, sampling air at every level. "We saw a number of interesting and puzzling things," said Atlas. "We saw masses of very, very clean air and also very, very polluted conditions." Although the group had weather and chemical forecasts before each flight, he noted, "the resolution is not as fine as it needs to be, and where you encounter certain chemical conditions is not always where you expected to find them." He anticipates that, with further analysis, "we'll understand whether the chemical features have origins in the stratosphere or come from ground-level sources in Europe, Asia, or maybe from U.S. pollution sources."

Very low altitude flight legs recorded areas of total ozone depletion that averaged about 50 km (30 mi) wide.

Data from the lowest-level flights may help to solve another mystery. Occasionally, surface air in some Arctic regions becomes completely depleted of ozone (see the UCAR Quarterly). The C-130 skimmed as low as 100 feet above Hudson and Baffin Bays and the Arctic Ocean to collect measurements in ozone-depleted areas. In February and April, flights were coordinated with the Polar Sunrise Experiment 2000 in Alert, Canada, which was also studying the issue. From their camp on the Arctic ice, PSE scientists recorded total ozone depletion similar to that observed in TOPSE over wider areas of the Arctic ocean. "When we went back [in mid-May] for the last flight," Atlas said, "We were seeing some evidence of recovery of that ozone depletion, and over other areas where we had seen widespread depletion, it was pretty much gone." Scientists don't yet know what brings on these episodes. With the TOPSE data, though, "We have a lot more information to look at than had been obtained before, and certainly over a larger region."

During the experiment, the ACD global and regional modeling groups ran chemical models driven by observed meteorology in near–real time to assist in the analysis of the TOPSE observations. Peter Hess and Andrzej Klonecki ran the regional model, while Xue-Xi Tie and Louisa Emmons compared results from the Model of Ozone and Related Trace Species (MOZART) with chemical and meteorological data from the TOPSE flights. "It's the first time that we could run the model at the same time that the measurements were being made," said Emmons. "The overall comparison between the observations and model results for many species has been good. However, some species show systematic differences, and we are investigating the causes so we can improve the model."

The experiment was also the first chance modelers had to run MOZART with observed winds that went along with observed chemistry, Emmons explained. "We've done quite a bit of comparisons of models and data, but always before we were able to say, 'Well, the model was run using climatological winds, and the real meteorology was probably different.' This time we didn't have that excuse." Additionally, they looked at the results in much more detail than usual. A chemical model creates enormous quantities of data, so it's common to examine only monthly or even seasonal averages. In TOPSE, modelers saved results every three hours. With this closer view, the modelers found that MOZART generates quite a lot of variability, "which is probably realistic," according to Emmons.

Although the Arctic cold was part of what the TOPSE scientists were studying, it was hard on them and their equipment. Before the experiment, Atlas recalled, "Scientifically speaking, we weren't that concerned with the weather; we were more concerned with the way the temperature and light conditions would affect the chemistry. We really didn't think about how bad the weather would be until we encountered it." And they encountered plenty of it, especially in the earlier legs. In February, the plane was grounded in Churchill by a blizzard with winds up to 100 kilometers per hour (60 miles per hour) and temperatures down to –30°C (–22°F). With no airplane hangar, these conditions meant that staff had to run heaters inside the cabin to protect the instruments—and took turns staying up all night to tend the heaters. After that, "When you get up to freezing, you think it's shirtsleeve weather."

All TOPSE results—both observations and model calculations—are still preliminary. Now, Atlas explained, "We'll look at the [results] for a while as people go back to their labs and computers and try to get final numbers and conditions." The data require final calibrations and further analysis, and the models will be run with reanalyzed dynamics and some improved processes. Around the end of September, the researchers hope to have a final data set, which will be linked to the TOPSE Web site. After that, said co-PI Chris Cantrell, "The hard work should lead to rich scientific rewards over the next few years."

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Fri Sep 1 16:44:56 MDT 2000