|The Stratospheric/Tropospheric Measurements Group is one of the observing teams within ACD that's been deluged by field assignments this summer and fall. In front: Audrey Specht. In rear, left to right: Frank Flocke, Sue Schauffler, Stephen Donnelly, Kristen Johnson, Elliot Atlas, Rich Lueb, Verity Stroud, and Maxime Claire. Not shown: Rachel Weaver. (Photo by Carlye Calvin.)|
Field projects are lined up like boxcars on a train for the Atmospheric Chemistry Division. Fortunately, everything is on track.
ACD's biggest-ever experiment, TOPSE (see sidebar), starts this coming February. Already, though, the demand on ACD's measurement groups is at record level. The pressure isn't confined just to the scientists and technicians. "It's busier than it's ever been," says Marilena Stone, the administrator for several ACD groups. Simply keeping up with travel arrangements has been a major task, she says.
|This map of potential vorticity shows an extreme example of Arctic air being carried into the mid-latitudes while mid-latitude air is stirred into the middle of the polar vortex. The frequency and amount of this exchange is key to one of SOLVE's goals: understanding the evolution of polar ozone levels. (Image courtesy NASA.)|
No one knows exactly why stratospheric ozone gradually decreases during the winter. This happens well before spring sunlight kick-starts the chlorine-based chemistry that causes the Antarctic's dramatic ozone hole (and more modest depletion elsewhere). Current models cannot account for the gradual wintertime drops. Data from SOLVE will help clarify the processes involved. The data also will be used to corroborate a soon-to-be-launched satellite for the Stratospheric Aerosol and Gas Experiment. SAGE is one of the three satellite systems that provide real-time ozone monitoring across the globe.
NASA's ER-2 and DC-8 research aircraft will fly out of Kiruna for SOLVE, carrying instruments from NCAR and other labs and complementing a balloon-based payload. The stratosphere above Kiruna is, on average, as cold as it gets in the Northern Hemisphere. A typical January temperature at 20 kilometers (12 miles) is about -74°C (-101°F). This extreme cold helps to generate polar stratospheric clouds (PSCs), which are essential in helping chlorine damage the ozone layer. However, the polar vortex that envelops the extreme cold behaves differently around the North Pole versus the South. The Arctic vortex tends to break up in spots and mix with surrounding air during the winter. In the Antarctic, the vortex often remains intact long enough for major ozone depletion the following spring.
Mountain ranges that line the polar edge of northern continents may contribute to the breakup of the Arctic vortex. But scientists also suspect that the ascent forced by these mountains creates isolated cold pockets where PSCs could form. SOLVE will be looking for and examining such pockets.
Aboard the ER-2 for SOLVE will be ATD's multiple-angle aerosol spectrometer probe, which locates and characterizes particles that can serve as cloud nuclei. Also on hand will be ACD's whole air sampler (WAS), which is getting a real workout this year. Just as new earth-based instruments need ground truth to verify their accuracy, satellites and other space-based sensors need "air truth" so researchers know what they're detecting. WAS helps fill the niche: it's the only device that collects air samples from aboard the ER-2 for later analysis on the ground. The WAS canisters, each holding about 4.5 standard liters of air, are express-shipped in special crates to ACD and then put through their paces using three different gas chromatographs.
"We've never had the aircraft instruments out on two simultaneous deployments before," says Stephen. One unit was on the ER-2 for early SOLVE activities while a second was traversing the subtropics of North America aboard the WB-57, sampling the air left behind by aircraft and rockets. That task was part of another NASA study, ACCENT (Atmospheric Chemistry of Combustion Emissions Near the Tropopause).
Soot and other particles emitted by aircraft aren't thought to have a large direct impact on the upper troposphere and lower stratosphere. However, they could have more subtle, indirect effects. For instance, they may tweak the planet's radiative balance by affecting cloud formation. On a more local scale, rocket plumes inject a concentrated mix of chemicals, including chlorine and nitrogen oxides, into the troposphere-stratosphere interface. These chemicals can have a significant impact on local ozone levels. Another ACCENT goal was to examine the distribution of short-lived organic halogen compounds--especially those containing bromine--in the tropics' upper troposphere. "Bromine really needs to be looked at pretty carefully," says Sue Schauffler, part of the WAS team. "Even a small amount of bromine can have a significant effect on ozone, especially in the lower stratosphere."
ACCENT studied all of these emissions by flying in and near rocket plumes, by sampling the air inside and outside the favored commercial flight tracks across North America, and by flying to the upper tropical troposphere.
A snapshot of spring over high latitudesMany of the threads running through this year's ACD projects are being woven into a massive field program early next year, the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment.
TOPSE will involve all of ACD's observational groups and many of its modelers. The division designed a innovative experiment to look at a specific and critical problem: how does the photochemistry of the polar atmosphere "spin up" during the arrival of spring sunlight?
NCAR's C-130 will engage in some Lindbergh-length flights for TOPSE between February and June. The main flight track will run from Denver through Canada to Thule, Greenland, and back, although the plane is likely to do some zigzagging to hit places of meteorological or chemical interest. The project has been allocated 225 hours of flight time, an unusually large amount that should permit flights about once every three weeks.
As they commute from the Rockies to the pole, ACD's scientists will be watching what the sun does to a winter's worth of pollution. The wintertime polar vortex tends to trap pollutants that drift north from Europe, Russia, and North America. Once spring arrives and the sun's photochemical machine is turned on, many of these chemicals are destroyed, and secondary pollutants like ozone reach a peak. However, ozone could also be descending from the stratosphere, contributing to the occurrence of the low-level peak.
According to ACD director Guy Brasseur, "This is the first time that so many universities and government laboratories are joining forces with ACD to try to solve a long-standing enigma. Together, we will be using the most advanced instrumentation to measure how the concentration of tropospheric reactive species changes as the sun returns over the Arctic region." BH
For the 1999 field phase of SOS, the WAS on board the NOAA P-3 collected samples in and near Nashville, which has unusually high ozone levels for a relatively small and compact city. "It's an urban area, but it's fairly isolated--you can get away from the city center and the urban heat island fairly quickly," notes Stephen.
Also for SOS 99, Frank Flocke and Andy Weinheimer brought a new instrument aboard the P-3 to measure peroxyacetyl nitrate (PAN) and two related compounds. PAN-type compounds are a byproduct of photochemical ozone formation, says Frank: "They can provide information on the relative contributions of anthropogenic and biogenic emissions to the ozone problem." The new instrument, a compact gas chromatograph, takes readings every three minutes.
Organic aerosols tend to absorb light rather than reflect it as do more common aerosols, such as sulfates. This throws a different light on the photochemical balance in the South, where organic aerosols may be especially abundant. Studying photochemical pathways is the goal of ACD's Atmospheric Radiation Investigation and Measurements Project (ARIMP). The group analyzes how reaction rates are controlled by environmental variables, such as pressure and temperature, and by the amounts of reactants available. "Light can be thought of as a reactant," notes Teresa Campos.
One of the group's key tools is an instrument developed by project head Rick Shetter in the mid-1990s, the scanning actinic flux spectroradiometer. Like a radiometer, the device measures the radiation impinging on a surface. In this case, the surface is a hemisphere encasing a set of concentric domes that can sense radiation coming from all directions. Like a spectrometer, the device breaks down the radiation by wavelength.
The ARIMP group's SOS 99 campaign, led by Sam Hall, marked one of the first deployments outside the tropics for the actinic flux sensors. The group is looking forward to comparing the data with that from the Indian Ocean Experiment earlier this year, where soot carbon readings were unusually high. ARIMP members are also going to Sweden at the end of the year for SOLVE, where they will help unravel a particular mystery. Hydrogen oxides are normally produced by daytime photochemistry, but for some reason HOx appears to be persisting into the high-latitude night.
The field measurements this winter will be at very low sun angles. Because the arriving sunlight passes through far more atmosphere than usual, the spectral range is altered and the amount of light is reduced. This pushes the abilities of the actinic flux sensor and other instruments. "We'll be operating in very challenging measurement regimes," Teresa says.
Bill Mankin and fellow senior scientist Mike Coffey are coinvestigators for this piece of the Network for Detection of Stratospheric Change. A worldwide set of institutions is placing ground-based sensors across five latitude bands straddling the globe. It's a unified attempt to build a useful data base on stratospheric chemistry and how that chemistry's changing. Once complete, the NDSC will be the only global network keeping regular watch on the stratosphere, especially ozone, from the ground. The devices will also help calibrate satellites, an important secondary goal, says Bill.
"If you've got a satellite, it gives you a nice global picture of the atmosphere, but if something changes, you don't know if it's the satellite or the atmosphere that's changing. Our thesis is that by having both of those networks [satellite and ground-based], you can do a better job."