Each August in southern Africa, literally thousands of people equipped with lighters or torches go out into the African savanna, a region dotted with villages and teaming with animals, and intentionally set the dry grasslands ablaze. The fires typically scorch an area the size of Montana, Wyoming, Idaho, and the Dakotas combined. Long plumes of smoke rise across the continent like hundreds of billowing smokestacks, and herds of animals are sent scurrying across open grassland.
But unlike in the United States or Europe, where such burning would cause a national panic, it is a welcome occurrence in Africa during the late dry season. In fact, together with fires touched off by lightning, those started by people are a necessary part of the ecosystem on the savanna. Nutrients are released from the dead plants and returned to the soil, and the land is cleared for new grass to grow the following year. Ranch animals and wild animals from antelope to oxen to zebra depend on the restoration of the land and the renewed growth for food.
At the same time the savannas are being cleared, seasonal cleanup is taking place in villages and cities of southern Africa. Locals set fire to small household plots and systematically clear roadside debris with fire. Charcoal for the coming year’s cooking needs is manufactured using a smoldering process.
Though all these activities are vital to life in Africa, they have a downside in the form of pollution. During fire season, a thick pall of smoke chokes the southern Africa skies for weeks each year. The smoke is laced with a number of gases such as nitrogen oxides, carbon monoxide, and hydrocarbons. These gases react with the intense heat and sunlight to form ozone. Unlike high-flying stratospheric ozone, ground level, tropospheric ozone can lead to a number of respiratory diseases, destroy lung tissue, and cause serious damage to crops. When present in the middle to upper troposphere, the molecules trap thermal radiation emanating from the Earth in the same way as carbon dioxide and other greenhouse gases do. In fact, up to 20 percent of the warming experienced by the Earth over the past 150 years is thought to be from ozone.
In September 2000, Anne Thompson, a researcher at NASA’s Goddard Space Flight Center, flew to southern Africa to measure the ozone and help improve scientists’ abilities to extract low-level ozone measurements from instruments located in space. Not only did she find ozone levels to be higher than anticipated, but she also discovered further evidence of unusual wind currents above Africa that scatter the pollution well beyond Africa’s boundaries.
Sampling the Smog
"We wanted to get smack into a place that’s on fire and take measurements,” says Thompson. She chose Lusaka, Zambia, a town of about 1.5 million stationed in the center of the burning, roughly 1,500 miles (2,400 kilometers) northeast of the Cape of Good Hope. The work was conducted under the umbrella of the Southern African Regional Science Initiative (SAFARI 2000). One of SAFARI’s two goals was to refine scientists’ ability to track ozone, aerosols, land cover types, and other features of the land and the atmosphere from space. The other goal was to study the impact of human activities on the atmosphere and ecosystem of southern Africa.
Thompson and her team, made up of herself, one Canadian, and one South African, set up their laboratory on the grounds of the Zambian Meteorological Department (ZMD). From September 6 to 11, 2000, the researchers acquired measurements of ozone from the Earth’s surface to many kilometers above sea level. They launched a rapid series of balloons carrying a lightweight ozone-measuring device called an “ozonesonde” along with a standard radiosonde, which gathers pressure, humidity and temperature readings. As the balloons climbed through the troposphere over a period of about an hour and a half, the instruments radioed measurements back to Thompson’s team. Typically, the balloons expanded to twenty times their original size before exploding. The instruments would then parachute to Earth.
Most days, winds were so light that the balloons did not drift far before popping, and surprised Lusakans would call local authorities to report that strange objects had parachuted into their gardens. ZMD issued rewards for the return of the instruments. One sonde, however, was sent in by the ministry that oversees national security. “The battery for the ozone pump was still operating and the finder thought it was a bomb,” Thompson explains. “The ZMD was quick to reassure the authorities.”
Though Thompson expected the ozone measurements to be elevated due to the fires, the readings she received were surprisingly high. In the absence of burning, ozone levels at ground level in Lusaka are typically less than 20 parts per billion by volume (ppbv). In other words, if one gathered one billion molecules from the area when the savanna was not burning, 20 would be ozone. During most of the days Thomson took readings, however, ground level ozone reached over 90 ppbv. They were the highest levels measured on the ground in southern Africa. “Not only were ozone levels much greater than we expected,” Thompson states, “it appeared that one third to half the ground level ozone came from the various local burning practices, trash fires, charcoal making, and thousands of poorly maintained cars.”
In the United States, ozone is considered to be Code Red if ozone levels are above 90 ppbv on average for eight hours, as they were in Africa. While U.S. cities such as Washington, DC, are chronically over the limit in the summer months, the Washington, DC, metropolitan area, which is four times larger than Lusaka, has many more cars per capita. At the same time, Thompson’s team detected smoke and aerosol levels over Lusaka worse than anything seen in the United States. The constant haze over the city causes chronic irritation to the eyes and throat. “This should be a big health concern,” says Thompson.
Perhaps even more surprising was that as high as the ozone levels were on the ground, they were even higher further up in the atmosphere. On any given day, ozone levels would remain relatively stable until the balloons reached an altitude of 2 to 3 kilometers (1.2 to 1.9 miles). Here the amount of ozone would shoot up to as high as 120 ppbv. The readings would stay this high until about 5 kilometers (3.1 miles) above sea level before sharply decreasing to lower values up through the remainder of the troposphere.
Thompson compared her Lusaka measurements to similar ones taken at the same time by SAFARI researchers in the larger cities of Nairobi, Kenya, and Pretoria, South Africa. The ozone levels in Lusaka were appreciably higher, especially in the 3 to 5 kilometer (1.9 to 3.1 mile) range. “So even with the places that we’ve watched using the sondes, Lusaka was a standout in pollution,” says Thompson.
The high ground-level ozone readings were easily explained given Lusaka’s position in the heart of the burning. As to the unusual spike in ozone from the 3 to 5 kilometer (1.9 and 3.1 mile) range, Thompson says she had seen this phenomenon before. “In the late 1980s, a NASA Langley researcher, Jack Fishman, noticed an ozone hotspot over the south Atlantic in August and September,” she said. There isn’t much in the way of industry, automobile traffic, or biomass burning in the middle of the ocean, and the high ozone values corresponded with the burning in southern Africa. Researchers came to the conclusion that some type of circulation pattern over Africa must be carrying the ozone out over the water.
In the early 1990s, Thompson traveled to Africa with Fishman to conduct more experiments. Thompson said that their research pointed to the existence of a large circular atmospheric motion over south central Africa. She says, however, they did not have enough evidence. It took SAFARI 2000 to prove the phenomenon existed. The spike in ozone at 3 to 5 kilometers (1.9 to 3.1 miles) above sea level could be explained by what Thompson describes as a gyre.
Centered roughly on northern Botswana and central Zambia, this southern African gyre is a large ring of swirling air that passes over Nambia, northern South Africa, southern Mozambique, Zambia, and Angola. The gyre can stay intact for weeks at a time, held in place by temperature inversions at the top and bottom of the ring. Ozone from fires all across southern Africa rise up and mix into the layer of air in the gyre, which can then carry the pollution hundreds of miles. Sometimes parcels of polluted air spin off, streaming out over the Atlantic, the Indian Ocean, and even the Pacific.
“Most of the ozone stays spinning in the inversion layer though,” Thompson explains. “And ozone from fires in western Zambia, Angola, and the other countries on fire slowly builds up. The peak location was around Lusaka.” Though ozone in the gyre is not a health threat, it may have an effect on climate. The gyre could be causing ozone to build up high in the troposphere where the ozone would be a very effective greenhouse gas.
For now, Thompson’s measurements of ozone will be used as part of a program to track low-level ozone from space. During the Lusaka campaign, NASA’s Total Ozone Mapping Spectrometer (TOMS) satellite was also measuring ozone. By comparing the balloon data taken in hazy conditions to satellite data, scientists hope to create new algorithms for extracting tropospheric ozone data from space measurements — a difficult task when one considers the overall thickness of the atmosphere. This information will be useful in coming years as more advanced ozone detectors, such as those aboard NASA’s Aura spacecraft, are launched. “In the end our efforts could help scientists track smog over the entire planet and predict where it is going in the next 24 hours,” says Thompson.
Earth Observatory story by John Weier
