Science News - November 6, 2010

But satellite images clearly show that smoke carried upward inside the clouds emerges from the top as if from a chimney, he notes.

Many high-altitude aerosol plumes detected by satellites in the late 1980s and early 1990s were mistakenly attributed to volcanoes. Take, for instance, a plume that wafted over the central United States in the summer of 1989. Several times during late July and early August, ground-based lasers near Manhattan, Kan., spotted the layer of aerosols in the lower stratosphere. Instruments based near Salt Lake City also recorded the plume. The first analyses of airflow patterns in the lower stratosphere suggested a Central American source. And indeed, Fromm notes, Guatemala’s Santiaguito volcano had erupted on July 19 of that year.

But other data didn’t support a volcanic origin, Fromm and his colleagues reported at the August meeting. For one thing, witness accounts and satellite images suggested that Santiaguito’s plume never rose more than six kilometers into the sky. Also, sensors at the Utah site saw signs that the particles weren’t spherical, as typical sulfate droplets from volcanoes would be. New analyses of weather patterns, including the location and speed of the jet stream at the time, track the aerosol plume to large wildfires sparked by lightning in Manitoba and Saskatchewan on July 17 of that year, Fromm said.

Similarly a thick plume of aerosols detected in the stratosphere over the

An aerosol plume (center) generated
by fires in central Russia in late July
of this year (red denotes fire at ground
level) traveled all the way to Alaska.

Atlantic Ocean north-west of Spain in late June 1991 was mistakenly linked to eruptions of the Philippines’ Mount Pinatubo earlier that month. Fromm’s team has traced that plume to wildfires in Quebec.

Satellite images from the 2002 fire season in North America suggest just how often pyroCbs may pollute the stratosphere. That year, 17 different plumes of stratospheric aerosols could be linked to massive wildfires, including Colorado’s Hayman Fire and Arizona’s Rodeo-Chediski Fire — the largest fires recorded in the histories of those states.

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Aerosols index patterns during 2002

Daily maximum aerosol index

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Fire cloud Aerosol peak

0May Jun Jul Aug Sep Oct High times for aerosols clouds generated by wildfires during the 2002 fire season have been linked to peaks in the aerosol index over North america. the index is a measure of the concentration of aerosols in all layers of the atmosphere.

Gas injection

In addition to aerosols such as smoke and soot, fire-fueled clouds inject unusually large quantities of chemically reactive gases such as carbon monoxide, methyl cyanide and hydrogen cyanide into upper layers of the atmosphere, data suggest. Scientists are somewhat concerned about the ultimate effects of these gases on atmospheric chemistry and the planet’s climate.

“These clouds are an important route to the stratosphere for smoke, soot and other pollutants,” Wang says.

The thickest plume of aerosols lofted from a pyroCb in recent years originated with fires that ravaged southeastern Australia and killed hundreds of people on February 7, 2009 — a day known as Black Saturday. These fires, fueled by high temperatures, low humidity and strong winds, slung an unprecedented amount of pollutants into the stratosphere, says Nathaniel Livesey, an atmospheric scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Satellite data hint that carbon monox-
ide concentrations in some parts of the
plume in the lower stratosphere reached
800 parts per billion, 10 to 15 times that
in normal air, he reported at the August
meeting. Preliminary analyses suggest
that concentrations of methyl cyanide
and hydrogen cyanide were between
2. 5 to 7 times normal, says atmospheric
physicist Hugh Pumphrey of the Uni-
versity of Edinburgh, who worked with
Livesey on the study.