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.
Aerosols index patterns during 2002
Daily maximum aerosol index
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.
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.
s mike fromm explains pyrocbs: http://
November 6, 2010 | science news | 29