Smoking trees

CLIMATE SCIENCES: aerosols (Credit: SURYA SEN / CSE)WHILE we are yet to ascertain whether
aerosols are warming or cooling our
planet, a scientific team has traced a new
source: deciduous trees. These are
plants that shed their leaves seasonally.
So far aerosols were described as particles
of pollutants like sulphur dioxide,
black carbon (soot) and sea salt that
remain suspended in the air.

Deciduous plants release around
500 teragrammes (1 teragramme equals
1012 grammes) of carbon each year in
the form of an organic compound called
isoprene (hydrocarbon). The chemistry
of what happens to the compound as it
forms aerosol particles has, so far, been
unclear.

The team from CalTech in the US,
University of Otago in the New Zealand
and University of Copenhagen, in
Denmark, found isoprene is repeatedly
oxidized in the atmosphere to form
an epoxide, hitherto unknown, called
dihydroxyepoxide.

Epoxides are generally synthesized
in the chemicals industry. It is rare to
find such huge quantities of an epoxide,
produced naturally by plants. "Nothing
is known about its fate in the atmosphere.
Given the tendency of this epoxide
to stick to acidic particles, it is likely
to form aerosols under pristine conditions,"
said Fabien Paulot of CalTech.

Paul Weinberg, atmospheric science
professor from the same insitute, said:
"If you mix emissions from the city with
emissions from plants, they interact to
alter the chemistry of the atmosphere."
Higher the concentration of
aerosols, human-induced or otherwise,
foggier is the visibility in that area.
Tadeusz Kleindienst, scientist at the US
Environmental Protection Agency, said
that air quality regulatory agencies
mainly pay attention to the effect of
emissions from cars and industries; less
is known about biogenic emissions.
Hence their visibility predictions as well
as other climatic predictions are not that
accurate.

This study published in the August 8
issue of Science explains from where the
Great Smoky Mountains derive their
name: the mountains are covered by
one of the largest patches of deciduous
forests in North America.