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10.5194/acpd-8-1159-2008
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http://www.atmos-chem-phys-discuss.net/8/1159/2008/acpd-8-1159-2008.pdf
1159
1190
2008-01-23
Halogenated organic species over the tropical rainforest
S. Gebhardt
A. Colomb
R. Hofmann
J. Williams
williams@mpch-mainz.mpg.de
J. Lelieveld
Max Planck Institute for Chemistry, Mainz, Germany
Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), Paris, France
Airborne measurements of the halogenated trace gases methyl chloride, methyl
bromide and chloroform were conducted over the Atlantic Ocean and 1000 km of
pristine tropical rainforest in Suriname and French Guyana (3–6° N,
51–59° W) in October 2005. In the boundary layer (0–1.4 km), maritime air
masses initially low in forest hydrocarbons, advected over the forest by
southeasterly trade winds, were measured at various distances from the
coast. Since the organohalogens presented here have relatively long
atmospheric lifetimes (0.4–1.0 years) in comparison to the transport times
(1–2 days), emissions will accumulate in air traversing the rainforest. The
distributions of methyl chloride, methyl bromide and chloroform were
analyzed as a function of forest contact time and the respective
relationship used to determine fluxes from the rainforest during the long
dry season.
<br></br>
Emission fluxes have been calculated for methyl chloride and chloroform as
9.4 (±4.0 2σ) and 0.34 (0.14± 2σ) μg m<sup>−2</sup> h<sup>−1</sup>, respectively. No significant flux from the rainforest
was observed for methyl bromide within the limits of these measurements. The
flux of methyl chloride was in general agreement with the flux measured over
the same region in March 1998 during the LBA Claire project using a
different analytical system. This confirms that the rainforest is a strong
source for methyl chloride and suggests that this emission is relatively
uniform throughout the year. In contrast the chloroform flux derived here is
a factor of three less than previous measurements made in March 1998
suggesting a pronounced ecosystem variation. The differences in chloroform
fluxes could not be attributed to either temperature or rainfall changes.
The global extrapolation of the derived fluxes led to 1.5 (±0.6 2σ) Tg yr<sup>−1</sup> for methyl chloride, which is in the range of the
missing source postulated by previous model studies and 55 (±22 2σ) Gg yr<sup>−1</sup> for chloroform.
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