Measurements and modelling of molecular iodine emissions, transport and photodestruction in the coastal region around Roscoff
Iodine emissions from the dominant six macroalgal species in the coastal regions around Roscoff, France, have been modelled to support the Reactive Halogens in the Marine Boundary Layer Experiment (RHaMBLe) undertaken in September 2006. A two-dimensional model is used to explore the relationship bet...
| Main Authors: | , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2010-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/10/11823/2010/acp-10-11823-2010.pdf |
| Summary: | Iodine emissions from the dominant six macroalgal species in the coastal
regions around Roscoff, France, have been modelled to support the Reactive
Halogens in the Marine Boundary Layer Experiment (RHaMBLe) undertaken in
September 2006. A two-dimensional model is used to explore the relationship
between geographically resolved regional emissions (based on maps of seaweed
beds in the area and seaweed I<sub>2</sub> emission rates previously measured in
the laboratory) and in situ point and line measurements of I<sub>2</sub> performed
respectively by a broadband cavity ringdown spectroscopy (BBCRDS) instrument
sited on the shoreline and a long-path differential optical absorption
spectroscopy (LP-DOAS) instrument sampling over an extended light path to an
off-shore island. The modelled point and line I<sub>2</sub> concentrations compare
quantitatively with BBCRDS and LP-DOAS measurements, and provide a link
between emission fields and the different measurement geometries used to
quantify atmospheric I<sub>2</sub> concentrations during RHaMBLe. Total I<sub>2</sub>
emissions over the 100 km<sup>2</sup> region around Roscoff are calculated to be
1.7×10<sup>19</sup> molecules per second during the lowest tides.
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During the night, the model replicates I<sub>2</sub> concentrations up to 50 pptv
measured along the LP-DOAS instrument's line of sight, and predicts spikes of
several hundred pptv in certain conditions. Point I<sub>2</sub> concentrations up
to 50 pptv are also calculated at the measurement site, in broad agreement
with the BBCRDS observations. Daytime measured concentrations of I<sub>2</sub> at
the site correlate with modelled production and transport processes. However
substantial recycling of the photodissociated I<sub>2</sub> is required for the
model to quantitatively match measured concentrations. This result
corroborates previous modelling of iodine and NO<sub>x</sub> chemistry in the
semi-polluted marine boundary layer which proposed a mechanism for recycling
I<sub>2</sub> via the formation, transport and subsequent reactions of the
IONO<sub>2</sub> reservoir compound.
<br><br>
The methodology presented in this paper provides a tool for linking spatially
distinct measurements to inhomogeneous and temporally varying emission
fields. |
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| ISSN: | 1680-7316 1680-7324 |