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| Titel |
Interpretation of In-Situ Measurements of Iodine Monoxide in Coastal Regions Using Laser-Induced Fluorescence Spectroscopy |
| VerfasserIn |
K. L. Furneaux, L. K. Whalley, D. E. Heard |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026107
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| Zusammenfassung |
| Iodine species are present in coastal and open ocean regions due to the release of I2 and
iodocarbons from macro and micro algae. The photolysis of these molecules yields iodine
atoms, which react with ozone to produce iodine monoxide (IO). IO is involved in ozone
depletion cycles, the partitioning of HOx and NOx, and the formation and growth of new
particles.
A novel point source Laser Induced Fluorescence (LIF) instrument was deployed to
measure IO in September 2006 at Roscoff, France as part of the Reactive Halogens in the
Marine Boundary Layer (RHaMBLe) programme (1Ï instrument uncertainty = 23%)1. The
maximum IO mixing ratio was 30 ± 7.1 pptV (10 s integration period, limit of detection =
1.4 pptV) at this semi-polluted coastal site (NOx levels = 1 – 5 ppbV). The closest
macroalgae beds known to strongly emit I2 (laminaria) were ~ 300 m from the
LIF instrument. IO displayed a strong anti-correlation with tidal height which is
consistent with previous studies. IO was also dependent on solar irradiation and
meteorological conditions. The dominant source of IO at this site was the photolysis of
I2.
The measurements provided by this instrument aim to address the main uncertainties
associated with iodine chemistry. Co-ordinated measurement of IO by point source (LIF) and
spatially averaged (Long Path Differential Optical Absorption Spectroscopy) instruments
confirm the presence of IO hotspots due to non-uniform macroalgae distribution at this
location (resulting in a spatially variable I2 source). The ratio of point source/spatially
averaged IO is determined by meteorological conditions and distance of the instrument from
macroalgae beds. Co-located point source I2 (Broadband Cavity Ringdown Spectroscopy)
and IO (LIF) measurements correlated on some days but cannot be explained by our current
knowledge of iodine chemistry.
The influence of NOx on IO has been investigated. The detection of IO by LIF at the
Roscoff site shows that IO can survive in a high NOx environment, at a distance from the
iodine source region. The LIF measurements display an anti-correlation with NOx. A
modelling study shows that this is due to an iodine recycling scheme via IONO2. The effect
of NO2 is to initially lead to IO suppression (displayed by an anti-correlation of IO with
NOx) and then to prolong the atmospheric lifetime of IO through decomposition of
IONO2.
Night-time IO was detected on two out of four nights at levels of 1 – 2 pptV, providing
further evidence for the presence of night-time IO. The night-time source of I atoms is
proposed to be via I2 + NO3 - I + INO3, 2,3.
Co-located point source measurements of IO and particle number provide clear evidence
for the link between IO and new particle formation. Over 18 days of simultaneous
measurements, IO and particle number displayed a linear relationship. On all occasions, IO
levels increased before an increase in particle number was observed.
The influence of IO on the important daytime oxidants OH and HO2 has been investigated
by modelling studies. IO was found to decrease the HO2/OH ratio due conversion of HO2 to
OH via (HO2 + IO - HOI, HOI + hν - OH + I). The high levels of IO detected at Roscoff
can result in an enhancement of OH at up to 50%. However, the impact of IO on HOx
decreases as NOx levels increase, until IO actually results in a decrease in OH (NOx >
1.55 ppbV).
Whalley et. al. (2007), J. Atmos. Chem. 58: 19 – 39.
Chambers, R. M. et al (1992), Journal of Physical Chemistry, 96, 3321–3331.
Saiz-Lopez, A. and J. M. C. Plane (2004), GRL, 31, 19215–19218. |
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