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| Titel |
Measurements of OH and HO2 Radicals and OH Reactivity at Tropical Locations Using Laser-Induced Fluorescence Spectroscopy |
| VerfasserIn |
K. L. Furneaux, L. K. Whalley, P. Edwards, A. Goddard, T. Ingham, M. J. Evans, 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 |
250019777
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| Zusammenfassung |
| The OH radical is the dominant daytime oxidant in the atmosphere. Together with the closely
coupled HO2 radical, these two species (termed HOx) play an important role in determining
the composition of the atmosphere. Tropical latitudes are active regions of atmospheric
chemistry due to high solar radiation, humidity and temperature. For these reasons, field
measurements of HOx in the tropics are crucial to improve understanding of atmospheric
chemistry through model – measurement comparisons. Due to the low number of HOx
measurements in the tropics, these comparisons are sparse. An aircraft campaign over the
pristine Amazon rainforest found HOx concentrations to be high1,2. It has been proposed that
this is due to a previously overlooked OH recycling mechanism via the oxidation of
isoprene1,2. The need to determine if this is ubiquitous across tropical rainforest regions is
necessary.
The Leeds FAGE instrument was deployed at the Bukit Atur Global Atmospheric Watch
Station, Borneo (5.0N, 117.8E) from April – July 2008 as part of the OP3 project
(Oxidant and Particle Photochemical Processes above a South-East Asian tropical rainforest)
to measure OH and HO2 concentrations and the OH chemical lifetime by Fluorescence Assay
by Gas Expansion (FAGE). These measurements represent the first ground based [HOx]
measurements in a tropical rainforest. Chemical activity differed significantly throughout the
measurement period. HOx concentrations were elevated in July (average peak [OH] = 5.3
×106 molecule cm-3) compared to April (average peak [OH] = 2.5 ×106 molecule cm-3),
attributed to higher OH sinks in April. Measurements of the OH chemical lifetime can
be used to quantify unknown OH sinks. The OH chemical lifetime displayed a
diurnal cycle that correlated with isoprene concentrations. At this site isoprene
represents the major OH loss route but there are significant unknown fractions.
Model calculations result in an under prediction of HOx when measured sinks are
included, indicating a missing HOx source. Both OH and HO2 were observed at
night.
Measurements of HOx at the Cape Verde Atmospheric Observatory (16.9N, 24.9W)
were made from May – June 2007 as part of the RHaMBLe (Reactive Halogens in the Marine
Boundary Layer) programme. The site is located adjacent to the ocean with an absence of
macro algae, providing conditions analogous to open ocean, clean marine air. However,
background tropical conditions were not dictated by simple chemistry. Peak OH and HO2
concentrations were 9 × 106 molecule cm-3 and 6 ×108 molecule cm-3, respectively. HO2
was observed at night between 5 – 20 × 106 molecule cm-3. Modelling studies
determined oxygenated-VOCs and halogen chemistry to play an important role in HOx
chemistry.
A comparison of HOx measurements at tropical open ocean and tropical rainforest
locations shows that HOx chemistry varies greatly throughout the tropics. Higher HOx sinks
in tropical rainforest environments result in a decrease of HOx compared to the tropical open
ocean.
1. Lelieveld, J., T. M. Butler, et al. (2008). Atmospheric oxidation capacity sustained by a
tropical forest, Nature, 452(7188): 737-740.
2. Martinez, M., Harder, H., et al. (2008). Hydroxyl radicals in the tropical troposphere
over the Suriname rainforest: airborne measurements, ACPD, 8, 15491-15536. |
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