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| Titel |
Global Modelling of the total OH reactivity: validation against measurements and atmospheric implications of the ‘missing’ sink |
| VerfasserIn |
Valerio Ferracci, Alexander T. Archibald, John A. Pyle |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138331
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| Publikation (Nr.) |
 EGU/EGU2017-1309.pdf |
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| Zusammenfassung |
| The removal of most trace gases emitted into the atmosphere is primarily initiated by reaction
with the hydroxyl radical, OH. A number of field campaigns over the last two decades have
observed the presence of a “missing” sink of the OH radical in a variety of regions across the
planet, from urban areas to remote forests: comparison of the direct measurements of the OH
loss rate, also known as total OH reactivity, with the sum of individual known OH sinks
(obtained via the simultaneous detection of species such as volatile organic compounds and
nitrogen oxides) indicated that, in some cases, up to 80% of the total OH loss rate was
unaccounted for. The implications of this finding are significant, as a potentially major OH
sink operating in the atmosphere is not currently accounted for in atmospheric models: the
presence of an additional OH sink might, for instance, lead to an increase in the atmospheric
lifetime of a number of trace species, including high-impact greenhouse gases such as
methane.
The only modelling of the total OH reactivity is currently performed on a regional scale; a
thorough assessment of the impact of the missing sink on the chemistry and climate of the
planet by global modelling is therefore highly desirable. In this work a chemistry-climate
model (the Met Office’s Unified Model with the United Kingdom Chemistry and
Aerosols scheme, UM-UKCA) was used to calculate the total OH reactivity at
the planetary boundary layer. The model output was validated against available
field measurements to verify that the known OH sinks observed in the field were
reproduced correctly by the model: a good agreement was found between the data from
more than 30 field campaigns and the model output. Following this, the effects of
introducing novel OH sinks in the chemistry scheme were investigated. The first step
was the introduction in the model of the newly characterised reactions of peroxy
radicals (RO2) with OH, the kinetics and products of which have only recently been
studied in the laboratory. Results from the UM-UKCA model show that reaction with
RO2 might represent a non-negligible OH sink in remote environments, but cannot
reconcile field measurements of the total OH reactivity with the sum of the individual
sinks. To address this, an unspecified additional sink was added to the model in a
series of simulations reproducing different scenarios (e.g., different OH recycling
probabilities through the oxidation of the additional sink) with a view to establishing the
impact of the additional OH sink on the oxidative capacity of the lower atmosphere. |
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