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| Titel |
Reporting the sensitivity of laser-induced fluorescence instruments used for HO2 detection to an interference from RO2 radicals and introducing a novel approach that enables HO2 and certain RO2 types to be selectively measured |
| VerfasserIn |
L. K. Whalley, M. A. Blitz, M. Desservettaz, P. W. Seakins, D. E. Heard |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 6, no. 12 ; Nr. 6, no. 12 (2013-12-09), S.3425-3440 |
| Datensatznummer |
250085129
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| Publikation (Nr.) |
 copernicus.org/amt-6-3425-2013.pdf |
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| Zusammenfassung |
Laboratory studies have revealed that alkene-derived RO2 and
longer chain alkane-derived RO2 (> C3) radicals rapidly
convert to HO2 and then to OH in the presence of NO in a fluorescence
assay by gas expansion (FAGE) detection cell (Fuchs et al., 2011). Three
different FAGE cells that have been used to make ambient measurements of OH
and HO2 in the University of Leeds ground-based instrument have been
assessed to determine the sensitivity of each cell, when operating in
HO2 detection mode, to RO2 radicals. The sensitivity to
this interference was found to be highly dependent on cell design and
operating parameters. Under the operating conditions employed, during
fieldwork undertaken in the Borneo rainforest in 2008, an OH yield of 17%
was experimentally determined for both ethene- and isoprene-derived RO2
radicals. The high pumping capacity of this system, resulting in a short
residence time in the cell, coupled with poor mixing of NO into the ambient
air-stream for the titration of HO2 to OH effectively minimised this
potential interference. An OH yield of 46% was observed for
ethene-derived RO2 radicals when a smaller detection cell was used, in
which the mixing of NO into the ambient air was improved and the cell
residence times were much longer. For a newly developed ROxLIF cell,
used for detection of HO2 and RO2 radicals an OH yield of 95% was observed for ethene-derived RO2
radicals, when running in
HO2 mode.
In experiments in which conditions ensured the conversion of RO2 to OH
were complete, the yields of OH from a range of different RO2 species
agreed well with model predictions based on the Master Chemical Mechanism
version 3.2. For ethene and isoprene-derived RO2 species, the relative
sensitivity of FAGE was found to be close to that for HO2, with an OH
yield of 100% and 92%, respectively. For the longer chain or cyclic
alkane-derived RO2 radicals (> C3), model predicted OH
yields were highly dependent upon temperature. A model predicted OH yield of
74% at 298 K and 36% at 255 K were calculated for cyclohexane-derived
RO2 radicals, and an experimental yield of 38% was observed
indicating that the temperature within the cell was below ambient owing to
the supersonic expansion of the airstream in the low pressure cell.
These findings suggest that observations of HO2 by some LIF instruments
worldwide may be higher than the true value if the instruments were sensitive
to these RO2 species. If this is the case, it becomes necessary to
compare atmospheric chemistry model simulations to HO2* observations,
where HO2* = [HO2] + Σi αi [RO2i], and
αi is the mean fractional contribution of the RO2 species that
interfere (RO2i). This methodology, however, relies on model simulations
of speciated RO2 radicals, as instrumentation to make speciated RO2
measurements does not currently exist. Here we present an approach that
enables the concentration of HO2 and RO2i to be selectively
determined by varying the concentration of NO injected into a FAGE cell.
Measurements of [HO2] and [RO2i] taken in London are presented. |
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