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| Titel |
HOx radical chemistry in oxidation flow reactors with low-pressure mercury lamps systematically examined by modeling |
| VerfasserIn |
Z. Peng, D. A. Day, H. Stärk, R. Li, J. Lee-Taylor, B. B. Palm, W. H. Brune, J. L. Jimenez |
| 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 ; 8, no. 11 ; Nr. 8, no. 11 (2015-11-20), S.4863-4890 |
| Datensatznummer |
250116690
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| Publikation (Nr.) |
 copernicus.org/amt-8-4863-2015.pdf |
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| Zusammenfassung |
| Oxidation flow reactors (OFRs) using OH produced from
low-pressure Hg lamps at 254 nm (OFR254) or both 185 and 254 nm (OFR185) are
commonly used in atmospheric chemistry and other fields. OFR254 requires the
addition of externally formed O3 since OH is formed from O3
photolysis, while OFR185 does not since O2 can be photolyzed to produce
O3, and OH can also be formed from H2O photolysis. In this study,
we use a plug-flow kinetic model to investigate OFR properties under a very
wide range of conditions applicable to both field and laboratory studies. We
show that the radical chemistry in OFRs can be characterized as a function
of UV light intensity, H2O concentration, and total external OH
reactivity (OHRext, e.g., from volatile organic compounds (VOCs), NOx, and SO2). OH
exposure is decreased by added external OH reactivity. OFR185 is especially
sensitive to this effect at low UV intensity due to low primary OH
production. OFR254 can be more resilient against OH suppression at high
injected O3 (e.g., 70 ppm), as a larger primary OH source from O3,
as well as enhanced recycling of HO2 to OH, make external perturbations
to the radical chemistry less significant. However if the external OH
reactivity in OFR254 is much larger than OH reactivity from injected
O3, OH suppression can reach 2 orders of magnitude. For a typical
input of 7 ppm O3 (OHRO3 = 10 s−1), 10-fold OH suppression
is observed at OHRext ~ 100 s−1, which is similar or
lower than used in many laboratory studies. The range of modeled OH
suppression for literature experiments is consistent with the measured
values except for those with isoprene. The finding on OH suppression may
have important implications for the interpretation of past laboratory
studies, as applying OHexp measurements acquired under different
conditions could lead to over a 1-order-of-magnitude error in the estimated
OHexp. The uncertainties of key model outputs due to uncertainty in all
rate constants and absorption cross-sections in the model are within ±25 % for OH exposure and within ±60 % for other parameters. These
uncertainties are small relative to the dynamic range of outputs.
Uncertainty analysis shows that most of the uncertainty is contributed by
photolysis rates of O3, O2, and H2O and reactions of OH and
HO2 with themselves or with some abundant species, i.e., O3 and
H2O2. OHexp calculated from direct integration and estimated
from SO2 decay in the model with laminar and measured residence time
distributions (RTDs) are generally within a factor of 2 from the plug-flow
OHexp. However, in the models with RTDs, OHexp estimated from
SO2 is systematically lower than directly integrated OHexp in the
case of significant SO2 consumption. We thus recommended using
OHexp estimated from the decay of the species under study when
possible, to obtain the most appropriate information on photochemical aging
in the OFR. Using HOx-recycling vs. destructive external OH reactivity
only leads to small changes in OHexp under most conditions. Changing
the identity (rate constant) of external OH reactants can result in
substantial changes in OHexp due to different reductions in OH
suppression as the reactant is consumed. We also report two equations for
estimating OH exposure in OFR254. We find that the equation estimating
OHexp from measured O3 consumption performs better than an
alternative equation that does not use it, and thus recommend measuring both
input and output O3 concentrations in OFR254 experiments. This study
contributes to establishing a firm and systematic understanding of the
gas-phase HOx and Ox chemistry in these reactors, and enables
better experiment planning and interpretation as well as improved design of
future reactors. |
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