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| Titel |
Pressure-dependent calibration of the OH and HO2 channels of a FAGE HOx instrument using the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) |
| VerfasserIn |
F. A. F. Winiberg, S. C. Smith, I. Bejan, C. A. Brumby, T. Ingham, T. L. Malkin, S. C. Orr, D. E. Heard, P. W. Seakins |
| 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. 2 ; Nr. 8, no. 2 (2015-02-03), S.523-540 |
| Datensatznummer |
250116118
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| Publikation (Nr.) |
 copernicus.org/amt-8-523-2015.pdf |
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| Zusammenfassung |
| The calibration of field instruments used to measure concentrations of OH
and HO2 worldwide has traditionally relied on a single method
utilising the photolysis of water vapour in air in a flow tube at
atmospheric pressure. Here the calibration of two FAGE (fluorescence assay
by gaseous expansion) apparatuses designed for HOx (OH and HO2)
measurements have been investigated as a function of external pressure using
two different laser systems. The conventional method of generating known
concentrations of HOx from H2O vapour photolysis in a turbulent
flow tube impinging just outside the FAGE sample inlet has been used to study
instrument sensitivity as a function of internal fluorescence cell pressure
(1.8–3.8 mbar). An increase in the calibration constants CHO and
CHO2 with pressure was observed, and an empirical
linear regression of the data was used to describe the trends, with
ΔCHO = (17 ± 11) % and ΔCHO2 = (31.6 ± 4.4)% increase per
millibar air (uncertainties quoted to 2σ). Presented here are the first
direct measurements of the FAGE calibration constants as a function of
external pressure (440–1000 mbar) in a controlled environment using the
University of Leeds HIRAC chamber (Highly Instrumented Reactor for
Atmospheric Chemistry). Two methods were used: the temporal decay of
hydrocarbons for calibration of OH, and the kinetics of the second-order
recombination of HO2 for HO2 calibrations. Over comparable
conditions for the FAGE cell, the two alternative methods are in good
agreement with the conventional method, with the average ratio of
calibration factors (conventional : alternative) across the entire pressure
range, COH(conv)/COH(alt) = 1.19 ± 0.26 and
CHO2(conv)/CHO2(alt) = 0.96 ± 0.18
(2σ). These alternative calibration methods currently have comparable
systematic uncertainties to the conventional method: ~ 28% and ~ 41% for the alternative OH and HO2
calibration methods respectively compared to 35% for the H2O vapour
photolysis method; ways in which these can be reduced in the future are
discussed. The good agreement between the very different methods of
calibration leads to increased confidence in HOx field measurements and
particularly in aircraft-based HOx measurements, where there are
substantial variations in external pressure, and assumptions are made
regarding loss rates on inlets as a function of pressure. |
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