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Field instrument development and calibration |
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The first pressure dependent calibrations of a FAGE HOx instrument |
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HIRAC has been used to complete the first calibration of a FAGE HOx instrument over a range of external pressures simulating
the conditions experienced during aircraft measurements. Atmospheric pressure changes with altitude resulting
in a concurrent change in the internal FAGE detection cell pressure and this can have significant affects on the instrument sensitivity to
OH and HO2. Currently the pressure dependence of FAGE instruments is determined by changing the size of the FAGE inlet pinhole
(to induce the range of internal detection cell pressures experiences during a flight) and calibrating using the typical 185 nm photolysis
of H2O vapour method described here. This method of pressure dependent calibration introduces a number of unquantifiable variations
compared to the ambient measurement setup such as changes in the loss of radicals upon sampling and the flow dynamics within the inlet.
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OH Calibration - Hydrocabon Decays |
| Measurement of the decay of a hydrocarbon (HC) in the presence of OH has been used in HIRAC to calibrate a FAGE instrument for external pressures between 300 and 1000 mbar inside HIRAC. |
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| OH was produced in the chamber by the photolysis of methyl nitrite, and the decay of 1 – 2 HCs with well known rates of reaction with OH, kOH+HC, were measured by GC-FID whilst simultaneously measuring the decay in OH by FAGE. A first order exponential fit to the measured [HC] decay was used to calculate [HC] at the frequency of the OH measurement (1 s). |
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By rearrangement of equation (2) the [OH] can be calculated from the rate of decay of [HC], -d[HC]/dt, and literature kOH+HC.
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Measured OH signals were plotted against calculated [OH], and the sensitivity to OH, COH, determined from the gradient at each chamber pressure. The results from the HC decay method were compared to data from the traditional H2O photolysis / changing pinhole technique giving an average ratio of Ctraditional / CHC decay = 1.07±0.07.
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| OH calibration plots taken at chamber between 300 and 1000 mbar.
| OH sensitivity as a function of pressure measured by the HC decay (red) and traditional (black) methods.
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HO2 Calibration - HCHO Photolysis |
| Measurements of the loss of HO2 through self-reaction following photolysis of HCHO in air have been used to determine the sensitivity of a FAGE instrument to HO2, CHO2 between 300 and 1000 mbar and with 0 – 0.8% [H2O]. The loss of HO2 following photolysis of HCHO is due to mixed first and second order process and can be analysed as follows: |
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| The paramater kloss in equation 5 was determined using a simple FACSIMILIE model to fit experimental HO2 data for experiments conducted with and without the chamber mixing fans switched on. Experiments without the chamber mixing fans on had kloss = 0 s-1 illustrating that kloss was due to loss of HO2 on the chamber walls. The average value of kloss determined from experimental runs with the mixing fans switch on was 0.048±0.005 s-1 independent of pressure, [HCHO] and %[H2O]. Using the model determination of kloss along with pressure and %[H2O] dependent kHO2+HO2 (Stone and Rowley, PCCP, 2005), experimental HO2 decays data were fitted using equation 5 yielding pressure dependent CHO2. The results from the HCHO photolysis method were compared to data from the traditional H2O photolysis / changing pinhole technique giving an average ratio of Ctraditional / CHCHO photolysis = 0.95±0.08. |
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| Experimental HO2 decay data taken inside HIRAC during a HCHO photolysis calibration of FAGE for HO2.
| HO2 sensitivity as a function of detection cell pressure from the HCHO photolysis (black) and traditional (red) calibration methods.
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