Instrument development

 1. CH₃O₂ Measurement

The methylperoxy radical CH₃O₂, along with other RO₂ species, is an important intermediate in atmospheric chemistry, but is currently not routinely measured in either chambers or in the field. We have recently obtained a grant from NERC, in conjunction with Grant Ritchie (University of Oxford) to develop techniques for CH₃O₂ monitoring.

Grant's group will focus on near IR cavity techniques for HO₂ and CH₃O₂ with HIRAC providing the ideal environment to test the new methods. We have also been working on developing the FAGE technique to provide an alternative selective method for CH₃O₂ detection via laser induced fluorescence (LIF). In this method, gas is sampled from HIRAC into a low pressure environment in the presence of NO which converts CH₃O₂ (no LIF spectrum) into CH₃O, the methoxy radical, which can be detected by LIF.

CH₃O₂ + HO₂ → CH₃O + NO₂

The technique appears to work well. We can record CH₃O LIF spectra which agree well with the literature and have developed calibration techniques based either around the water photolysis method used for FAGE:

H₂O + hν → OH + H

OH + CH₄ → H₂O + CH₃

or from the direct photolysis of methanol:

CH₃OH + hν → CH₃O + H

CH₃O LIF spectrum

CH₃O₂ calibration plot

The detection limits of the method offer hope that it could be a realistic field instrument in the future and, as can be seen from the figure on CH₃O₂ recombination, the method provides excellent sensitivity for HIRAC experiments.

Temporal profile of CH3O2 self-reaction in HIRAC

This work has been published in Atmospheric Measurement Techniques in 2017.

2. Intercomparisons of HO₂ Measurements

HO2 has been measured in the field using the FAGE technique for many years. Here HO2 (not detectable via LIF) is converted into OH by titration with NO:

HO₂ + NO → OH + NO₂

Whilst a sensitive technique, FAGE detection of HO₂ is not a direct method and requires calibration (see below). However, HO₂ can be detected by NIR absorption and Grant Ritchie's group have developed a Cavity Ring Down Spectroscopy (CRDS) method to directly measure HO₂ absorption. This instrument has recently been installed in HIRAC and the results of the first simultaneous detection of HO₂ using different techniques has been reported in Atmospheric Measurement Techniques.

CRDS and FAGE locations in the HIRAC Instrument

Details of CRDS instrument

The results of the intercomparison are excellent with good agreement (within experimental error) between CRDS and FAGE validating the techniques. The CRDS technique is of course dependent on knowledge of the HO₂ absorption cross-section and the figure below illustrates the comparison with FAGE for two cross-section values.

Intercomparison of CRDS and FAGE HO₂ concentrations

Similar exercises are currently underway to compare CH₃O₂ detection with the FAGE and CRDS methods.

3. The first pressure dependent calibrations of a FAGE HOx instrument

HIRAC has been used to complete the first calibration of a FAGE HO_x instrument over a range of external pressures simulating the conditions experienced during aircraft measurements. This work was published in Atmospheric Measurement Techniques in 2015.

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.

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.

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,k_OH+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).

By rearrangement of equation (2) the [OH] can be calculated from the rate of decay of [HC], -d[HC]/dt, and literature k_OH+HC.

Measured OH signals were plotted against calculated [OH], and the sensitivity to OH, C_OH, 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 C_traditional / C_{HC decay} = 1.07±0.07.

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.

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, C_HO2 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:

The paramater k_loss 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 k_loss = 0 s^-1 illustrating that k_losswas due to loss of HO₂ on the chamber walls. The average value of k_loss 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 k_loss along with pressure and %[H₂O] dependent k_HO2+HO2 (Stone and Rowley, PCCP, 2005), experimental HO₂ decays data were fitted using equation 5 yielding pressure dependent C_HO2. The results from the HCHO photolysis method were compared to data from the traditional H₂O photolysis / changing pinhole technique giving an average ratio of C_traditional / C_{HCHO photolysis} = 0.95±0.08.

Experimental HO₂ decay data taken inside HIRAC during an 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.