Experimental

In the present work a discharge could be struck between two tungsten electrodes having a gap of 2 - 20 mm. The electrodes had a diameter of 4 mm, narrowing down to 2 mm diameter hemispheres 5 mm from their ends. The pyrex reaction vessel had a volume of approximately 250 cm³ and was generally spherical apart from two optical flat windows to allow the passage of a laser beam across the discharge.

Electrical discharges of energy 31 to 113 J and energy per unit length of 0.4 to 1.1×10⁴ J m^-1 were examined. A 2 microFarad capacitor was connected across the electrodes and could be charged up to 15 kV, with the discharge being struck by increasing the voltage until breakdown occurred. However, there was a slight run-to-run variation in the breakdown voltage using this technique so a thyratron trigger device was put in the circuit between the capacitor and one of the electrodes, to achieve a more reproducible discharge for the NO freeze out mixing ratio experiments. The capacitor could then be reproducibly charged to potentials above the breakdown voltage, with the discharge occurring when the thyratron was triggered. The thyratron was not used in the other experiments however, because of the possibility that some of the energy from the capacitors was dissipated in the thyratron and not between the electrodes. All the discharges were fired in dry air (BOC), except for those described in (figures 3 and 5, to which known quantities of NO (Matheson) and water vapour were added.

NO or NO_x (sum of NO and NO₂) was detected by flushing the contents of the reaction vessel through a chemiluminescence NO_x detector (Thermoelectron Corporation, Model 10A). The detector was calibrated by analyzing for known concentrations of NO or NO₂. There was an uncertainty in the NO_x measurements of ca. 2-4 ppb NO_x, which translates to the error bars shown in figure 3 figures 4 and figures 5. There was also an uncertainty in measuring the breakdown voltage of ca. 200 V, which contributes a ca. 5% uncertainty to the measurements of NO_x per unit energy and energy per unit length.

The radial velocity of shock fronts generated by electrical discharges was examined using a simple laser deflection technique. A narrow beam (0.3 mm diameter) from a 633 nm HeNe laser was directed across the reaction vessel in the plane centred on the mid-point of the spark gap, perpendicular to the axis of the electrodes and at known distances from the axis of the electrodes. The shock front consists of a shell of dense gas which causes a change in refractive index of the air and hence a deflection of the laser beam, when the front reaches the beam. The laser beam was directed onto the edge of a photodiode 2 m from the reaction vessel. The small angle of deflection of the beam by the shock front caused the beam to move slightly off the diode, giving a reduction in signal. The light intensity recorded by the diode was stored on a digital oscilloscope. The examples given in figure 1 show the diode signal for the laser at a distance of 6 and 12 mm from the axis of the electrodes. The signal in the first 2 microseconds is due to light from the spark reaching the diode and can be used to estimate an upper limit for the duration of the current flow in the discharge. With the laser beam 6mm from the axis of the electrodes there was a decrease in signal due to the arrival of the shock front 3.5 microseconds after the start of the discharge, taken to be when the light from the discharge starts. With the laser beam 12 mm from the electrode axis the decrease in signal due to the arrival of the shock front at the beam occurs 7.8 microseconds after the start of the discharge. The delay between the start of the discharge and the deflection of the beam increases as the distance of the beam from the discharge increases (figure 2); the velocity of the shock front is then determined from the gradient.

The NO_x freeze out mixing ratio was determined by adding varying concentrations of NO to an N₂:O₂ mixture and monitoring the increase or decrease in NO_x formed when a discharge was fired. Ideally, these freeze out mixing ratio experiments would have been conducted at pressures at which lightning occurs (200 mbar to 1.0 bar). However for two reasons the experiments had to be conducted at lower pressures. Since the NO freeze out mixing ratio is proportional to the difference between two large numbers (NO_x measured with and without a discharge), the uncertainty in [NO]_{freeze out} is proportional to pressure and reliable measurements could not be determined above 70 mbar. Secondly, with higher pressures of NO, the NO added to the N₂:O₂ mixture undergoes significant conversion to NO₂ (via the termolecular reaction NO + NO + O₂) before a discharge can be fired. Where NO₂ was present in measurable amounts, there was an anomalous slight increase in NO_x produced by the discharge, as more NO_x was added. Two possible reasons for this are, firstly, any NO₂ present can thermally dissociate to NO and O atoms at lower temperatures than the NO freeze out temperature, or secondly there is photolysis of NO₂ to NO and O by UV light from the discharge. These additional sources of O atoms could lead to the additional NO_x observed.