Results
The velocity of shock fronts propagating away from the electrical discharges was measured at 200 and 500 mbar and with spark gaps of 4 to 20 mm. Figure 2 gives the distance travelled by the front against time for the 500 mbar cases. The measured velocity close to the discharge is similar for both the 4 and 20 mm spark gaps, but the velocity of the former decreases with increasing distance more than in the 20 mm case. For the latter, the highest shock front velocity measured was (2200±200) m s-1, found by using the time for the front to travel from 2 to 4 mm from the spark gap centre. The shock front is unlikely to be faster at earlier times than those measured, as this would require the front to start with zero radius sometime after the start of the discharge. The front velocities for the other pressures and spark gaps examined were all less than or similar to this value. A radius for the luminous channel of approximately 2.5±1.0 mm was estimated by photographing the discharge through optically thick filters (20 mm spark gap, 500 mbar pressure). This value is ca 70±30% of the radius of the shock front at 1.9 µs, the end of the luminous discharge.
The NOx freeze out mixing ratio was determined as (2.8±0.4)% (figure 3) and (2.3±0.3)% at 27 and 67 mbar; however, as discussed in the next section, there is some evidence of interference from the reactor walls in the higher pressure experiment.
The variation with spark gap of the NOx produced per unit energy (stored on the capacitor) was significant as shown in figure 4 (previous work is shown for comparison and is discussed below). The variation in the yield of NOx with pressure was also examined, with the yield of NOx per joule increasing by 45% on increasing the pressure from 0.13 to 1.0 bar and the [NO2]/[NOx] ratio remained constant at 0.46±0.04 (8 mm spark gap). The variation in the NOx and NO2 yields with water content is given in figure 5. For dry air the [NO2]/[NOx] ratio is high at 0.52±0.07, decreasing significantly with increasing water content. The addition of atmospheric concentrations of CO2, N2O and CH4 was found to have no measurable effect on NOx or NO2 production by the discharge.