The formation of NO2 by electrical discharges and the effect of water
Water is present in widely varying concentrations in the air in which lightning occurs. The effect of water vapour on NOx formation has been examined experimentally by Chameides et al. [1977] who found no change in total NOx yield for moist or dry air (95% and 5% relative humidity). Levine et al. [1981] found that no NO2 is formed within experimental error, although the water content of the air was not specified. Peyrous and Lapeyre [1982] examined the effect of humidity and found a significant NO2/NOx ratio, with humid air giving a lower ratio than dry air. However, it is not clear whether the NO2 was produced promptly by the spark discharges, or whether it was produced by NO from the spark reacting with the large amounts of ozone produced by the corona discharges that ran concurrently with the spark discharges.
Hill et al [1980] suggested that NO2 formation from lightning could be significant due to NO from the discharge reacting with ozone in the ambient air. As the air in the hot channel entrains the surrounding cooler air, the ozone from the entrained air reacts with NO generated by the Zel'dovich mechanism to give NO2. This behaviour was demonstrated in their computer simulations of lightning: however, to simplify the reaction scheme, the reactions of water were omitted.
The experiments reported here show both significant NO2 formation and a strong effect of water on the NO2/NOx ratio (figure 5), although water content has little effect on the total NOx formed. As a test to see if ozone in ambient air could convert NO to NO2, discharges were fired in dry air containing O3, the resulting NOx consisted mostly of NO2. If it is accepted that NO2 is formed by the reaction of O3 and NO, then the presence of water must be having a significant effect on the concentration of O3 in the air surrounding the discharge. Lightning occurs in air that has a significant water content, so it would be expected that in practice the NO2/NOx ratio would be low for atmospheric discharges.