Formation of nitrogen oxides by electrical discharges and implications for atmospheric lightning

Abstract

The mechanism of production of nitrogen oxides by electrical discharges has been examined. The velocity of shock fronts generated by laboratory scale discharges have been measured and are found to be too slow to raise the air temperature to the ∼3000 K necessary for nitrogen fixation by the Zel'dovich mechanism. The freeze‐out mixing ratio of NO x in air has been measured directly for low‐pressure discharges and is found to be of the order expected from the Zel'dovich mechanism for gas cooling over a timescale far longer than the duration of the shock front. Therefore it is concluded that NO x is formed in the gas in the slowly cooling hot channel region and not in the rapidly cooling shock front. Also, it is argued that NO x formation occurs by a freeze out mechanism due to a rapid drop in temperature, not density as has been suggested. NO 2 production is found to be significant, with the [NO 2 ]/[NO] ratio being strongly dependent on the water content of the air. Discrepancies between previous experimental studies of the [NO 2 ]/[NO] ratio and the quantity of NO x formed per unit energy ( P ) are discussed. P is also found to vary with spark gap and ambient pressure. It is thought that these effects may be due to a significant loss of heat from the spark gap to the electrodes. The inclusion of atmospheric levels of N 2 O, CH 4 , and CO 2 are found to have no measurable effect on the yields of NO or NO 2 .