Abstract
Abstract. It is well known that lightning produces NOx as a result of the high temperatures in discharge channels. Since most viable proposed electrification mechanisms involve ice crystals, it is reasonable to assume that lightning discharge channels frequently pass through fields of ice particles of various kinds. We address the question of whether ice crystals may serve as catalysts for the production of NOx by lightning discharges. If so, and if the effect is large, it would need to be taken into account in estimates of global NOx production by lightning. In this theoretical study, we make a series of plausible assumptions about the temperature and concentration of reactant species in the environment of discharges and we postulate a mechanism by which ice crystals are able to adsorb nitrogen atoms. We then compare production rates between uncatalyzed and catalyzed reactions at 2000 K, 3000 K, and 4000 K, which are reasonable temperatures in lightning channels as they cool down. Ice crystal catalysis is expected to produce 2.7 times more NO than if ice crystals were not present. Catalyzed NO production rates are greater at 2000 K, whereas uncatalyzed production rates are greater at 4000 K. Thus, temperatures that favor rapid NO production without ice crystals adsorbing nitrogen atoms are unfavorable for NO production in the presence of ice crystals, and vice versa. The density of atmospheric ice crystals is much larger at 10 km where intracloud (IC) flashes peak than at 5 km where cloud to ground (CG) flashes peak, thus catalytic processes are expected to be more important for IC flashes than CG flashes, perhaps explaining a portion of the discrepancy in IC and CG production rates.
Highlights
Though it has been known for a long time that lightning produces oxides of nitrogen (NOx), serious research on the subject has been undertaken only in the last 30 yr or so
It should again be noted that total conversion of nitrogen atoms does not imply total conversion of all nitrogen to NO; the quantity of NO produced is limited by the dissociation of nitrogen molecules
It is reasonable to conclude that NOx may be produced in the atmosphere through a wide variety of reaction mechanisms
Summary
Though it has been known for a long time that lightning produces oxides of nitrogen (NOx), serious research on the subject has been undertaken only in the last 30 yr or so. NO forms an equilibrium with molecular nitrogen and molecular oxygen at high temperatures, such as those that are generated within the lightning channel (Price et al, 1997). Rakov et al (1998) found return-stroke channel peak temperatures exceeded 30 000 K. Uman and Voshall (1968) found that temperatures in the lightning channel cool at rates depending on channel radius. For a channel of radius 1 cm the central temperature cools to 4000 K in about 7 ms, 3000 K in about 12 ms, and 2000 K in 50 ms. For a channel of radius 8 cm the central temperature has only cooled to about 5700 K in 100 ms, the longest time duration for which the cooling process was modeled. Rakov and Uman (2003) estimate the lightning channel to be 1 to 2 cm in radius. The degree of dissociation of nitrogen and oxygen molecules is dependent
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