Possible catalytic effects of ice particles on the production of NO_x by lightning discharges

Abstract. It is well known that lightning produces NO_x 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 NO_x by lightning discharges. If so, and if the effect is large, it would need to be taken into account in estimates of global NO_x 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.