Spatially separate production of hydrogen oxides and nitric oxide in lightning

Abstract. The atmosphere’s most important oxidizer, the hydroxyl radical (OH), is generated in abundance by lightning, but the contribution of this electrically generated OH (LOH) to global OH oxidation remains highly uncertain. Part of this uncertainty is due to the abundant nitric oxide (NO) also generated in lightning, which could rapidly remove the LOH before it can oxidize other pollutants in the atmosphere. However, evidence from a previous laboratory study indicated LOH is not immediately consumed by NO, possibly because LOH’s production is spatially separated from the NO production in lightning flashes. This hypothesis of spatially separate OH and NO production is further tested here in a series of laboratory experiments, where the OH decays were measured from spark discharges in air which had increasing amounts of NO added to it. The LOH decayed faster as more NO was added to the air, indicating that the LOH was reacting with the added NO, and not the spark NO. Thus, LOH from lightning flashes is not immediately consumed by the electrically generated NO but is available to oxidize other pollutants in the atmosphere and contribute to global OH oxidation. Subsequent modelling of the laboratory data also supports the spatially separate production of LOH and NO, and further suggests that substantial HONO is also produced by sparks and lightning in the atmosphere.