Atmospheric production of nitrous oxide from excited ozone and its significance

Today our understanding of the sources and sinks of nitrous oxide (N₂O) may be at a turning point. Currently, it is believed that there are no atmospheric photochemical sources of N₂O and that microbial activity at the earth's surface (soil, lake, ocean, etc.) is the major source of atmospheric N₂O. Anthropogenic activities are thought to release N₂O into the atmosphere, but their magnitude is uncertain and probably minor. Here we present estimates of atmospheric production of N₂O from excited ozone (O₃) based on comprehensive laboratory experiments. These experiments covered a large range of pressures from 1 to 1000 torr to distinguish between the various possibilities on the basis of their pressure dependencies, and used two reaction vessels of widely varying surface-to-volume ratios to distinguish between surface and gas phase reactions. Never before in the history of the experimental studies of N₂O under atmospherically significant conditions has such a comprehensive coverage of the parameter space been attempted. From this data, the atmospheric production is substantial, being around 40% of its “classical” source strength. In order to put the atmospheric production in proper perspective, we also present those considerations that led us to look into the atmospheric sources. If we accept the IPCC’s 1990 position on the N₂O source-sink inventory, then the atmospheric production of N₂O bridges the source deficits. On the other hand, if the later IPCC positions of a nearly balanced inventory is accepted, then the new source means that either the post-1990 IPCC methodology for establishing national inventories of greenhouse gas emissions overestimates N₂O emissions or there exists some hitherto unrecognized sinks of N₂O.