Reexamination of the Impact of Nitrous Oxide on Stratospheric Ozone

Ozone depleting chlorofluorocarbons (CFCs) are decreasing in the atmosphere because of the implementation of the Montreal Protocol. In contrast, nitrous oxide remains as a major potential ozone-depleting molecule for which there are significant anthropogenic sources. IPCC (2007) estimates that more than 1/3 of the source of nitrous oxide is of anthropogenic origin. Observations show that N2O atmospheric levels have steadily increased over the last century and are projected to increase throughout the 21st century. The primary source of NOy (odd nitrogen, leading to ozone loss) is N2O. We report an analysis of model simulations using the GEOS Chemistry Climate Model (CCM) in which standard scenarios of greenhouse gases and ozone-depleting substances were assumed between the years 1950 and 2100. During the 1950-2004 time period, the CCM was forced with observed levels of nitrous oxide that increased from 290 ppb to 320 ppb. After 2004, the CCM was forced with IPCC scenario A1b, increasing N2O from 320 to 370 ppb by the year 2100. Over the 150 year period, this was an increase of more than 25 percent. In spite of this N2O increase, the total global amount of NOy decreased by more than 5 percent in the simulation. The major reason for the decrease is the cooling of the stratosphere by increasing carbon dioxide. NOy is produced by the reaction of excited oxygen atoms with nitrous oxide. It is destroyed by reaction of N atoms with NO. When N atoms are created they can either react with NO to form N2, destroying NOy, or they can react with O2 to reform NO. The reaction of N atoms with O2 is strongly temperature dependent (exp(-3600/T)). Thus a cooling slows down this reaction and favors the path of N atoms reacting with NO to destroy NOy. We will explore this and other mechanisms that impact the future amount of ozone-depleting nitrogen compounds in the stratosphere.