Trends and variability in stratospheric NOx derived from merged SAGE II and OSIRIS satellite observations

Nitrogen oxides (NOx) in the stratosphere are produced from N2O, which is the dominant emission contributing to stratospheric ozone depletion in the 21st century and an important anthropogenic greenhouse gas. Decades worth of observations are required in order to quantify the variability and trends in stratospheric NOx so that we can better understand their impact on climate. Here we use the Stratospheric Aerosol and Gas Experiment (SAGE) II, a solar occultation instrument that measured NO2 from 1984 to 2005, and the Optical Spectrograph and InfraRed Imager System (OSIRIS), a limb-scattering instrument that began measuring NO2 in 2001. By taking advantage of the 4-year overlap between these instruments it was possible to produce a merged data set of stratospheric NO2, spanning over 34 years. In order to merge the data a photochemical correction was applied to account for the different times of day at which the instruments measure, and to convert the NO2 to NOx. A linear regression model was applied to the merged, deseasonalized data set to identify variability associated with long-term trends, the quasi-biennial oscillation (QBO), and volcanic aerosols. High levels of aerosol associated with large volcanic eruptions were found to greatly influence the calculated trend; when volcanic periods are excluded the trend in NOx is around 10% per decade in the tropical lower stratosphere. In this case, the observed trends and variability from the satellite measurements show overall good agreement with simulations from the whole atmosphere community climate model (WACCM).