Trends and Variability in Merged SAGE II and OSIRIS NOx

Nitrogen oxides (NO_x) in the stratosphere are produced by N₂O, which is the dominant emission contributing to stratospheric ozone depletion in the 21^st century. Decades worth of observations are required in order to quantify the variability and trends in stratospheric NO_x 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 four year overlap between these instruments it was possible to produce a merged dataset 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 NO_x. A linear regression model was applied to the merged dataset to identify variability associated with long-term trends, the Quasi-Biennial Oscillation (QBO), the El-Nino Southern Oscillation, the solar cycle, and volcanic aerosols. The dominant source of variability in the mid-stratosphere tropics is the QBO, while aerosol is the largest source of variability in the lower stratosphere. After accounting for these natural sources of variability, the largest trends in NO_x, reaching up to 16% per decade, were observed in the tropical lower stratosphere. Observed results show overall good agreement with simulations from the Whole Atmosphere Community Climate Model (WACCM).