Effect of volcanic aerosol on stratospheric NO₂ and N₂O₅ from 2002–2014 as measured by Odin-OSIRIS and Envisat-MIPAS

Abstract. Following the large volcanic eruptions of Pinatubo in 1991 and El Chichón in 1982, decreases in stratospheric NO₂ associated with enhanced aerosol were observed. The Optical Spectrograph and Infrared Imaging Spectrometer (OSIRIS) measured the widespread enhancements of stratospheric aerosol following seven volcanic eruptions between 2002 and 2014, although the magnitudes of these eruptions were all much smaller than the Pinatubo and El Chichón eruptions. In order to isolate and quantify the relationship between volcanic aerosol and NO₂, NO₂ anomalies were calculated using measurements from OSIRIS and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). In the tropics, variability due to the quasi-biennial oscillation was subtracted from the time series. OSIRIS profile measurements indicate that the strongest anticorrelations between NO₂ and volcanic aerosol extinction were for the 5 km layer starting  ∼  3 km above the climatological mean tropopause at the given latitude. OSIRIS stratospheric NO₂ partial columns in this layer were found to be smaller than background NO₂ levels during these aerosol enhancements by up to  ∼  60 % with typical Pearson correlation coefficients of R ∼ −0. 7. MIPAS also observed decreases in NO₂ partial columns during periods affected by volcanic aerosol, with percent differences of up to  ∼  25 % relative to background levels. An even stronger anticorrelation was observed between OSIRIS aerosol optical depth and MIPAS N₂O₅ partial columns, with R ∼ −0. 9, although no link with MIPAS HNO₃ was observed. The variation in OSIRIS NO₂ with increasing aerosol was found to be consistent with simulations from a photochemical box model within the estimated model uncertainty.