Detection of stratospheric HNO₃ and NO₂ response to short-term solar ultraviolet variability

Abstract

Variations in the solar ultraviolet irradiance with a period equal to or approximately one-half of the rotation period of the Sun (27 days) are currently observed by satellite monitoring. These variations have typical peak-to-peak amplitudes of ∼3% between 170 and 208 nm and <1.5% at longer wavelengths. Detection of the response of stratospheric species to solar ultraviolet variability is crucial for understanding the photochemical behaviour of the middle atmosphere. Understanding the natural variations of the stratosphere is also a prerequisite for isolating possible anthropogenic effects; up to now most work has concerned stratospheric ozone^1–8. Recent analysis⁵ of measurements by the Nimbus 7 satellite's LIMS (limb infrared monitor of the stratosphere) and SBUV (solar backscatter ultraviolet) experiments has established very high correlations between ozone mixing ratios (detrended and corrected for temperature effects) and short-term variations in 205-nm solar radiation. A similar approach is used here to detect a relation between LIMS measurements of HNO₃^9,10 and NO₂¹¹ and the SBUV measurements of short-term variations in 205-nm radiation¹². Observations show that the response of HNO₃ is much stronger than, but in the opposite sense to the ozone response, and that the NO₂ response is in the opposite sense to the HNO₃ response. This is the first detection of such a response for species other than ozone. Model calculations are in fair agreement with observed short-term variations and predict large variations in HNO₃ over the 11-yr solar cycle.