Abstract
Abstract. The Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched in February 2001, observes thermal emissions of stratospheric nitric acid (HNO3) originating from the Earth limb in a band centred at 544.6 GHz. Height-resolved measurements of the global distribution of nitric acid in the stratosphere were performed approximately on two observation days per week. An HNO3 climatology based on more than 7 years of observations from August 2001 to April 2009 covering the vertical range between typically ~19 and 45 km (~1.5–60 hPa or ~500–1800 K in terms of potential temperature) was created. The study highlights the spatial and seasonal variation of nitric acid in the stratosphere, characterised by a pronounced seasonal cycle at middle and high latitudes with maxima during late fall and minima during spring, strong denitrification in the lower stratosphere of the Antarctic polar vortex during winter (the irreversible removal of NOy by the sedimentation of cloud particles containing HNO3), as well as large quantities of HNO3 formed every winter at high-latitudes in the middle and upper stratosphere. A strong inter-annual variability is observed in particular at high latitudes. A comparison with a stratospheric HNO3 climatology, based on over 7 years of UARS/MLS (Upper Atmosphere Research Satellite/Microwave Limb Sounder) measurements from the 1990s, shows good consistency and agreement of the main morphological features in the potential temperature range ~465 to ~960 K, if the different characteristics of the data sets such as the better altitude resolution of Odin/SMR as well as the slightly different altitude ranges are considered. Odin/SMR reaches higher up and UARS/MLS lower down in the stratosphere. An overview from 1991 to 2009 of stratospheric nitric acid is provided (with a short gap between 1998 and 2001), if the global measurements of both experiments are taken together.
Highlights
Nitric acid (HNO3) is an important chemical constituent in the stratosphere
Heterogeneous chemical processes involving HNO3 on the surfaces of polar stratospheric cloud (PSC) particles lead to the activation of chlorine (ClOx=Cl+ClO+2Cl2O2) from its reservoir gases in the cold polar vortices during winter and to ozone loss when sunlight returns in late winter and spring
We combine the tropical HNO3 time-series observed by Upper Atmosphere Research Satellite (UARS)/Microwave Limb Sounder (MLS) (1991–1998) and Odin/Sub-Millimetre Radiometer (SMR) (2001–2009) in Fig. 8, showing the time evolution of HNO3 interpolated on selected levels of potential temperature between 520 and 960 K
Summary
Nitric acid (HNO3) is an important chemical constituent in the stratosphere. It is one of the most abundant species of the NOy family (NOy=HNO3, NO2, NO, N2O5, ClONO2, . . . ) and a major reservoir of active odd nitrogen (NOx=NO+NO2) which is responsible for the main catalytic ozone loss cycle in the middle stratosphere. Heterogeneous chemical processes involving HNO3 on the surfaces of polar stratospheric cloud (PSC) particles lead to the activation of chlorine (ClOx=Cl+ClO+2Cl2O2) from its reservoir gases in the cold polar vortices during winter and to ozone loss when sunlight returns in late winter and spring. Taking measurements from both experiments (UARS/MLS and Odin/SMR) together provides a unique overview from 1991 to 2009 of stratospheric nitric acid in the range ∼465 to ∼960 K, with a short gap between 1998 and 2001. A second paper (Orsolini et al, 2009) is dedicated to a thorough discussion of the vertically well resolved Odin/SMR observations of enhanced HNO3 in the middle and upper stratosphere during polar winter, based on the unique and nearly continuous Odin/SMR high-latitude HNO3 time-series starting in 2001
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