The impact of mesospheric dynamics and chemistry on key chemical species: 20 years of Odin/SMR satellite observations

Doctoral thesis, 2022

Coupling mechanisms between different atmospheric layers are such that changes in middle atmospheric dynamics and composition have an effect on what happens at lower altitudes and on the climate. There is therefore a need to extend climate models to include higher altitudes and to perform measurements of the middle atmosphere. Carbon monoxide (CO), water vapour (H2O) and nitric oxide (NO) are species of high scientific interest due to their aptness to being used as circulation tracers in the middle atmosphere, due to their long photochemical lifetime (for NO, this is only true during polar winter). Moreover, NO plays a decisive role for ozone (O3) chemistry. In fact, the downward branch of the middle atmospheric residual circulation is responsible for the descent of NO generated by energetic particle precipitation (EPP) in the mesosphere and lower thermosphere (MLT) to lower altitudes where it is involved in catalytic destruction of O3. However, the estimates on the amount of EPP NO thus descending still present large uncertainties. In addition to this, NO observations allow us to estimate its oscillation in concentration due to atmospheric solar tides in the middle atmosphere. Among the satellite instruments currently performing remote sensing of the middle atmosphere, the Sub-Millimetre Radiometer (SMR) on board the Odin satellite is one of the most long-lived. SMR has indeed been performing limb sounding of the middle atmosphere for 21 years. However, the CO and H2O data sets are both affected by instrumental artifacts that resulted in a misestimation of the two species’ concentration and, in the case of CO, also caused failure of the retrieval process. Papers 1 and 2 included in this thesis focus on identifying the causes and correcting such artifacts, leading to two new long-term and global data sets that are now available to the scientific community to study middle atmospheric dynamics. In Paper 3, high latitude mesospheric NO observations from SMR were used to measure the flux of EPP NO transported down through the mesosphere during each polar winter, in both hemispheres. Such collection of EPP NO mesospheric fluxes is unique - considering the longevity of Odin/SMR and that it is the only instrument currently observing mesospheric NO globally. It can help reduce the uncertainties in the above mentioned estimates of descending EPP NO, providing the possibility to further study the impact of EPP on the atmosphere. Finally, the SMR NO data set was also used in Paper 4 with the aim of investigating the tidal signature in lower thermospheric NO concentration at low latitudes, with a particular focus on how NO diurnal variations are affected by nonmigrating semidiurnal atmospheric solar tides.