Long-term evolution of stratospheric aerosol extinction and particle sizes at northern mid and high latitudes

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Stratospheric sulfate aerosols are of great importance for the climate system, because they scatter solar radiation, enhance the Earth's planetary albedo and lead to a surface cooling. They are important for stratospheric chemistry, because they facilitate chlorine activation even outside the polar vortex and they are known to play an important role in the formation of polar stratospheric clouds. Stratospheric aerosols are one of the contributors to the current global warming hiatus - according to the 5th assessment report of the Intergovernmenal Penal on Climate Change (IPCC). One of the most accurate methods to remotely sense stratospheric aerosols is based on ground-based lidar observations. Within the research project proposed here, high-quality lidar data sets at different locations - that have to date not been used to study stratospheric aerosols - will be employed for this purpose. The lidar systems are operated by the Leibniz-Institute of Atmospheric Physics (IAP) at Rostock University, Kühlungsborn (Germany) and are located at the ALOMAR observatory in Andenes (Norway), at Davis Station (Antarctica) and at Kühlungsborn itself. Two of the lidar time series cover 20 decades and the measurements at ALOMAR provide lidar backscatter observations at multiple wavelengths. A unique feature of the lidar systems is that they are operated also during daytime. One of the existing lidar system has a spectral resolution that allows separating aerosol and molecular scatter by its Doppler broadening. The lidar raw observations will be used in the first part of the project to determine stratospheric aerosol backscatter and extinction profiles, as well as stratospheric aerosol optical depth, which can be converted to aerosol radiative forcing. In addition, the multi-wavelength observations at the ALOMAR observatory allow the retrieval of aerosol particle size. In the second part of the project, the retrieved time series of stratospheric aerosol extinction, optical depth and particle size will be used to characterize the variability and trends in the stratospheric aerosol load and the associated radiative forcing at mid and high northern latitudes over a period of more than 2 decades. In this context, seasonal variations, QBO and volcanic effects will be of crucial importance. The particle size retrievals will provide essential constraints for satellite retrievals of stratospheric aerosol extinction based on the limb-scatter observation technique during a period with several weak to medium volcanic eruptions. The satellite limb-scatter technique was/is applied by the SCIAMACHY instrument on Envisat, OSIRIS on Odin as well as the OMPS-LP instrument on the Suomi satellite, and relies on a priori knowledge of the aerosol particle size. The expected outcome of the proposed research will provide important new information on the variability and trends in stratospheric aerosol parameters at mid and high latitudes at decadal time scales.