Year-round stratospheric aerosol backscatter ratios calculated from lidar measurements above northern Norway

Atmos. Meas. Tech., 12, 4065–4076, 2019 https://doi.org/10.5194/amt-12-4065-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Year-round stratospheric aerosol backscatter ratios calculated from lidar measurements above northern Norway Arvid Langenbach1 , Gerd Baumgarten1 , Jens Fiedler1 , Franz-Josef Lübken1 , Christian von Savigny2 , and Jacob Zalach2 1 Leibniz-Institut für Atmosphärenphysik an der Universität Rostock, Schlossstraße 6, 18225 Kühlungsborn, Germany 2 Institut für Physik, Universität Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald, Germany Correspondence: Arvid Langenbach () Received: 22 February 2019 – Discussion started: 7 March 2019 Revised: 13 June 2019 – Accepted: 1 July 2019 – Published: 24 July 2019 Abstract. We present a new method for calculating backscatter ratios of the stratospheric sulfate aerosol (SSA) layer from daytime and nighttime lidar measurements. Using this new method we show a first year-round dataset of stratospheric aerosol backscatter ratios at high latitudes. The SSA layer is located at altitudes between the tropopause and about 30 km. It is of fundamental importance for the radiative balance of the atmosphere. We use a state-of-the-art Rayleigh–Mie– Raman lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) station located in northern Norway (69◦ N, 16◦ E; 380 m a.s.l.). For nighttime measurements the aerosol backscatter ratios are derived using elastic and inelastic backscatter of the emitted laser wavelengths 355, 532 and 1064 nm. The setup of the lidar allows measurements with a resolution of about 5 min in time and 150 m in altitude to be performed in high quality, which enables the identification of multiple sub-layers in the stratospheric aerosol layer of less than 1 km vertical thickness. We introduce a method to extend the dataset throughout the summer when measurements need to be performed under permanent daytime conditions. For that purpose we approximate the backscatter ratios from color ratios of elastic scattering and apply a correction function. We calculate the correction function using the average backscatter ratio profile at 355 nm from about 1700 h of nighttime measurements from the years 2000 to 2018. Using the new method we finally present a year-round dataset based on about 4100 h of measurements during the years 2014 to 2017. 1 Introduction The importance of stratospheric sulfate aerosol (SSA) for the radiative balance and the ozone chemistry of the atmosphere is widely accepted. Long-term observations of the stratospheric aerosol layer are crucial for the analysis of global atmospheric temperature and ozone layer variability (Thomason and Peter, 2006; Solomon et al., 2011). The first in situ measurements of SSA were performed by Christian Junge and co-workers (Junge and Manson, 1961). They found a distinct layer between 15 and 25 km altitude with a peak at 20 km (Junge et al., 1961a, b). The stratospheric aerosol layer is therefore often referred to as the Junge layer. Remote sensing of the aerosol layer by lidar was started by Bartusek and Gambling (1971). Global satellite observations of SSA began in the late 1970s (e.g., Thomason and Peter, 2006; Kremser et al., 2016). The upper boundary of the SSA layer is determined by the evaporation of the aerosol particles due to rising temperatures in the stratosphere as well as sedimentation (Hofmann et al., 1985). The tropopause is the base of the aerosol layer since the upper tropospheric aerosol loads are often much lower than in the stratosphere (Kremser et al., 2016). Understanding the formation and life cycle of SSA is impossible without understanding the processes controlling sulfur in the stratosphere. Stratospheric sulfur can be found in a broad variety of molecules, such as carbon disulfide (CS2 ), sulfur dioxide (SO2 ), carbonyl sulfide (OCS) and sulfuric acid (H2 SO4 ) (English et al., 2011). SSA typically consists of 75 % sulfuric acid / water (H2 SO4 –H2 O) solution droplets (Thomason and Peter, 2006). In volcanically quiescent periods, the main sources for these droplets are CS2 and OCS, Published by Copernicus Publications on behalf of the European Geosciences Union. 4066 A. Langenbach et al.: Stratospheric aerosol backscatter ratios from lidar which are emitted at the Earth’s surface and lifted into the stratosphere by deep convection and the Brewer–Dobson circulation (Khaykin et al., 2017). They then react in multiple steps via SO2 into sulfuric acid (Kremser et al., 2016). Stratospheric aerosols are primarily washed out by sedimentation and through the quasi-isentropic transport of air masses in tropopause folds (Thomason and Peter, 2006). Moreover, the SSA variability is dominated by major volcanic eruptions injecting sulfur directly into the stratosphere. These episodic, but powerful eruptions, can overlay the permanent stratospheric aerosol layer (referred to as “background” aerosol) for years and have a global cooling effect on the surface of the order of a few tenths of a degree Celsius (Robock and Mao, 1995). The fact that aerosols from large volcanic eruptions have global effects was first determined by worldwide observations of optical phenomena following the eruption of Krakatoa in 1883 (Symons, 1888). After the Mount Pinatubo eruption in 1991 the stratospheric sulfur burden was increased by a factor of 60 above background levels and remained elevated by a factor of 10 well into 1993 (McCormick et al., 1995). The long-term development of SSA has been discussed in various studies (Kremser et al., 2016). Ignoring periods with volcanically enhanced SSA, observations covering the time span between 1970 and 2004 did not show significant changes in the background aerosol (Deshler et al., 2006). Newer studies show rising levels of SSA since 2002 (Hofmann et al., 2009; Vernier et al., 2011; Trickl et al., 2013; von Savigny et al., 2015). The reason for this increase is being debated. Originally the rise of the aerosol levels was connected to a fast increase in Asian sulfur emissions (Hofmann et al., 2009). More recent studies show an increase in non-volcanic aerosol inside of the Asian Tropopause Aerosol Layer (ATAL). This layer occurs during the northern summer above the Asian monsoon (Vernier et al., 2015; Yu et al., 2015). Vernier et al. (2011) showed, with the help of global satellite observations, that weaker eruptions also influence the stratospheric aerosol layer. These moderate eruptions are much less powerful compared to those of El Chichón or Mount Pinatubo, and the effect on stratospheric aerosol levels is much smaller. Nevertheless, several studies have shown that they have an impact on global surface temperatures (Solomon et al., 2011; Fyfe et al., 2013; Santer et al., 2014, 2015; Andersson et al., 2015). Accurate long-term measurements are essential to quantify changes to the background, as well as volcanically and anthropogenically driv (...truncated)