Six months of stratospheric aerosol observations with the European Aerosol Research Lidar Network (EAR-LINET)
Abstract. Knowledge of the particle lidar ratio (Sλ) and the particle linear depolarisation ratio (δλ) for different aerosol types allows for aerosol typing and aerosol-type separation in lidar measurements. Reference values generally originate from dedicated lidar observations but might also be obtained from the inversion of AErosol RObotic NETwork (AERONET) sun/sky radiometer measurements. This study investigates the consistency of spectral Sλ and δλ provided in the recently released AERONET version 3 inversion product for observations of undiluted mineral dust in the vicinity of the following major deserts: Gobi, Sahara, Arabian, Great Basin, and Great Victoria. Pure dust conditions are identified by an Ångström exponent <0.4 and a fine-mode fraction <0.1. The values of spectral Sλ are found to vary for the different source regions but generally show an increase with decreasing wavelength. The feature correlates to AERONET, retrieving an increase in the imaginary part of the refractive index with decreasing wavelength. The smallest values of Sλ=35–45 sr are found for mineral dust from the Great Basin desert, while the highest values of 50–70 sr have been inferred from AERONET observations of Saharan dust. Values of Sλ at 675, 870, and 1020 nm seem to be in reasonable agreement with available lidar observations, while those at 440 nm are up to 10 sr higher than the lidar reference. The spectrum of δλ shows a maximum of 0.26–0.31 at 1020 nm and decreasing values as wavelength decreases. AERONET-derived δλ values at 870 and 1020 nm are in line with the lidar reference, while values of 0.19–0.24 at 440 nm are smaller than the independent lidar observations by a difference of 0.03 to 0.08. This general behaviour is consistent with earlier studies based on AERONET version 2 products.
A lidar for measuring fluorescence from atmospheric aerosols was constructed with a third harmonic Nd:YAG laser, a 1-m diameter telescope, and a 32-channel time-resolved photon-counting spectrometer system. Fluorescence spectrum and vertical distribution of fluorescent aerosols in the lower atmosphere were observed during the nighttime with excitation at 355 nm. Relatively strong broad fluorescence was observed from Asian dust and air-pollution aerosols transported from urban and industrial areas. Rough estimates of the fluorescence efficiency were given for these aerosols. The intensity of the total fluorescence over the spectral range from 420 to 510 nm was comparable to that of nitrogen vibrational Raman scattering. That indicates the possibility of making a compact Raman-Mie-fluorescence lidar for aerosol monitoring.
<p><strong>Abstract.</strong> Six months of stratospheric aerosol observations with the European Aerosol Research Lidar Network (EARLINET) from August 2017 to January 2018 are presented. The decay phase of an unprecedented, record-breaking stratospheric perturbation caused by wild fire smoke is reported and discussed in terms of geometrical, optical, and microphysical aerosol properties. Enormous amounts of smoke (mainly soot particles) were injected into the upper troposphere and lower stratosphere over fire areas in western Canada on 12 August 2017 during strong thunderstorm-pyrocumulonimbus activity. The stratospheric smoke plumes spread over the entire northern hemisphere in the following weeks and months. Twenty-eight European lidar stations from northern Norway to southern Portugal and the Eastern Mediterranean monitored the strong stratospheric perturbation on a continental scale. The main smoke layer (over central, western, southern, and eastern Europe) was found between 15 and 20&#8201;km height since September 2017 (about two weeks after entering the stratosphere). Thin layers of smoke were detected to ascent to 22&#8211;24 km height. The stratospheric aerosol optical thickness at 532&#8201;nm decreased from values >&#8201;0.25 on 21&#8211;23 August 2017 to 0.005&#8211;0.03 until 5&#8211;10 September, and was mainly 0.003&#8211;0.004 from October to December 2017, and thus still significantly above the stratospheric background (0.001&#8211;0.002). Stratospheric particle extinction coefficients (532&#8201;nm) were as high as 50&#8211;200&#8201;Mm<sup>&#8722;1</sup> until the beginning of September and of the order of 1&#8201;Mm<sup>&#8722;1</sup> (0.5&#8211;5&#8201;Mm<sup>&#8722;1</sup>) from October 2017 until the end of January 2018. The corresponding layer mean particle mass concentration was of the order of 0.05&#8211;0.5&#8201;&#956;g&#8201;cm<sup>&#8722;3</sup> over the months. Soot is an efficient ice-nucleating particle (INP) at upper tropospheric (cirrus) temperatures and available to influence cirrus formation when entering the tropopause from above. We estimated INP concentrations of 50&#8211;500&#8201;L<sup>&#8722;1</sup> until the first days in September and afterwards 5&#8211;50&#8201;L<sup>&#8722;1</sup> until the end of the year 2018 in the lower stratosphere for typical cirrus formation temperatures of &#8722;55&#8201;&#176;C and ice supersaturation values of 1.15. The measured profiles of the particle linear depolarization rato indicated the predominance of non-spherical soot particles. The 532&#8201;nm depolarization ratio decreased with time in the main smoke layer from values of 0.15&#8211;0.25 (August&#8211;September) to values of 0.05&#8211;0.10 (October&#8211;November) and <&#8201;0.05 (December&#8211;January). The decrease of the depolarization ratio is consistent with the steady removal of the larger smoke particles by gravitational settling and changes in the particle shape with time towards a spherical form. An ascending layer with a vertical depth of 500&#8211;1000&#8201;m was detected (over the Eastern Mediterranean at 32&#8211;35&#176;&#8201;N) that ascended from about 18&#8211;19&#8201;km to 22&#8211;23&#8201;km height from the beginning of October to the beginning of December 2017 (about 2&#8201;km per month) and may be related to the increasing build up of the winter-hemispheric Brewer&#8211;Dobson circulation system.</p>
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