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We report the recovery and processing methodology of the first ever multi-year lidar dataset of the stratospheric aerosol layer. A Q-switched ruby lidar measured 66 vertical profiles of 694 nm attenuated backscatter at Lexington, Massachusetts, between January 1964 and August 1965, with an additional nine profile measurements conducted from College, Alaska, during July and August 1964. We describe the processing of the recovered lidar backscattering ratio profiles to produce mid-visible (532 nm) stratospheric aerosol extinction profiles (sAEP 532) and stratospheric aerosol optical depth (sAOD 532) measurements, utilizing a number of contemporary measurements of several different atmospheric variables. Stratospheric soundings of temperature and pressure generate an accurate local molecular backscattering profile, with nearby ozone soundings determining the ozone absorption, which are used to correct for two-way ozone transmittance. Two-way aerosol transmittance corrections are also applied based on nearby observations of total aerosol optical depth (across the troposphere and stratosphere) from sun photometer measurements. We show that accounting for these two-way transmittance effects substantially increases the magnitude of the 1964/1965 stratospheric aerosol layer's optical thickness in the Northern Hemisphere mid-latitudes, then ∼ 50 % larger than represented in the Coupled Model Intercomparison Project 6 (CMIP6) volcanic forcing dataset. Compared to the uncorrected dataset, the combined transmittance correction increases the sAOD 532 by up to 66 % for Lexington and up to 27 % for Fairbanks, as well as individual sAEP 532 adjustments of similar magnitude. Comparisons with the few contemporary measurements available show better agreement with the corrected two-way transmittance values. Within the January 1964 to August 1965 measurement time span, the corrected Lexington sAOD 532 time series is substantially above 0.05 in three distinct periods, October 1964, March 1965, and May–June 1965, whereas the 6 nights the lidar measured in December 1964 and January 1965 had sAOD values of at most ∼ 0.03. The comparison with interactive stratospheric aerosol model simulations of the Agung aerosol cloud shows that, although substantial variation in mid-latitude sAOD 532 are expected from the seasonal cycle in the stratospheric circulation, the Agung cloud's dispersion from the tropics would have been at its strongest in winter and weakest in summer. The increasing trend in sAOD from January to July 1965, also considering the large variability, suggests that the observed variations are from a different source than Agung, possibly from one or both of the two eruptions that occurred in 1964/1965 with a Volcanic Explosivity Index (VEI) of 3: Trident, Alaska, and Vestmannaeyjar, Heimaey, south of Iceland. A detailed error analysis of the uncertainties in each of the variables involved in the processing chain was conducted. Relative errors for the uncorrected sAEP 532 were 54 % for Fairbanks and 44 % Lexington. For the corrected sAEP 532 the errors were 61 % and 64 %, respectively. The analysis of the uncertainties identified variables that with additional data recovery and reprocessing could reduce these relative error levels. Data described in this work are available at 10.1594/PANGAEA.922105 (Antuña-Marrero et al., 2020a). [ABSTRACT FROM AUTHOR]

We report the recovery and processing methodology of the first ever multi-year lidar dataset of the stratospheric aerosol layer. A Q-switched Ruby lidar measured 66 vertical profiles of 694nm attenuated backscatter at Lexington, Massachusetts between January 1964 and August 1965, with an additional 9 profile measurements conducted from College, Alaska during July and August 1964. We describe the processing of the recovered lidar backscattering ratio profiles to produce mid-visible (532nm) stratospheric aerosol extinction profiles (sAEP532) and stratospheric aerosol optical depth (sAOD532) measurements, utilizing a number of contemporary measurements of several different atmospheric variables. Stratospheric soundings of temperature, and pressure generate an accurate local molecular backscattering profile, with nearby ozone soundings determining the ozone absorption, those profiles then used to correct for two-way ozone transmittance. Two-way aerosol transmittance corrections were also applied based on nearby observations of total aerosol optical depth (across the troposphere and stratosphere) from sun photometer measurements . We show the two-way transmittance correction has substantial effects on the retrieved sAEP532 and sAOD532, calculated without the corrections resulting in substantially lower values of both variables, as it was not applied in the original processing producing the lid ar scattering ratio profiles we rescued. The combined transmittance corrections causes the aerosol extinction to increase by 67 % for Lexington and 27 % for Fairbanks, for sAOD532 the increases 66 % and 26 % respectively. Comparing the magnitudes of the aerosol extinction and sAOD with the few contemporary available measurements reported show a better agreement in the case of the two way transmittance corrected values. The sAEP and sAOD timeseries at Lexington show a surprisingly large degree of variability, three periods where the stratospheric aerosol layer had suddenly elevated optical thickness, the highest sAOD532 of 0.07 measured at the end of March 1965. The two other periods of enhanced sAOD532 are both two-month periods where the lidars show more than 1 night where ret rieved sAOD532 exceeded 0.05: in January and February 1964 and November and December 1964. Interactive stratospheric aerosol model simulations of the 1963 Agung cloud illustrate that although substantial variation in mid-latitude sAOD532 is expected from the seasonal cycle in the Brewer-Dobson circulation, the Agung cloud dispersion will have caused much slower increase than the more episodic variations observed, with also different timing, elevated optical thickness from Agung occurring in winter and spring. The abruptness and timing of the steadily increasing sAOD from January to July 1965 suggests this variation was from a different source than Agung, possibly from one or both of the two VEI3 eruptions that occurred in 1964/65: Trident, Alaska and Vestmannaeyjar, Heimey, south of Iceland. A detailed error analysis of the uncertainties in each of the variables involved in the processing chain was conducted, relative errors of 54 % for Fairbanks and 44 % Lexington for the uncorrected sAEP532, corrected sAEP532 of 61 % and 64 % respectively. The analysis of the uncertainties identified variables that, with additional data recovery and reprocessing could reduce these relative error levels. Data described in this work are available at https://doi.pangaea.de/10.1594/PANGAEA.922105 (Dataset in Review) (Antuña-Marrero et al., 2020a). [ABSTRACT FROM AUTHOR]