Recovery of the first ever multi-year lidar dataset of the stratospheric aerosol layer, from Lexington, MA, and Fairbanks, AK, January 1964 to July 1965
Authors: Antuña-Marrero Juan-Carlos , Mann Graham W. , Barnes John , Rodríguez-Vega Albeht , Shallcross Sarah , Dhomse Sandip S. , Fiocco Giorgio , Grams Gerald W. 
Published: September 2021

Abstract. 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 (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, 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 sAOD532 by up to 66 % for Lexington and up to 27 %
for Fairbanks, as well as individual sAEP532 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 sAOD532 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 sAOD532 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 sAEP532 were 54 % for
Fairbanks and 44 % Lexington. For the corrected sAEP532 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 https://doi.org/10.1594/PANGAEA.922105 (Antuña-Marrero et al., 2020a).