  Ground-based astronomy has to correct astronomical observations from the
impact of the atmospheric transparency and its variability.The current
objective of several observatories is to achieve a sub-percent level monitoring
of atmospheric transmission. A promising approach has been to combine internal
calibration of the observations with various external meteorological data
sources, upon avail-ability and depending on quality. In this paper we
investigate the use of the NASA Modern-Era Retrospective Analysis for Research
and Applications, version 2 (MERRA-2) which is a general circulation model
(GCM) and data assimilation system that renders freely available for any given
site, at any time, all the parameters constraining atmospheric transmission.
This paper demonstrates the extraction of the relevant atmospheric parameters
for optical astronomy at two sites: Mauna Kea in Hawaii and Cerro Tololo
InternationalObservatory in Chile. The temporal variability for the past eight
years (annual, overnight and hourly), as well as the spatial gradients of
ozone, precipitable water vapor, and aerosol optical depth is presented and
their respective impacts on the atmospheric transparency is analyzed.
