A global evaluation of daily to seasonal aerosol and water vapor relationships using a combination of AERONET and NAAPS reanalysis data
Authors: Rubin Juli I. , Reid Jeffrey S. , Xian Peng , Selman Christopher M. , Eck Thomas F. 
Published: April 2023

Abstract. The co-transport of aerosol particles and water vapor has
long been noted in the literature, with a myriad of implications such as air
mass characterization, radiative transfer, and data assimilation. Here, the
relationship between aerosol optical depth (AOD) and precipitable water
vapor (PW) is evaluated to our knowledge for the first time globally, at
daily to seasonal levels using approximately 20 years of NASA Aerosol Robotic Network (AERONET)
observational data and the 16-year Navy Aerosol Analysis Prediction System (NAAPS) reanalysis v1.0 (NAAPS-RA) model
fields. The combination of AERONET observations with small uncertainties and
the reanalysis fields with global coverage is used to provide a best
estimate of the seasonal AOD and PW relationships, including an evaluation
of correlations, slope, and PW probability distributions for identification
of statistically significant differences in PW for high-AOD events. The
relationships produced from the AERONET and NAAPS-RA datasets were compared
against each other and showed consistency, indicating that the NAAPS-RA
provides a realistic representation of the AOD and PW relationship. The
analysis includes layer AOD and PW relationships for proxies of the
planetary boundary layer and the lower, middle, and upper free troposphere. The
dominant AOD and PW relationship is positive, supported by both AERONET and
model evaluation, which varies in strength by season and location. These
relationships were found to be statistically significant and present across
the globe, observed on an event-by-event level. Evaluations at individual
AERONET sites implicate synoptic-scale transport as a contributing factor in
these relationships at daily levels. Negative AOD and PW relationships were
identified and predominantly associated with regional dry-season timescales
in which biomass burning is the predominant aerosol type. This is not an
indication of dry-air association with smoke for an individual event but is
a reflection of the overall dry conditions leading to more biomass burning
and higher associated AOD values. Stronger correlations between AOD and PW
are found when evaluating the data by vertical layers, including the boundary
layer and the lower, middle, and upper free troposphere (corresponding to typical water
vapor channels), with the largest correlations observed in the free
troposphere – indicative of aerosol and water vapor transport events. By
evaluating the variability between PW and relative humidity in the NAAPS-RA,
hygroscopic growth was found to be a dominant term to (1) amplify positive
AOD–PW relationships, particularly in the midlatitudes; (2) diminish
negative relationships in dominant biomass burning regions; and (3) lead to
statistically insignificant changes in PW for high-AOD events for maritime
regions. The importance of hygroscopic growth in these relationships
indicates that PW is a useful tracer for AOD or light extinction but not
necessarily as strongly for aerosol mass. Synoptic-scale African dust events
are an exception where PW is a strong tracer for aerosol transport shown by
strong relationships even with hygroscopic effects. Given these results, PW
can be exploited in coupled aerosol and meteorology data assimilation for
AOD, and the collocation of aerosol and water vapor should be carefully taken
into account when conducting particulate matter (PM) retrievals from space
and in evaluating radiative impacts of aerosol, with the season and location
in mind.