A global evaluation of daily to seasonal aerosol and water vaporrelationships using a combination of AERONET and NAAPS reanalysis data

The co-transport of aerosol particles and water vapor haslong been noted in the literature, with a myriad of implications such as airmass characterization, radiative transfer, and data assimilation. Here, therelationship between aerosol optical depth (AOD) and precipitable watervapor (PW) is evaluated to our knowledge for the first time globally, atdaily 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) modelfields. The combination of AERONET observations with small uncertainties andthe reanalysis fields with global coverage is used to provide a bestestimate of the seasonal AOD and PW relationships, including an evaluationof correlations, slope, and PW probability distributions for identificationof statistically significant differences in PW for high-AOD events. Therelationships produced from the AERONET and NAAPS-RA datasets were comparedagainst each other and showed consistency, indicating that the NAAPS-RAprovides a realistic representation of the AOD and PW relationship. Theanalysis includes layer AOD and PW relationships for proxies of theplanetary boundary layer and the lower, middle, and upper free troposphere. Thedominant AOD and PW relationship is positive, supported by both AERONET andmodel evaluation, which varies in strength by season and location. Theserelationships were found to be statistically significant and present acrossthe globe, observed on an event-by-event level. Evaluations at individualAERONET sites implicate synoptic-scale transport as a contributing factor inthese relationships at daily levels. Negative AOD and PW relationships wereidentified and predominantly associated with regional dry-season timescalesin which biomass burning is the predominant aerosol type. This is not anindication of dry-air association with smoke for an individual event but isa reflection of the overall dry conditions leading to more biomass burningand higher associated AOD values. Stronger correlations between AOD and PWare found when evaluating the data by vertical layers, including the boundarylayer and the lower, middle, and upper free troposphere (corresponding to typical watervapor channels), with the largest correlations observed in the freetroposphere - indicative of aerosol and water vapor transport events. Byevaluating the variability between PW and relative humidity in the NAAPS-RA,hygroscopic growth was found to be a dominant term to (1) amplify positiveAOD-PW relationships, particularly in the midlatitudes; (2) diminishnegative relationships in dominant biomass burning regions; and (3) lead tostatistically insignificant changes in PW for high-AOD events for maritimeregions. The importance of hygroscopic growth in these relationshipsindicates that PW is a useful tracer for AOD or light extinction but notnecessarily as strongly for aerosol mass. Synoptic-scale African dust eventsare an exception where PW is a strong tracer for aerosol transport shown bystrong relationships even with hygroscopic effects. Given these results, PWcan be exploited in coupled aerosol and meteorology data assimilation forAOD, and the collocation of aerosol and water vapor should be carefully takeninto account when conducting particulate matter (PM) retrievals from spaceand in evaluating radiative impacts of aerosol, with the season and locationin mind.