Comment on acp-2022-594

Abstract

The co-transport of aerosol particles and water vapor has long been noted in the literature, with a myriad of implications from air mass characterization to radiative transfer. In this study, 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 AERONET observational data and the 16-year NAAPS reanalysis v1.0 model fields. The combination of AERONET observations with small associated uncertainties and the reanalysis fields with full 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 reanalysis datasets were compared against each other and showed consistency, indicating that the NAAPS reanalysis provides a realistic representation of the AOD and PW relationship. The initial analysis is then extended to layer AOD and PW relationships for proxies of the planetary boundary layer, and lower, middle and upper free troposphere. It was found that 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 aerosol for an individual event, but is more 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 boundary layer, lower/middle/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 variability between PW and relative humidity in the NAAPS-RA, hygroscopic growth was found to 1) amplify positive AOD-PW relationships, particularly in the mid-latitudes; 2) diminish negative relationships in dominant biomass burning regions; and 3) leads to statistically insignificant changes in PW for high AOD events in ocean regions. The importance of hygroscopic growth in these relationships indicates that PW is a useful tracer for AOD, but not necessarily as strongly for aerosol mass. African dust is an exception where PW is a strong tracer for aerosol shown by strong relationships even with hygroscopic affects. Given this 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 evaluating radiative impacts of aerosol, with the season and location in mind.