A Global Evaluation of Daily to Seasonal Aerosol and Water Vapor Relationships Using a Combination of AERONET and NAAPS Reanalysis Data

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https://acp.copernicus.org/articles/23/4059/2023/

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https://doi.org/10.5194/acp-23-4059-2023
 http://hdl.handle.net/11603/26267

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https://orcid.org/0000-0001-9801-1610

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2023-04-05

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journal articles
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Rubin, J. I., Reid, J. S., Xian, P., Selman, C. M., and Eck, T. F.: A global evaluation of daily to seasonal aerosol and water vapor relationships using a combination of AERONET and NAAPS reanalysis data, Atmos. Chem. Phys., 23, 4059–4090, https://doi.org/10.5194/acp-23-4059-2023, 2023.

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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract

The co-transport of aerosol particles and water vapor has long been noted in the literature, with a myriad of 14 implications from air mass characterization to radiative transfer. In this study, the relationship between aerosol optical 15 depth (AOD) and precipitable water vapor (PW) is evaluated to our knowledge for the first time globally, at daily to 16 seasonal levels using approximately 20 years of AERONET observational data and the 16-year NAAPS reanalysis 17 v1.0 (NAAPS-RA) model fields. The combination of AERONET observations with small associated uncertainties and 18 the reanalysis fields with full global coverage is used to provide a best estimate of the seasonal AOD and PW 19 relationships, including an evaluation of correlations, slope, and PW probability distributions for identification of 20 statistically significant differences in PW for high AOD events. The relationships produced from the AERONET and 21 NAAPS-RA datasets were compared against each other and showed consistency, indicating that the NAAPS-RA 22 provides a realistic representation of the AOD and PW relationship. The initial analysis is then extended to layer AOD 23 and PW relationships for proxies of the planetary boundary layer, and lower, middle and upper free troposphere. It 24 was found that the dominant AOD and PW relationship is positive, supported by both AERONET and model 25 evaluation, which varies in strength by season and location. These relationships were found to be statistically 26 significant and present across the globe, observed on an event by event level. Evaluations at individual AERONET 27 sites implicate synoptic-scale transport as a contributing factor in these relationships at daily levels. Negative AOD 28 and PW relationships were identified and predominantly associated with regional dry season timescales in which 29 biomass burning is the predominant aerosol type. This is not an indication of dry air association with smoke aerosol 30 for an individual event, but is more a reflection of the overall dry conditions leading to more biomass burning and 31 higher associated AOD values. Stronger correlations between AOD and PW are found when evaluating the data by 32 vertical layers, including boundary layer, lower/middle/upper free troposphere (corresponding to typical water vapor 33 channels), with the largest correlations observed in the free troposphere-indicative of aerosol and water vapor transport 34 events. By evaluating variability between PW and relative humidity in the NAAPS-RA, hygroscopic growth was 35 found to 1) amplify positive AOD-PW relationships, particularly in the mid-latitudes; 2) diminish negative 36 relationships in dominant biomass burning regions; and 3) leads to statistically insignificant changes in PW for high 37 AOD events in ocean regions. The importance of hygroscopic growth in these relationships indicates that PW is a 38 useful tracer for AOD, but not necessarily as strongly for aerosol mass. Synoptic-scale African dust events are an 39 exception where PW is a strong tracer for aerosol shown by strong relationships even with hygroscopic affects. Given 40 this results, PW can be exploited in coupled aerosol and meteorology data assimilation for AOD and the collocation 41 of aerosol and water vapor should be carefully taken into account when evaluating radiative impacts of aerosol, with 42 the season and location in mind.