Satellite retrievals of aerosol water uptake and their validation using ground-based, in-situ, and airborne campaign data

Aerosols affect climate in two main ways: directly, through scattering and absorption of solar radiation, and indirectly, through affecting cloud formation and cloud properties. Combined, these effects result in a net cooling, partially offsetting the warming caused by greenhouse gases (GHGs). While there is consensus about the existence of this cooling effect, its magnitude remains uncertain. This is problematic; as anthropogenic aerosol emissions decrease, we expect the cooling effect to diminish, leading to an enhanced warming effect from GHGs in the near future. Therefore, the accuracy of future warming projections strongly depends on our understanding of aerosols.

In particular, the hygroscopicity (i.e., efficiency of water uptake) of aerosols is a poorly understood property, yet highly influential on both cloud droplet nucleation capacity and light scattering. Typically, aerosols are complex mixtures of particles which themselves are a mixture of various materials, some of which are hydrophilic, and others hydrophobic. The distribution of these species, both within and among aerosol particles, is a key factor determining the effect of water uptake. As a result, different model treatments of aerosol compositions produce widely varying radiative forcing estimates. This strongly contributes to the uncertainty on the aerosol cooling term.

Global hygroscopicity data is crucial to inform model choices and thereby improve forcing estimates, except the available data is sparse and limited. Our goal is to address this gap by compiling polarimetric satellite observations from POLDER-PARASOL (for 2006-2010) and SPEXone-PACE (2024+) into the first ever global climatology of aerosol hygroscopicity. We use the RemoTAP algorithm to retrieve the aerosol refractive index, among other properties, from the multi-angle polarimeters. Through comparison of the retrieved refractive index to the known refractive indices for dry material and of water, a volume fraction of water in the aerosol is derived.

Given the novelty of our approach, validation of the retrievals is of particular importance. First, we compare the satellite retrievals to ground-based measurements such as refractive indices derived from AERONET observations. Furthermore, to validate our method of deriving the water fraction from the refractive index in general, we compare the retrievals to aerosol water fractions derived from ground-based in-situ nephelometer measurements of particle growth in response to changes in humidity, combined with ambient relative humidity measurements.

Another fundamental pillar of this validation is the PACE-PAX campaign, which was conducted in September 2024 with the express purpose of validating PACE and EarthCARE results. The campaign involved a high-altitude aircraft serving as a direct satellite proxy, which included the SPEX airborne instrument. Furthermore, one low-altitude aircraft gathering in-situ measurements, two boats, and a glider participated in the campaign. Observation targets included AERONET stations and satellite overpasses, providing ample opportunities for intercomparison between high-altitude aircraft observations and ground-based, aircraft in-situ, and satellite measurements.

We discuss our validation strategies and results, demonstrating the accuracy and limitations of our approach