Remote Sensing of Aerosol Water: First Look at a Climatology on Water Uptake

Aerosols play an important role in governing the Earth's radiation budget. In addition to scattering and absorbing radiation themselves, they affect the formation and properties of clouds. In both of these processes, and especially in the latter, are affected by the uptake of water. Nevertheless, the particulars of the uptake of water by aerosols remain poorly understood. The efficiency of water uptake (i.e., hygroscopicity) for a given aerosol is highly sensitive to its composition, history, and mixing state, making it a difficult property to model or predict. This leads to stark disagreements between models using different aerosol prescriptions, which greatly contributes to the large uncertainties in the resulting radiative forcing estimates. A better understanding of aerosol water uptake is therefore crucial for accurate warming predictions.

This understanding is currently held back by a lack of data. In most cases, in-situ measurements of aerosol properties are preceded by a drying step that removes any information about water content, so hygroscopicity data is only available for the small subset of studies where it is explicitly targeted. As such the spatial and temporal coverage of these data are very limited. To properly inform and constrain model choices, then, a satellite dataset would be incredibly valuable.

While the aerosol water uptake is a difficult property to measure from space, the rich information content of multi-angle polarimeter (MAP) nstruments such as POLDER-PARASOL and SPEXone-PACE presents new opportunities. We aim to use these instruments to produce a satellite dataset of the aerosol water content, and use it to assemble a first-of-its-kind global climatology on aerosol hygroscopicity. To retrieve a volume water fraction we compare the retrieved real component of the refractive index to an average refractive index for dry material and the known refractive index of pure water, assuming a linear scaling with the volume fraction. Here, refractive indices are retrieved from MAP measurements using the RemoTAP algorithm. The resulting water content measurements can then be used in combination with ambient relative humidity data from reanalysis products to estimate the hygroscopicity.

Recent years have seen efforts to validate these volume water fraction retrievals using both airborne (campaign) and ground-based in-situ measurements, with promising results. We now feel sufficiently confident to begin assembling the data into a global climatology, beginning with the POLDER era. We investigate regional and seasonal trends in the data, and compare them to the corresponding average relative humidities from ERA5 reanalysis to get an indication of hygroscopicity. Initial findings include a clear land/ocean divide, as well as a north-south contrast consistent with pollution patterns. Regions known for biomass burning are additionally investigated for seasonal patterns.

We briefly review the results of the aforementioned validation process, and present a first look at a global climatology on water uptake for the years 2006 through 2009.