Abstract
Aerosol hygroscopicity is a critical parameter for predicting radiative forcing and climate sensitivity, particularly under sub-saturated regimes where it drives complex aerosol–water interactions. Here, we show that externally mixed aerosols exert a stronger influence on direct radiative forcing than is currently represented in models. Incorporating our findings into radiative forcing calculations indicates a stronger aerosol cooling effect, especially at suburban sites, highlighting the importance of representing regional differences in mixing state. The conventional bulk-chemistry approach, which assumes volume-based mixing with limited spatial variability, exhibits low predictive performance for aerosol hygroscopicity (R² ≈ 0.61) at urban and suburban sites. Using an interpretable machine learning framework trained on geographically diverse, region-specific datasets can capture this variability with higher accuracy (R² ≈ 0.97), identifying key chemical compositional and mixing-state drivers. (Figure presented.) | PFC, JMB, and JGV work was funded by (grant nos. PID2022-138155OB-I00 and MCIN/AEI/10.13039/501100011033 by ERDF A way of making Europe), Spain, and by (grant nos. 2021SGR-01059 and AGAUR 2023 CLIMA 0097) AGAUR regional projects, Spain. PK acknowledges support from the RECLAIM Network Plus (EP/W034034/1) project. PF and BD acknowledge the AEROCLO-sA project supported by the French National Research Agency (ANR) under (grant no. ANR-15-CE01-0014-01), the French national program LEFE/INSU, the Programme national de Télédetection Spatiale (PNTS, grant no. PNTS-2016-14), the French National Agency for Space Studies (CNES), and the South African National Research Foundation (NRF) under (grant UID 105958. PEGASUS receives funding as a national facility of the Institut National des Sciences de l’Univers du Centre national de la recherche scientifique (CNRS INSU) within the research infrastructure ACTRIS-FR. MDP acknowledges support from the U.S. Department of Energy, Office of Science, Biological and Environmental Research grant no. DE-SC0021074. A.C. and S.Bwork was supported by the European Union s Horizon 2020 research and innovation programme under grant agreement no.856612 (EMME-CARE) and by the STDF-AUF-IRD launched in 2016 (POLCAIR ‘Air Pollution in Cairo: sources and impact). The Deutsche Forschungsgemeinschaft (DFG) has supported this research (grant no. WE 2757/4-1). SD, MP, LP, BR, SH, and BW are funded and supported by the DFG. Managed by Leibniz Institute for Tropospheric Research (TROPOS). | Peer reviewed