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.) ... Read more

The goal of the CAINA (Cloud-Aerosol Interactions in a Nitrogen-dominated Atmosphere) project is to investigate multiple aspects of aerosol-cloud interactions under high concentrations of reactive nitrogen. This chemical regime is starting to emerge in many regions following the strong reduction of SO2 emissions, but is already firmly established at our study location in the Netherlands. CAINA is a consortium project that aims to combine in-situ and remote sensing observations of aerosols and clouds with chamber experiments and high-resolution modelling to study the formation of CCN, cloud chemistry, and aerosol effects on clouds.

This talk will present first highlights of the CAINA project focussing on the cloud chamber
experiments and the field campaign conducted in March/April 2025.

Extensive studies in the AIDA cloud chamber have shown that substantially more secondary organic aerosol is formed under high humidity (80-90%) than at dry conditions, when liquid seed particles are present. This is accompanied with distinct differences in the chemical composition of the formed SOA. We can show considerable formation of secondary organic aerosol in the aqueous phase and that the presence of ammonium nitrate in the particles causes the formation of organic nitrogen species and other higher-order reaction products.

First results from the field campaign at a coastal and a regional background site in the Netherlands highlight the high ammonium nitrate contributions to the aerosol mass concentration and especially high gas-phase NH3 concentrations (up to 50 mg m-3) during the field campaign, indicating a chemical regime dominated by reactive nitrogen and relatively high aerosol pH. Further highlights include strong new particle formation events, as well as distinct differences in particle chemical composition between the ground and at 250 m height, particularly when clouds were overhead. A potential effect of nitrogen pollution on cloud properties will be investigated, combining ground-based data, remote sensing by cloud profilers, and in-situ cloud measurements using the helicopter-borne cloud probe ACTOS.
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Aerosol absorption profiling is crucial for radiative transfer calculations and climate modelling. Here, we utilize the synergy of lidar with sun-photometer measurements to derive the absorption coefficient and single scattering albedo profiles during the ACTRIS-2 campaigns held in Germany, Greece and Cyprus. The remote sensing techniques are compared with in situ measurements in order to harmonize and validate the different methodologies and reduce the absorption profiling uncertainties. ... Read more