Cloud–aerosol interactions in a nitrogen-dominated atmosphere – particle formation, activation, and turbulence (CAINA-NAT)

The formation and properties of clouds in the atmosphere are not determined solely by water vapor saturation, but to a large extent also by the presence of suitable aerosol particles. In particular, cloud edges often provide favorable conditions for new particle formation, resulting in a close coupling between aerosol and cloud microphysics. For reliable predictions of regional weather and climate scenarios, not only accurate input data—such as particle size distributions and meteorological parameters—are crucial, but also a sound understanding of the underlying processes.

In spring 2025, an international field experiment is planned at the Dutch coast to investigate the influence of nitrogen compounds on cloud properties. In spring, the region exhibits particularly high near-surface nitrate concentrations caused by agricultural emissions. The Leibniz Institute for Tropospheric Research (TROPOS) will participate in the campaign with the helicopter-borne ACTOS platform. Within the funded CAINA project (Cloud–Aerosol Interactions in a Nitrogen-dominated Atmosphere), cloud water will be collected for chemical analyses.

The ACTOS platform also offers the possibility to integrate additional sensors for the detailed characterization of aerosol particles and cloud droplets and to deploy them during the flight campaign. This allows, among other things, the measurement of the vertical distribution of aerosols and the targeted identification of regions with enhanced concentrations of small, presumably freshly formed particles. Parameters recorded include, among others, the particle number size distribution (PNSD), the droplet number size distribution (DNSD), the total particle number concentration, the liquid water content, and relevant meteorological parameters. A particular advantage of the ACTOS platform is its low flight speed of approximately 20 m/s, which enables very high spatial resolution of the measurements.

The collected PNSD and DNSD data will be jointly analyzed to investigate, among other aspects, relationships with the origin of air masses. For example, maritime air masses are expected to contain fewer aerosol particles, which would be reflected in clouds with fewer but larger droplets. Measured parameters such as water vapor content, temperature, and particle profiles will also serve as input data for the spectral cloud microphysics model SPECS, which allows the simulation of cloud formation under varying vertical wind speeds and particle compositions. The simulation results will subsequently be compared with the observations.

Within the project team, a doctoral candidate, who is currently completing her Master’s thesis at TROPOS, will be responsible for conducting and analyzing the aerosol measurements. Cloud microphysics and modeling will be handled by a postdoctoral researcher. The final integration and interpretation of the data will be the responsibility of the doctoral candidate. Comparable measurement campaigns focusing on aerosol and cloud microphysics have already been carried out in Barbados and the Azores, where, for example, new particle formation at cloud edges has frequently been documented. However, a comprehensive investigation of droplet formation processes is still lacking and will be systematically conducted for all collected data sets within the framework of this project.