ORACLE Project – Organic Aerosol–Cloud Interaction Studies

Cloud formation remains one of the largest uncertainties in climate modelling due to a limited understanding of how organic compounds influence aerosol-cloud activation. The ORACLE project addresses this uncertainty by investigating how semi-volatile organic compounds modify aerosol hygroscopic growth and CCN activity via co-condensation.

The project combines controlled chamber experiments with advanced aerosol instrumentation to simulate atmospheric multiphase interactions. Experiments were conducted in the CESAM atmospheric simulation chamber, where monodisperse aerosol particles were equilibrated with organic vapours under controlled relative humidity conditions. HTDMA measurements quantified hygroscopic growth, while CCN activation experiments provided complementary information on droplet formation efficiency.

TROPOS participated in one month of chamber experiments at CESAM (Fig. 1A). A key focus of the study was understanding how organic compounds influence particle growth and surface tension, both critical parameters for cloud droplet formation. Additional experiments evaluated evaporation losses in commercial CCN counters using a custom-built diffusion chamber operated in parallel with HTDMA measurements (Fig. 1B). Quantifying the uptake onto the aerosol at RH 90% (HTDMA) and monitoring the loss of water and the increase in organic gas-phase concentration when RH is reduced to 45%, where particles are still deliquesced but at drier conditions—repeating this for different mixtures of SVOCs and SDS (Fig. 1C).

Scientific outcomes:

  • Mechanistic understanding of organic aerosol co-condensation effects on particle growth
  • Improved quantification of aerosol surface tension and its impact on CCN activation
  • Enhanced predictive capability for cloud droplet number concentration and precipitation processes in climate models

Figure 1: Detailed schematic of the experimental setup of the CESAM chamber and the instrumentation used for measurements during the ORACLE campaign (A). The abbreviations stand for: Aerosol Aerodynamic Classifier (AAC), Scanning Mobility Particle Sizer (Wet SMPS) for determining the wet particle diameter, and Mobility Particle Size Spectrometer (MPSS, referred to as dry SMPS) for determining the dry diameter. Blue rectangles indicate the positions of sensors measuring pressure (P), temperature (T), and relative humidity (RH). The relative humidity in the chamber is controlled by the HUMidificAtioN System (HUMANS). An Aerodyne HR-ToF-AMS continuously monitors the chemical composition of the particles. Inset from the campaign showing instrumentation connected to the CESAM chamber (B). Addition of SDS (sodium dodecyl sulfate) for surface tension experiments, visible through bubble formation in the vessel (C). Complete manual cleaning of the CESAM chamber, view into the interior of the chamber (D).