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 for the CESAM chamber and instrumentation for ORACLE campaign measurements (A). Acronyms are as follows: Aerosol aerodynamic classifier (AAC), Scanning mobility particle sizer (Wet SMPS) for wet diameter, Mobility particle size spectrometer (MPSS, referred to as dry SMPS) for dry diameter. Blue rectangle symbols indicate the locations of sensors measuring pressure (P), temperature (T), and relative humidity (RH). The chamber RH is controlled by the HUMidificAtioN System (HUMANS). An Aerodyne HR-ToF-AMS continuously monitors the particle chemical composition. Snippet from the campaign and instrument attached to the CESAM chamber (B). Injecting SDS (Sodium Dodecyl Sulfate) for surface tension experiments showing the bubble in the jar (C). Complete manual cleaning on the CESAM chamber, snippet from inside the chamber (D).