Peering inside common atmospheric particles is providing important clues to their climate and health effects, according to a new study by chemists. Secondary organic aerosol (SOA) particles are ubiquitous in the atmosphere and play an important role in air quality and climate. They can add to air pollution and damage the lungs, as well as help deflect solar radiation or aid cloud formation. Different types of SOA can mix together in a single particle and their environmental impacts are governed by the new particles’ physical and chemical properties, particularly the number of phases –or states– it can exist in. In a new research letter published in the European Geosciences Union’s open access journal Atmospheric Chemistry and Physics, an international team of researchers found that particles with two phases can form when different types of SOA mix. The finding could help improve current models that predict SOA climate and health effects.