These droplets have volumes a billion times smaller than a teaspoon of water. In Earth’s atmosphere, droplets of this size are commonly found in clouds, fog and sea-spray, and they also form when organic particles from wildfires or human pollution condense water vapour from the surrounding air.

The PHAERO (Ultrafast Photochemistry in Organic Aerosols) project will explore how organic compounds in these microdroplets are affected by the ultraviolet A and ultraviolet B radiation in sunlight. The work will deepen our understanding of how organic aerosol particles change their chemical composition and physical properties over time in the Earth’s atmosphere. These changes determine how these airborne particles influence the Earth’s climate by absorption or scattering of sunlight, causing net warming or cooling respectively.

Using cutting-edge laser technology in a new laboratory funded by the ERC grant, the research will observe chemical changes in single droplets on ultrafast timescales corresponding to the breaking of chemical bonds or collisions between molecules in solution. By developing a better understanding of how UV light interacts with molecules confined in small droplets, the project will help to unravel the fates of the particulate matter adversely affecting air quality in our cities, and emitted from the growing incidences of wildfires.

The outcomes will also find broader applications, for example in developing photochemistry in small droplets as a new strategy for sustainable synthesis of fine chemicals, or use of germicidal ultraviolet C irradiation to prevent the spread of pathogens in exhaled respiratory droplets.

Professor Orr-Ewing is one of 281 leading researchers across Europe to be awarded an Advanced Grant in the latest competition. This prestigious scheme gives senior researchers the opportunity to pursue ambitious, curiosity-driven projects that could lead to major scientific breakthroughs.