Abstract: Low energy collisions between molecules in the atmosphere lead to about 50% of the particles that act as the seeds for cloud droplets. Many of these molecules, and many of the other particles, are the result of human activity. Therefore cloud droplet concentrations have increased over the industrial period. The increase has led to a poorly quantified cooling effect on Earth that has offset perhaps a third of historical warming from greenhouse gases. The CLOUD experiment at CERN is a laboratory facility for the study of atmospheric particle formation. In my talk I will show how we are using results from this facility to represent this process better in climate models. As particle formation in the atmosphere is strongly dependent on meteorology, it is also critical to study how it happens in situ and to test our models with real observations. I will show how we are characterizing the process using aircraft measurements, in areas including the Front Range and the remote Southern Ocean.
Bio: Hamish Gordon is an associate professor in the Department of Chemical Engineering and the Center for Atmospheric Particle Studies at Carnegie Mellon University. His research interests are focused on the effects of air pollution and natural airborne particles on clouds and climate. He received his first degree from the University of Cambridge in 2009, and his doctorate from the University of Oxford in experimental high energy physics in 2013. After postdoctoral positions first at CERN and then at the University of Leeds, he moved to Carnegie Mellon in 2019. In 2025 he was awarded an NSF CAREER grant for a proposal titled "Role of new particle formation in pre-industrial climate" and served as a forecaster and mission scientist for the HALO-South aircraft campaign from Christchurch, New Zealand.