Ultrafast Photochemistry in Organic Aerosols

Much of the current discussion of the causes of global warming and mitigation of its impacts focuses on greenhouse gas emissions and net-zero CO2 targets. However, atmospheric aerosols also contribute significantly, and in ways that remain more poorly quantified, to the radiative balance that determines Earth’s climate. The PHAERO project addresses a major gap in our knowledge of the sunlight-driven photochemistry of organic aerosol particles in the lower atmosphere. For the first time, PHAERO will apply cutting-edge time-resolved spectroscopy and mass spectrometry methods to explore directly the photochemistry of organic compounds dissolved in aqueous aerosol droplets with picolitre volumes, and with femtosecond to microsecond time resolution. The chosen organic compounds represent chemical species proposed to be responsible for organic aerosol growth, as well as those present in brown-carbon aerosol particles formed by the burning of plants and trees, for example in wildfires and agricultural land clearance. This project is timely because of the increasing incidences worldwide of such fires and their associated emissions of particulate matter, as exemplified by poor air quality in many cities and major fires in Europe, north America and the Arctic. PHAERO will explore how the photochemical pathways of atmospherically important organic molecules are changed by confinement in micron-dimension aqueous aerosol droplets, for example through solute accumulation at the water-air interface, supersaturated concentrations in evaporating aerosols, and inhibited diffusion in viscous aerosol particles. The outcomes will advance our understanding of climate science and air quality. Applying the same methods to the photochemistry of droplets containing molecules of biological importance, the project will also provide new insights into the UV-induced degradation of the molecular components of pathogens in exhaled respiratory droplets by sunlight or germicidal UVC radiation.