Exposure to atmospheric particles has been shown to be related to significant negative health effects and the World Health Organisation identified air pollution particles as the worldwide most severe and urgent public health issue, linked to over three million premature deaths per year worldwide. Despite the decade-long evidence (based on epidemiological and laboratory studies) that link inhalation of ambient aerosol particles with diseases such as respiratory and cardiovascular diseases and lung cancer, it is unclear which particle properties and components are responsible for their toxic effects. It has been proposed that a wide range of oxidising particle components such as peroxides and radicals (so-called ROS, reactive oxygen species), which are present in the particles or which are generated when particles get deposited on the surface of the lung, might be key in explaining atmospheric particle toxicity. However, firm data about oxidising properties of atmospheric aerosol particle is scarce due the unavailability of suitable analytical methods and instruments suitable for field measurements. In addition to the lack of a fundamental understanding of ROS sources in the atmosphere, we recently showed that a up to half or more of the total ROS is short-lived (due to their highly reactive nature) with a lifetime of only a few minutes and therefore conventional filter-based offline analysis methods to quantify ROS are likely underestimating strongly true ROS concentrations. A better understanding of the fundamental particle properties that cause negative health effects and improved measurement capabilities would allow to identify the most toxic particle sources and would ultimately provide the basis for future more effective and efficient air pollution reduction policies. Recognising the potential key importance of ROS in explaining atmospheric particle toxicity but also the significant shortcomings of current methods quantifying and characterising ROS, we therefore aim:(1)to quantify the true total levels of ROS concentrations in ambient atmospheric aerosol in extended field campaigns using our novel online ROS instrument,(2)to identify sources of ROS in aerosol particles and assess the toxicity of these particle sources using lung cell cultures by performing a range of laboratory experiments, (3)to develop a fundamental molecular-level understanding of components contributing to the total particle-bound ROS concentrations, especially organic peroxides, and to develop new methods to characterise and quantify peroxides in laboratory and field experiments.The project proposed here is a strongly interlinked lab and field project, which will significantly advance our knowledge about reactive oxygen species in atmospheric particles, their formation processes, sources and toxicity. This will be essential for a comprehensive understanding of air pollution particle toxicity and to devise improved and efficient air pollution mitigation policies.