Organic material is the most abundant component in atmospheric particles. The composition and potential toxicity of the organic material is only poorly understood, partly due to the high complexity of the organic compounds present in aerosol particles. This project develops novel high pressure liquid chromatography ultra-high resolution mass spectrometry methods to identify and quantify organic aerosol components that are characteristic for specific sources and which are potentially causing their toxicity, with a focus on peroxides and organic radicals.