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The chemistry of the formation of nitrogen-containing compounds in secondary organic aerosols and their impact on aerosol optical properties

Subject Area Atmospheric Science
Term from 2014 to 2017
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 252333154
 
Final Report Year 2017

Final Report Abstract

One of the main open questions regarding organic compounds in atmospheric chemistry today is related to the formation of light-absorbing compounds and the occurrence of socalled brown carbon. Ambient secondary organic aerosols have been assumed to not absorb solar radiation and to have a cooling effect on climate for decades. However, it is now generally accepted that a continuum of light-absorbing carbonaceous species is present in these fine particles. In this project, we studied the formation of nitrogencontaining compounds in organic aerosols. In systematic experiments, reactions between dicarbonyl compounds and reduced inorganic and organic amine components were found to be efficient pathways for the formation of a variety of nitrogen-containing heterocyclic products, which can absorb light in the actinic flux region of the UV/Vis spectrum. These products appear to be common structural motifs of secondary brown carbon and could potentially affect the absorption properties of secondary organic aerosols in the atmosphere. As of now, they have not been included in corresponding model simulations of aerosol-induced radiative forcing. Furthermore, mixtures of different dicarbonyl species in ammonium sulfate solutions yielded light-absorbing cross-reaction products. These synergistic effects of cross-reactions on secondary brown carbon formation were observed for mixtures of small carbonyl compounds, e.g., acetylacetone and acetaldehyde forming a strongly absorbing dihydropyridine. Mixtures of small carbonyl compounds with an atmospherically relevant monoterpene ozonolysis intermediate, e.g., the second-generation ozonolysis product of limonene ketolimononaldehyde, also showed synergistic effects for brown carbon formation due to cross-reaction products. Our results indicate that secondary brown carbon formation via the imine pathway may be a ubiquitous process with potential effects on particle properties such as absorbance, viscosity, and particle growth. Future research may focus the identification of suitable marker substances for dicarbonyl amine reactions in ambient aerosol samples and SOA particle heterogeneous chemistry induced by the photosensitization of nitrogencontaining brown carbon constituents.

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