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

• Applicant: Dr. Christopher Johannes Kampf
• Subject Area: Atmospheric Science
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 252333154

The formation of light-absorbing organic compounds, also termed brown carbon (BrC) chromophores, in secondary organic aerosols (SOA) can alter the optical properties of atmospheric fine particulate matter (sub-micrometer particles). Nitrogen-containing compounds produced by secondary reactions between reactive carbonyl species and amine compounds following an imine pathway, are subject of recent research activities and receive increasing attention in the literature. In contrast to local emissions by incomplete combustion processes, secondary BrC chromophores can be formed throughout the Troposphere, potentially affecting the global radiation budget and heterogeneous aerosol chemistry.In the initial project, analytical methods for bulk and molecular level analysis of BrC chromophores in reaction systems mimicking aqueous atmospheric aerosol particles, containing 1,2-, 1,3-, 1,4-, or 1,5-dicarbonyls and ammonium sulfate or the amino acid glycine, have been established and applied, also in further studies on nitrogen containing compounds in biogenic aerosols. The methods include spectrophotometry and state of the art chromatography and mass spectrometry. Several classes of nitrogen heterocycles with pH dependent yields have been identified in the studied reaction systems, including imidazoles, pyrroles, pyrrole dimers (dipyrromethenes), and (dihydro-)pyridines, demonstrating that nitrogen containing heterocycles are common structural motifs of BrC chromophores. Analysis of air filter samples confirmed the presence of imidazoles in atmospheric aerosols collected in Brazil, Europe, and India. Furthermore, experiments with mixtures of different dicarbonyls and amine nucleophiles revealed that cross-reactions can enhance bulk absorption and can yield new BrC chromophores absorbing at wavelengths near the visible region. This highlights the need to identify potentially important absorbers produced by such cross-reactions in atmospherically relevant dicarbonyl mixtures.Therefore, further laboratory experiments are necessary investigating the cross-reaction mechanisms, kinetics, and products as well as influencing factors, e.g., relative ratios of dicarbonyls in solution, amine species, and pH. Potential candidates for photosensitizers promoting heterogeneous radical chemistry and for marker compounds of atmospheric brown carbon shall be identified. The acquired process knowledge shall enable numerical modeling of BrC formation under atmospheric conditions using a kinetic multilayer model. Finally, kinetic modeling results should enable quantitative estimations of BrC formation from secondary reactions in aqueous aerosol particles.The new insights gained during this project will help to improve our understanding of how chemistry in aqueous aerosol particles affects aerosol optical properties and heterogeneous chemistry.

• DFG Programme: Research Grants