Investigation of key aerosol nucleation mechanisms in the tropical troposphere with a CI-APiTOF-Mass Spectrometer above Amazonia during the HALO mission CAFE-Brazil

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New particle formation (NPF) occurs frequently in the upper tropical troposphere but the formation mechanisms and the chemical species that initiate the nucleation have not been measured so far. In Amazonia NPF is absent in the Boundary Layer and in the lower parts of the troposphere, therefore NPF in the upper troposphere is probably a main source for Cloud Condensation Nuclei in the entire troposphere. It is also assumed to be an important source region for the stratospheric aerosol layer. We discovered that NPF from the highly oxygenated molecules (HOMs) that form from the oxdation of monoterpenes is very efficient even without participation of well-known inorganic nucleators such as sulfuric acid. Monoterpenes are emitted in large quantities by trees in the Amazon rain forest, and the monoterpenes are hardly water soluble and therefore survive transport into the upper troposphere by deep convection. We propose that the NPF in the upper troposphere above Amazonia is dominated by HOMs. To prove this mechanism we plan to operate a nitrate- Chemical Ionization Atmospheric Pressure interface Time of Flight mass spectrometer (nitrate-CI-APiTOF) onboard the German research aircraft HALO during the CAFE-Brazil mission (Chemistry of the Atmosphere: Field Experiment in Brazil) in Manaus, Brazil, in winter 22/23. The CI-APiTOF is able to measure quantitatively minute quantities of the key nucleating species such as HOMs and sulfuric acid. In combination with measurements of the ultrafine aerosol concentration we will be able to derive aerosol nucleation rates and describe the initial condensational growth of the newly formed particles.