The CAFE‐Pacific mission, coordinated by the Max Planck Institute for Chemistry and the Goethe Univ. Frankfurt, will be the third in a triad of measurement campaigns with the High Altitude – Long Range (HALO) aircraft to investigate photochemical and aerosol processes in the tropical troposphere. HALO has been uniquely equipped with a custom designed payload by three departments of the MPIC and partner institutes. It consists of highly sensitive mass spectrometric and optical instrumentation, to comprehensively characterize atmospheric oxidation mechanisms, as well as their control of particle production and growth throughout the troposphere; from reactive radicals to large volatile organic compounds and their reaction products, and from small clusters to stable aerosol particles.

Scientific rationale
The tropical region is of particular relevance on Earth because it represents a large fraction of the troposphere where oxidation processes are most efficient, and directly connects to the stratosphere through the upward motion generated by the formation of deep convective clouds. For example, about 80 – 90% of atmospheric methane oxidation takes place in the tropics, even though the largest fraction is emitted at higher latitudes. The tropical troposphere is referred to as the “wash room” of the atmosphere. Moreover, tropical atmospheric chemistry and aerosol processes have been much less studied compared to the extra‐tropics.

Objectives
The main objective of the CAFE missions is to investigate tropical tropospheric oxidant  photochemistry in combination with aerosol particle formation and growth under clean, pristine conditions over land, and contrast the results with those in marine and polluted conditions. The measurements constrain computer models that represent chemical processes, environmental feedback mechanisms up to the Earth system, in order to generalize the results to regional and global scales. The data support studies of the atmospheric oxidation capacity and the abundance of fine particulate matter in relation to clouds, climate and planetary health. The studies of unpolluted, pristine conditions provide a reference for human‐induced perturbations in the Anthropocene, as well as constraints on atmospheric conditions in the Earth’s history.