The Leibniz Institute for Tropospheric Research (TROPOS, www.tropos.de) is an independent research institution with about 150 employees and is member of the Leibniz Association, one of the German non-university research organizations. The institute has a well-defined research profile with focus on aerosol and clouds. The TROPOS expertise ranges from physical and chemical laboratory studies, large field experiments in which in-situ and remote-sensing techniques from ground and on aircraft are applied, to numerical modelling of aerosol and cloud processes. TROPOS has been an active or leading partner in many international projects related to atmospheric clouds and aerosols in the research fields of air pollution and climate. The TROPOS Research Station Melpitz is
jointly operated by the Departments of Atmospheric Chemistry, Experimental Aerosol and Cloud Microphysics, and Remote Sensing of Atmospheric Processes, which all contribute to ACTRIS.

TROPOS is a world-leading centre for aerosol and cloud in-situ and remote observations. Over the past two decades, it has contributed to the field with new and innovative technologies and methodologies, e.g., for in-situ measurements of particle size distribution, humidity-dependent and absorption properties, and mixing state of aerosols as well as for remote sensing of particle optical and microphysical properties with multi-wavelength Raman and polarization lidars and of cloud-aerosol interactions with combined lidar and radar profiling techniques.

The institute has been involved in a large number of international field studies and performs continuous observations at stations worldwide. TROPOS is an active member of EARLINET since 2000 and of Cloudnet since 2011 and contributes to the respective ACTRIS databases on the four-dimensional distribution of aerosols and clouds over Europe. At the ACTRIS Melpitz station, a more than 20-year-long record of near-surface aerosol and trace-gas data has been established. The institute also conducts measurements at GAW stations such as Cape Verde Observatory, Point Barrow, and Danum Valley and within the German Ultrafine Aerosol Network (GUAN), which all deliver data to the World Data Centre for Aerosols at NILU, Norway. All instruments are integrated in the ACTRIS quality-assurance activities and participate in the regular round-robin tests, site inter-comparisons, and internal quality checks. Furthermore, TROPOS hosts the World Calibration Centre for Aerosol Physics (WCCAP) of the World Meteorological Organization – Global Atmosphere Watch program (WMO-GAW), which is responsible for the quality assurance of microphysical aerosol parameters in the networks GAE, ACTRIS, and GUAN. TROPOS operates two simulation chambers, the Leipziger Aerosolkammer (LEAK) and the Leipzig Aerosol and Cloud Interaction Simulator (LACIS), to study chemical and physical aerosol and cloud processes under controlled conditions. Both chambers were active members in the EU infrastructure projects EUROCHAMP and EUROCHAMP-2.

Within the current ACTRIS-2 project, TROPOS is actively involved in all networking and joint research activities
related to aerosol and trace-gas in-situ measurements and vertical profiling of aerosol and clouds. In ACTRIS-2,
TROPOS leads NA2 on Aerosol and Cloud Profiling and coordinates the European Centre for Aerosol Calibration.
In addition, TNA to the research station Melpitz is provided.