"Clouds and aerosols are the most important component of the global climate system, but the least understood. The general goal of EXCATRO is to enhance the understanding of TROPICAL clouds/aerosols at high altitudes, and in particular to quantify the ORGANIC and ANTHROPOGENIC contributions. This is of fundamental importance for the global climate and ozone chemistry since (1) the highest solar energy input occurs in the tropics, (2) the fastest (convective) transport of trace substances and pollutants from the ground towards the stratosphere takes place here, and (3) the highest tropical clouds largely control how much water vapor and aerosols enter the global stratosphere.Most of the processes and effects of tropical high clouds/aerosols critically depend on the chemical composition of individual particles. As very few direct in-situ measurements are available, I propose with EXCATRO: (1) to devise state-of-the-art in-situ instrumentation for the study of cloud/aerosol particle chemical composition from the Russian high altitude aircraft M-55 ""Geophysica"", and (2) to perform research flights within the Indian Monsoon and other tropical regions. In-situ, online-remote-controlled aerosol particle mass spectrometers as well as specific particle collection devices will allow measurements inside tropical (cirrus) clouds and aerosols as high as 20 km so as to gain essential insight into the processes underlying their origin, the formation of the stratospheric aerosol and of precipitation. Major emphasis is placed on the ORGANIC content of the measured aerosol, its sources and anthropogenic fraction, in particular from ground releases in the Indian Monsoon region and Southeast Asia.The in-situ data will provide key input for the numerical simulation of clouds, and satellite product verification. EXCATRO will thus provide knowledge crucial to gaining a full picture of the role of clouds and aerosols in influencing the planet's atmospheric chemistry and climate."