A new instrument, COPAS, has been designed, constructed, certified, laboratory characterized, implemented on a high altitude researchlaireraft (Geophysika) and, flown up to 20 km altitude during test flights and research flights. This instrument detects the smallest atmospheric aerosol Particles with sizes above 10 nanometers in 4 separate channels. Each channel treats the sampled air containing, the particles differently before the actual measurement in order to enable differentiated statements concerning the atmospheric particle population. The COPAS instruments have been constructed in two disjunct units, one for standalone operation on the aircraft-,.one as integrated part of the CVI package. After the design and construction of the hardware very extensive and laborious schock and vibration tests as well as electromagnetic compatibility tests were performed. These resulted in the aircraft readiness certification according to DO 160C aircraft industry standards and were a necessary condition for implementing the instrument units on Geophysika. Additionally extensive laboratory experiments with sophisticated particle generation and detection instrumentation were performed in order to characterize the operation and the quality of the data obtained by the COPAS units. The instruments were implemented on Geophysika and successfully test-flown during the Extended Test Campaign (APE-ETC) in Forll, Italy, 1998. The COPAS units then participated in the APE-TliESEO campaign conducted with Geophysika on the Seychelles for cloud physics and atmospheric chemistry measurements in the tropical upper troposphere and lower stratosphere.