A small system for the unattended measurement of total odd nitrogen (NO<sub>y</sub>, i.e., the sum of NO and its atmospheric oxidation products) aboard civil in-service aircraft in the framework of MOZAIC is described. The instrument employs the detection of NO by its chemiluminescence with O<sub>3</sub> in combination with catalytic conversion of the other NO<sub>y</sub> compounds to NO at 300°C on a gold surface in the presence of H<sub>2</sub>. The instrument has a sensitivity of 0.4-0.7cps/ppt and is designed for unattended operation during 1-2 service cycles of the aircraft (400-800 flight hours). The total weight is 50kg, including calibration system, compressed gases, mounting, and safety measures. The layout and inlet configuration are governed by requirements due to the certification for passenger aircraft. Laboratory tests are described regarding the conversion efficiency for NO<sub>2</sub> and HNO<sub>3</sub> (both >98%). Interference by non-NO<sub>y</sub> species is <1% for CH<sub>3</sub>CN and NH<sub>3</sub>, <5x10<sup>-5</sup>% for N<sub>2</sub>O (corresponding to <0.2ppt fake NO<sub>y</sub> from ambient N<sub>2</sub>O) and 100% for HCN. The time response of the instrument is <1s (90% change) for NO<sub>2</sub>. The response for HNO<sub>3</sub> is nonlinear: 20s for 67%, 60s for 80%, and 150s for 90% response, respectively.