In this paper we introduce the new TropoPause Composition TOwed Sensor Shuttle (TPC-TOSS), which constitutes an advanced development of the AIRcraft TOwed Sensor Shuttle (AIRTOSS), introduced by Frey et al. (2009). As part of a tandem measurement platform with a Learjet 35A, both platforms were equipped with redundant instruments for collocated measurements of aerosol size distribution (Ultra-High Sensitivity Aerosol Spectrometer, UHSAS), ozone (2BTech model 205), cloud particles (Back-Scatter Cloud Probe, BCP), as well as relative humidity, temperature and pressure. To measure the exact position of the two platforms as well as the relative distance of the TPC-TOSS to the Learjet a Global Positioning System (GPS) is installed on both platforms. Two identical Inertial Navigation Systems (INS) further allow to monitor attitude angles (roll, pitch, and heading) and accelerations.</p> <p>Laboratory tests before and ground tests as well as inflight tests during the intensive operation period show a good agreement of the ozone and temperature measurements of better than 4.2 ppbv + 1.1 % (ozone) and 0.5 &deg;C (temperature) at a noise level of &plusmn; (2 ppbv + 0.5 %) for 2 s data (ozone) and 0.1 K for 1 Hz data (temperature). Stability of the ozone monitor mounted in the TPC-TOSS has been tested and is estimated to be 2.2 ppbv (offset, 1 &sigma;) and 0.7 % (gain, 1 &sigma;), respectively, based on the drift of offset and gain during regular calibrations between measurement flights in the two weeks operation period.</p> <p>The new TPC-TOSS was successfully flown during the TPEx I (TropoPause composition gradients and mixing Experiment) mission in June 2024 and performed four flights covering the altitude range between 6 and 12 km. The tropopause was crossed several times as evident from different temperature and ozone gradients as well as gradients of the aerosol number density. With the setup we are able to resolve transient stability and composition gradients ranging from almost zero or even negative to strong positive gradients of up to 25 K km^&minus;1 for potential temperature and from inverted to strong positive vertical gradients of ozone of up to 800 ppbv km^&minus;1, respectively. These gradients are caused by transport and mixing due to convection or shear induced turbulence at the tropopause.