The supersaturation in the thermal gradient CCN counter can be disturbed by changes in pressure in the column. There is a slight influence of the column internal pressure on the supersaturation under stable conditions (Roberts and Nenes, Aerosol Science and Technology, 39, 206-221, 2005). With rapid changes in pressure, when an aircraft ascends or descends, the supersaturation in the column will drop and the CCN measurements will not be reliable until the pressure has stabilized. The CCN constant pressure inlet (CPI) accessory provides constant pressure at the inlet of the CCN when the CCN is operated from an aircraft platform. A hose clamp pinching the inlet conductive tubing is used to provide a pressure drop, and the CPI provides a variable vacuum to maintain a constant pressure in the CCN instrument.