A Continuous - Flow Streamwise Thermal - Gradient CCN Chamber for Atmospheric 4 Measurements 5 Short Title : Continuous - Flow Streamwise CCN Chamber

24 We have addressed the need for improved measurements of cloud condensation nuclei 25 (CCN) by developing a continuous-flow instrument that provides in situ measurements of CCN. 26 The design presented in this paper can operate between 0.1% and 3% supersaturation, at 27 sampling rates sufficient for airborne operation. The design constitutes a cylindrical continuous28 flow thermal-gradient diffusion chamber employing a novel technique of generating a 29 supersaturation: by establishing a constant streamwise temperature gradient so that the difference 30 in water vapor and thermal diffusivity yield a quasi-uniform centerline supersaturation. Our 31 design maximizes the growth rate of activated droplets, thereby enhancing the performance of 32 the instrument. The temperature gradient and the flow through the column control the 33 supersaturation and may be modified to retrieve CCN spectra. 34 35 The principle of the CCN instrument has been validated in controlled laboratory 36 experiments at different operating conditions using a monodisperse aerosols with known 37 composition and size. These experiments yield sharp activation curves, even for those 38 kinetically-limited particles that have not exceeded their critical diameter. The performance of 39 the CCN instrument was also assessed using polydisperse laboratory-generated aerosol of known 40 composition and size distributions similar to ambient particulate matter. In all tests, the 41 measured CCN concentrations compared well with predicted values and highlight the 42 instrument’s ability to measure CCN at various size distributions. 43 44 The full potential of the new design has yet to be explored; however, model simulations 45 suggest that direct measurements in the climatically important range of supersaturations of less 46