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RESEARCH PRODUCT

Application of an O-ring pinch device as a constant pressure inlet (CPI) for airborne sampling

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Abstract. We present a novel and compact design of a constant pressure inlet (CPI) developed for use in airborne aerosol mass spectrometry. In particular the inlet system is optimized for aerodynamic lenses commonly used in aerosol mass spectrometers, where efficient focusing of aerosol particles into a vacuum chamber requires a precisely controlled lens pressure, typically of a few hPa. The CPI device can also be used in gas phase sampling instruments in a large range of altitude and inlet pressure. The constant pressure is achieved by changing the inner diameter of a properly scaled O-ring that acts as a critical orifice. The CPI control keeps air pressure and thereby mass flow rate (≈ 0.1 l/min) upstream of an aerodynamic lens constant, deviating at most by only ± 2 % from a pre-set value. In our setup a pressure sensor downstream of the O-ring controls the pinch mechanism via a feedback loop and setpoint conditions are reached within seconds. The device was implemented in a few instruments, which were successfully operated on different research aircraft covering a wide range of ambient pressures, from sea level up to about 55 hPa. Details of operation and quality of aerosol particle transmission are evaluated by laboratory experiments and in-flight data with a single particle mass spectrometer.