An Evaluation of the Community Aerosol Inlet for the NCAR C-130 Research Aircraft

Abstract

Based on both in-flight measurements and a fluid dynamics model, airflow in the National Center for Atmospheric Research (NCAR) Community Aerosol Inlet (CAI) is similar to fully developed pipe flow. Distortions of the velocity field were pronounced when suction to inlet tubes was shut off, but conditions were otherwise insensitive to all flight parameters but airspeed. The principal value of the multiuser CAI system for NCAR's C-130 is that it decelerates air with no curves until the velocity has been reduced to 10 m s?1. It then supplies uniformly modified air (after turbulent losses) to all users, enabling valid closure experiments. Chemical data from both the First Aerosol Characterization Experiment (ACE-1) and the Second Community Aerosol Inlet Evaluation Program (CAINE-II) clearly indicate that while passing efficiency for submicron aerosol is acceptable, very little of the sea salt mode mass is transmitted by the CAI to instruments inside the aircraft. Comparisons between chemical samples from an external total aerosol sampler and samplers behind the CAI indicate that 70%?90% of the sea salt mass is unable to pass the CAI. The 50% cut size is about 3 ?m, but the precise details of the efficiency curve are obscured by the difficulty of measuring a reference ambient aerosol distribution. The loss of particle mass becomes very significant above 3 ?m, but the size cut is not sharp. These conclusions are supported by calculated particle transmission efficiencies for the CAI.