Development of an Airborne Open Polar/Imaging Nephelometer for Ice Particles in Cirrus Clouds and Aerosols

Description

Cirrus clouds and aerosol particles play important roles in the Earth’s energy balance and climate. However, simply knowing that cirrus contrails are increasing and pollution type aerosols are decreasing tells you nothing about how these trends will affect climate without first knowing the specific optical properties of those cloud and aerosol types. The optical properties of cloud ice particles and natural aerosols including their scattering phase function are very difficult to model due to the complex crystal structure of ice particles or the uncertainty of an aerosol particle’s size, shape and composition. Remote sensing retrievals of particle phase function are nearly impossible for cirrus clouds and have never been validated with ambient in situ measurements for aerosols. A fundamental problem with current airborne nephelometers used to measure particle phase function is the need to sample ice crystals and aerosol particles through inlet tubes. In particular ice crystals shatter upon sampling via current methods, and thus, the phase function of the ambient cloud is never observed. With ARM/ASR/ACRF/AAF funding we have developed an Open Path Imaging Nephelometer to measure the ambient phase function of cloud and aerosol particles. The system has been integrated to the NASA P3 aircraft and has flown its first campaign during the DISCOVER AQ mission in Colorado. This mission represented the first engineering test flights for this instrument and was also an attempt to produce the first science results from the Open I-Neph. The Open I-Neph instrument uses a narrow beam laser source and a wide field of view imaging camera to capture the entire scattering phase function in one image quasi-instantaneously. TheOpen I-Neph was developed based on the heritage of the Polarized Imaging Nephelometer that has been used in the laboratory and airborne campaigns with enough sensitivity for fully resolved phase functions measurements of gas molecules (Rayleigh scattering) and low concentration background aerosol particles. The Open I-Neph is now being considered for other airborne and ground based measurements where it is important to get the measurement of truly ambient/unbiased phase functions.