Measurement of Interstitial Particles During the PLOWS and the GRIP Aircraft Campaigns

Two airborne inlets were developed for representative sampling of interstitial (or non-activated) particles in clouds. Computational fluid dynamics (CFD) studies, combined with numerical particle trajectory calculations, have been utilized in the design process for both inlets. Special focus in both design projects is to avoid splash artifacts from impaction of cloud, drizzle, and precipitation droplets on the surface of the inlet. The Clarkson Interstitial Inlet (CII) has a sampler housing that was designed to avoid flow separation around the body's curvature and provides for a location of the interstitial inlet that can selectively sample particles over a size range of non-activated particles in cloud systems. The inlet design avoids the contamination of the interstitial aerosol sample by particles formed from droplet splashing and liquid film breakup by appropriate shaping of the blunt body and controlling the boundary layer flows over the sampler. A full scale model was fabricated and flown on the NCAR C-130 aircraft during the PLOWS campaign (November 2009-March 2010). The GRIP-CII (GRIP – Clarkson Interstitial Inlet) was originally designed to sample gases which has been modified to sample aerosols. This inlet contains a shroud with interchangeable flow constricting nozzles, which are used to align and decelerate the flow and particles. The main sample is taken at an angle with the direction of the main flow inside the shroud to prevent the large activated particles from entering the aerosol sample. This inlet was designed to operate at higher speeds observed in flights on the NASA DC-8 aircraft and has been flown during the GRIP campaign (August-September 2010). The performance of both inlets during different cloud/precipitation conditions will be presented and compared with other standard inlets operated simultaneously during the campaigns. Analysis of the results of the CII data showed promise at conditions of warm, small cloud droplets, but at cold conditions or precipitation the inlet was generating splash. The flight results for performance of the GRIP-CII will be presented and discussed after preliminary analysis. Both inlets were connected to CN counters and size distribution measurement instruments (SMPS and UHSAS) during the airborne campaigns. Size distribution of interstitial particles at conditions where the inlets had satisfactory performance will be also discussed.