PELTI: Measuring the passing efficiency of an airborne low turbulence aerosol inlet

In an effort to improve the accuracy of airborne aerosol studies, we compared a new porous-diffuser low-turbulence inlet (LTI) with three other inlets on the NSF/NCAR C-130, using both dust and sea salt as test aerosols. Analysis of bulk filters behind the LTI and an external reference total aerosol sampler (TAS) found no significant differences, while both the NASA shrouded solid diffuser inlet (SD) and NCAR community aerosol inlet (CAI) passed smaller amounts. However, scanning electron microscopic analyses of particles behind the LTI and TAS confirmed the model prediction that the LTI porous diffuser (PD) enhanced 7 mum particle concentrations by about 60%. Aerodynamic particle size distributions behind the other inlets began to diverge from enhancement-corrected LTI values above 2 mum, with mass concentrations of larger particles lower by as much as a factor of ten behind the CAI and a factor of 2 behind the SD. We conclude that the corrected LTI distributions were closer to ambient values than those from either the CAI or the SD. Since tubing losses contributed the most uncertainty when deducing ambient supermicron size distributions from LTI data, minimizing them should be a high priority for future experiments. Measured transfer tubing losses were larger than model estimates, in part because of some complex pieces for which no suitable model exists. The LTI represents a significant advance in our ability to sample populations of large particles from aircraft. A necessary part of using an LTI is the calculation of and correction for large-particle enhancement using a computational fluid dynamics (CFD) program. Although the solid diffuser inlet performed well under some conditions, its large-particle efficiency cannot be modeled, varies with humidity and particle morphology, and involves wall contact that has the potential to modify some particles.