Application-Specific Calibration of Condensation Particle Counters under Low Pressure Conditions

Condensation Particle Counters (CPC) are research instruments that are commonly used to measure fine, airborne particles. They are able to determine the particle number concentration of both, the smallest nanoparticles and also larger particles in the accumulation mode, which cannot be measured with optical methods at all or with sufficient accuracy. In the last four decades, CPCs have been increasingly used todetermine particle number concentrations also at non-standard atmospheric pressurein both scientific research as well as for regulatory purposes. Measurement conditionsat reduced pressure require a dedicated calibration of the CPCs to ensure correctresults. The aim of my thesis was to investigate the performance of CPCs at variouslevels of low-pressure by means of a comprehensive and detailed calibration study.In the first part, this thesis reviews details of the CPC technology, explains applicationswith a need for low-pressure calibration and presents an extensive literature review ofthe field. It provides a critical analysis of the papers most relevant to this research,building on a survey of more than 60 peer-reviewed publications from the past forty years. The literature review points to contradictions in the understanding of thebehavior of CPCs at low pressure and highlights opposing findings. In a subsequentsection, a numerical model is applied to investigate how reduced pressure conditionsimpact CPC performance in theory. The model delivers a prediction of CPC countingefficiencies as well as d90, d50 and d10 cut-sizes at various levels of reduced pressure.The second part of this thesis describes extensive laboratory experiments and providesa detailed analysis of the performance at reduced pressure. Two different CPC modelswere calibrated for their particle counting efficiency, smallest particle size detectionlimit, and concentration linearity. The calibrations were performed with test aerosols oftwo very different particle sources. One of the calibration aerosols is representative ofinorganic particles in ambient air not impacted by anthropogenic activity. The otheraerosol is freshly generated soot from an inverted-flame burner, which serves as aproxy for combustion emissions found in urban environments. Additionally, all countingefficiency experiments were done with the relative humidity of the calibration aerosolvaried from almost dry to moderate levels of humidity. This aspect was included toconsider that humidity might play a role in the activation of particles inside a CPC.This thesis provides new insights into the experimental method, operating parametersand effective practices to calibrate CPCs for their use under low-pressure conditions.Results demonstrate that in general CPCs work as specified down to a thresholdpressure. Below that point, their performance depends on the exact measurementconditions and the specific CPC design. These findings are an important advancementin understanding the behavior of CPCs at low pressure and can serve as guidancewhen correcting the reported number concentration for the impact of reduced pressure.They also show that a dedicated calibration of CPCs is advisable for every type of lowpressureapplication that requires highly accurate particle number concentration data