A reference number concentration generator for ultrafine aerosols based on Brownian coagulation

A system for delivering a reference number concentration (calibration source) is proposed as an alternative to using reference methods for calibrating instrumentation of ultrafine particle number concentration. The underlying physical principle is the asymptotic behaviour of Brownian coagulation of liquid particles. We describe and characterize a simple set-up based on aerosol formation by homogeneous nucleation of an organic vapour and subsequent growth by Brownian coagulation in a closed chamber. Controlling parameters are the coagulation time and, to a much lesser extent, the volume fraction of the condensed aerosol. The system can be used in a time window starting from 100 s, the time required to reach the asymptotic state, and ending at 1000 s after initiation of the nucleation process, beyond which wall losses become significant. In this regime, the temporal development of the number concentration in the coagulation vessel is exclusively determined by the physical process of Brownian coagulation. The system delivers predictable reference concentrations in the range between 106106 and 107cm-3.