Design and calibration of a low volume fog sampler

In this work a small, portable fog droplet sampler is presented. The modified low volume round jet impactor consists of a Teflon® inlet and a Teflon® cup as impaction surface, where the fog elements are collected, form bigger drops and then flow into a receiver vessel. The sampling apparatus has been designed in order to establish constant impaction conditions and a sharp cut-off at a droplet diameter of 10 μ, such conditions differentiating between fog droplets and non-activated aerosol particles. The apparatus has been calibrated with monodisperse DOP aerosol as a model for water droplets at a constant flow rate of 1.6 m³ h^-1. The calibration resulted in an effective cut-off diameter of 10.2 μm and a sharp cut-off characteristic. To verify if evaporation of the collected droplets or condensation of water vapour during sampling occurs, the impactor was tested with water droplets of well-defined sodium fluorescein concentration. The liquid collected in the recipient did not exhibit a statistically significant change in concentration. A field test in a dense fog showed that it is possible to collect a volume of fog-water sufficient for ion chromatographic analysis within 1 h.