A relative humidity processing method for the sampling of aerosol particles with low growth-ability

A method for the fractionation of aerosol particles with respect to size and ability to grow with an increased relative humidity has been developed. The system consists of cascade impactors, diffusion driers, a humidifier and a temperature stabiliser. Diffusion driers were designed and the vapour penetration was modelled below 20%. A humidifier which can be operated with an output relative humidity above 95% was developed. Flow-rates up to 51/min can be used and the relative humidity can be controlled within approximately 1%. The ability of the system to fractionate aerosol particles with respect to growth with relative humidity was investigated. The equivalent aerodynamic diameter growth factor for sodium chloride was determined to 2 at a relative humidity of 98%, in good agreement with theory. The growth is used to collect particles with no, or limited growth with increased relative humidity in separate fractions for chemical characterisation. This is obtained with an impactor stage operated at high relative humidity followed by a diffusion drier and an impactor stage with a factor of 1.4 lower cut-off diameter operated at low relative humidity, where the particles with low growth-ability are collected. The system was in operation during the EUROTRAC sub-project Ground-based Cloud Experiment (GCE) at Po Valley, supplying important information on the particle size related scavenging and the elemental composition of particles with a low growth-ability.