Ambient particles at an urban, semi-urban and rural site in Central Europe: hygroscopic properties

Measurements on hygroscopic properties of sub-micrometer atmospheric particles have been carried out using a tandem differential mobility analyser (TDMA). It measures the growth of initially dry monodisperse particles with sizes of 50,100,150 and 250 nm at a relative humidity (RH) of 0.85. Measurements have been carried out at an urban, semi-urban and rural location in Bavaria (Germany) each during 1 week in winter, summer and fall. Two fractions of particles are found, hygroscopic and non-hygroscopic particles with mean growth factors of 1.37 and 1.03, respectively. Growth factors are nearly independent of location and season, but increased significantly with initial particle size from 1.31 to 1.47 for 50 and 250 nm particles, respectively. The mean number concentration of non-hygroscopic particles is strongly dependent on location and increases by a factor of 2.4 for the semi-urban site and by a factor of 9 for the urban site, compared to the rural location. The corresponding differences for hygroscopic particles are less than a factor of three, respectively. Individual measurements strongly differ in number concentration from each other. Assuming that the hygroscopic material is ammonium sulphate, the non-hygroscopic particles and the hygroscopic particles contain about 3% and 54% ammonium sulphate, respectively. For the hygroscopic particles the mean increase of ammonium sulphate was 38-74% for 50 and 250 nm particles, respectively.