Atmospheric Chemistry and Physics Mode resolved density of atmospheric aerosol particles

Abstract. In this study, we investigate the mode resolved density of ultrafine atmospheric particles measured in bo-real forest environment. The method used here enables us to find the distinct density information for each mode in at-mospheric fine particle population: the density values for nu-cleation, Aitken, and accumulation mode particles are pre-sented. The experimental data was gained during 2 May 2005–19 May 2005 at the boreal forest measurement station “SMEAR II ” in Hyytiälä, Southern Finland. The density val-ues for accumulation mode varied from 1.1 to 2 g/cm3 (av-erage 1.5 g/cm3) and for Aitken mode from 0.4 to 2 g/cm3 (average 0.97 g/cm3). As an overall trend during the two weeks campaign, the density value of Aitken mode was seen to gradually increase. With the present method, the time dependent behaviour of the particle density can be investi-gated in the time scale of 10 min. This allows us to fol-low the density evolution of the nucleation mode particles during the particle growth process following the nucleation burst. The density of nucleation mode particles decreased during the growth process. The density values for 15 nm particles were 1.2–1.5 g/cm3 and for grown 30 nm particles 0.5 –1 g/cm3. These values are consistent with the present knowledge that the condensing species are semi-volatile or-ganics, emitted from the boreal forest.