Vertical and horizontal variation of aerosol number size distribution in the boreal environment

Abstract. This study explores the vertical and horizontal variability of the particle number size distribution from two flight measurements campaigns over a boreal forest in Hyytiälä, Finland during May–June 2013 and March–April 2014, respectively. Our other aims were to study the spatial extent of new particle formation events and to compare the airborne observation with the ground measurements from the SMEAR II (Station for Measuring Ecosystem-Atmosphere Relations) field station located in Hyytiälä. The airborne measurements extended vertically 3.8 km and horizontally 30 km from the station. A Cessna 172 aircraft was used as a measurement platform. The measured parameters included the particle number concentration (> 3 nm) and particle number size distribution (10–400 nm). The airborne data used in this study were equal to 111 flight hours. The measurements showed that despite local fluctuations there was a good agreement between the on-ground and airborne measurements inside the planetary boundary layer. On median, the airborne total number concentration was found to be 10 % larger than at the ground level. The seasonal and meteorological differences between the campaigns were reflected in aerosol properties. NPF days showed areas of intensified NPF on a scale from kilometres up to couple of tens of kilometres in the planetary boundary layer. NPF was also observed frequently in the free troposphere.