Airborne observations of newly formed boundary layer aerosol particles under cloudy conditions

Abstract. This study describes the formation of freshly formed boundary layer aerosol particles under classical called non favourable conditions by the existence of low-level strato cumulus clouds. Airborne measurements for vertical profiling atmospheric boundary layer properties and aerosol particles in the diameter range between 5 nm and 10 μm were repeatedly performed with the unmanned aerial system ALADINA (Application of Light-weight Aircraft for Detecting IN-situ Aerosol) at the research site of TROPOS (Leibniz Institute for Tropospheric Research) in Melpitz during three seasons between October 2013 and July 2015. More than 100 measurement flights were performed during this period on 23 different days with a total flight duration of 53 h. On 26 % of the measurement days, new particle formation was measured close to the inversion layer and was observed to be transported downwards during short time intervals of cloud gaps. However, the typical banana shape of new particle formation and growth was not seen at ground, only sporadic events appeared with low particle growth rate and thus might not have been classified as NPF by pure surface studies. This presentation focuses on two cases influenced by the passage of a low pressure system and minimal concentrations of SO2 as indicator for precursor gases at ground; I) on April 4, 2014 by east wind and II) on June 21, 2015 under south-west wind. For a closure, an LES-model output was used for the second study in order to derive a full analysis of atmospheric boundary layer growth over the measurement site at small-scale with high spatial resolution. Pronounced turbulent fluxes of sensible and latent heat in the vertical distribution initiated fast mixing processes of freshly formed boundary layer aerosol. Furthermore, CO2 and particle fluxes indicated downward transport and high deposition during sporadic nucleation events, so that NPF occurrence above with subsequent downward transport was clearly identified.