Spatial distribution and variability of boundary layer aerosol particles observed in Ny-Ålesund during late spring in 2018

Abstract. This article aims to improve the understanding of the small scale aerosol distribution affected by different atmospheric boundary layer (ABL) properties. In particular, transport and mixing of ultrafine aerosol particles (UFP) are investigated, as an indicator for possible sources triggering the appearance of new particle formation (NPF) at an Arctic coastal site. For this purpose, flexible measurements of unmanned aerial systems (UAS) are combined with continuous ground based observations at different altitudes, the observatory Gruvebadet close to the fjord at an altitude of 67 m above sea level (a.s.l.), and the observatory at the Zeppelin Mountain at an altitude of 472 m a.s.l.. The two unmanned research aircraft called ALADINA and MASC-3 were applied for field activities at the polar research site Ny-Ålesund, Svalbard, between 24 April 2018 and 25 May 2018. The period was at the end of Arctic haze during the snow melt season. A high frequency of occurrence of NPF was observed, namely on 55 % of the airborne measurement days. With ALADINA, 230 vertical profiles were performed between the surface and the main typical maximum height of 850 m a.s.l., and the profiles are connected to surface measurements, in order to obtain a 4-D picture of aerosol particle distribution. Analyses of potential temperature, water vapour mixing ratio and aerosol particle number concentration of UFP in the size range of 3–12 nm (N₃₋₁₂) indicate a clear impact of the ABL’s stability on the vertical mixing of the measured UFP, which results in systematical differences of particle number concentrations at the two observatories. In general, higher concentrations of UFP occurred near the surface, suggesting the open sea as the main source for NPF. Three different case studies show that the UFP were rapidly mixed in the vertical and horizontal scale depending on atmospheric properties. In case of temperature inversions, the aerosol population stayed confined to specific altitude ranges, and was not always detected at the observatories. However, during another case study that was in relation to a persistent NPF event with subsequent growth rate, the occurrence of UFP was identified to be a wide spreading phenomenon in the vertical scale, as the observed UFP exceeded the height of 850 m a.s.l.. During a day with increased local pollution enhanced equivalent black carbon mass concentration (eBC) coincided with an increase of the measured N₃₋₁₂ in the lowermost 400 m, but without subsequent growth rate. The local pollution was transported to higher altitudes, as measured by the UAS. Thus, emissions from local pollution may play a role for potential sources for UFP in the Arctic as well. In summary, a highly variable spatial and temporal aerosol distribution was observed with small scales at the polar site Ny-Ålesund, determined by atmospheric stability, contrasting surface and sources, and topographic flow effects. The UAS provides the link to understand differences measured at the two observatories at close distance, but different altitudes.