ON THE POSSIBLE CONNECTION BETWEEN AFRICAN DUST OUTBREAKS AND STRATOSPHERE-TO-TROPOSPHERE EXCHANGE AS MEASURED IN THE IBERIAN PENINSULA

The FRESA (“Impacto de las intrusiones de masas de aire con polvo aFRicano y de masas de aire EStratosférico en la Península Ibérica. Influencia de El Atlas”) Spanish national project aims to study the interaction between upper level disturbances and the Atlas Mountains in promoting low-level instability and cyclone formation in the lee of the Atlas Mountains, responsible of dust uplift over the Sahara Desert and also of upper tropospherelower stratosphere (UT-LS) to troposphere transport. This work presents an overview of the results obtained during a preliminary campaign of the FRESA project (June-October 2016) at Sierra Nevada (Granada) (37.096 N, 3.387 W, 2550 m a.s.l.) analysing weekly samples of 7Be, 210Pb (tracers of UTLS and continental origin, respectively) and PM10, as well as hourly particle number size distribution from a Grimm OPC. 96-hour 3D backward trajectories starting every 6 h (00, 06, 12, and 18Z) were calculated using the HYSPLIT4 model with the meteorological input from the ERA-Interim 0.5 deg meteorological dataset. Moreover, potential vorticity (PV) data was associated to each of the 96 trajectory endpoints by performing a 3D nearest-neighbour interpolation to the 27 isobaric levels between 100 and 1000 hPa of the ERA-Interim reanalysis. During this pre-campaign, 12 Saharan dust and 14 stratospheric intrusions days were identified analysing back-trajectories, BSC-DREAM model, particle number concentrations and synoptic charts: the meteorological situation leading to these outbreaks will be further studied in detail. The analysis of the source-receptor relationship for 7 Be, 210Pb and PM10 shows the influence of the Sahara Desert and of air parcels which passed over the North Sea with strong subsidence and then at lower heights over the western Mediterranean basin on all the three parameters. The analysis also shows that the Atlantic air masses are more associated to 7 Be than 210Pb increases. The fine and coarse fractions (smaller and larger than 1 µm in diameter) were analysed on both a weekly and 6-hourly basis. Locations to the south of the Atlas and the western Mediterranean were identified as important sources of the coarse fraction while the origin of the fine fraction was quite similar to that of 7 Be, probably because of the attachment of 7 Be nuclide to accumulation-mode aerosols soon after its production. The comparison between 7 Be/210Pb potential sources with those of high PV crossing shows that the Atlas is connected both to events of high PV values and to a strong PSCF probability of the 7 Be/210Pb ratio. This result suggests that the high PV areas located around the Atlas Mountains are not linked to friction and turbulence over the mountains themselves, but are rather associated to tropopause crossing and Stratosphere-TroposphereExchange (STE) occurring in the leeward side of mountain ranges, in agreement with the recent work of Brattich et al. (submitted). This work was supported by the Spanish MINECO under grant CGL2015-70741-R (FRESA). Brattich et al. (submitted to JGR), Influence of stratospheric air masses on radiotracers and ozone over the central Mediterranean.