The western Mediterranean basin as an aged aerosols reservoir. Insights from an old-fashioned but efficient radiotracer

The long-term contemporary 210Pb time series acquired during the period 2004-2011 at two distant sites of different altitude in the Mediterranean basin, El Arenosillo (40 m a.s.l. in southwestern Spain) and Mt. Cimone (2165 m a.s.l. in northern Italy), are analyzed and compared. Besides being considered a tracer of continental air masses, 210Pb radionuclide is also a proxy of fine stable aerosol. For this reason, the measurements of PM10 mass concentrations collected at the same time and the corresponding 210Pb/PM10 ratio at the two sites are considered to gain better insights into the origin and size of the particles. Three statistical trajectory methods are applied to identify and characterize the 210Pb source regions at the two sites. The three methods yield similar outcomes in the source identification, which strengthens the robustness of our results. In addition to the importance of the transport from areas of continental Europe, this study highlights the relevant role of the Mediterranean Sea as a major 210Pb reservoir layer associated to the aged air masses that accumulate in the western Mediterranean basin. The analysis of the sources points out the significant influence of northern Africa to 210Pb increases at both sites as well, even though the most intensive episodes are not of Saharan origin

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

The FRESA (\u201cImpacto de las intrusiones de masas de aire con polvo aFRicano y de masas de aire EStratosf\ue9rico en la Pen\uednsula Ib\ue9rica. Influencia de El Atlas\u201d) 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 \ub5m 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

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

The FRESA (\u201cImpacto de las intrusiones de masas de aire con polvo aFRicano y de masas de aire EStratosf\ue9rico en la Pen\uednsula Ib\ue9rica. Influencia de El Atlas\u201d) 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 \ub5m 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

UPPER-LEVEL DISTURBANCES AND THE IMPACT OF DUST OUTBREAKS IN SPAIN

The interaction between upper-level disturbances and major orographic features in north-western Africa is a key element for cyclogenesis and the development of convective storms, which promote instability at lower levels and lead to dust mobilization. It may also induce stratospheric to tropospheric exchange. The analysis of these processes and their subsequent impact over the Iberian Peninsula (IP) is the objective of the FRESA (\u201cImpacto de las intrusiones de masas de aire con polvo aFRicano y de masas de aire EStratosf\ue9rico en la Pen\uednsula Ib\ue9rica. Influencia de El Atlas\u201d) research project. After a long term (2004-2016) analysis of jet streams in NW Africa and the analysis of a number of representative cases, in this work we present a description of the largescale meteorological processes leading to massive dust mobilization in NW Africa and transport to the IP after passing over the Saharan Atlas for three episodes (October 2008, September 2007, and February 2016). The three events had a strong impact either at the ground level or in the vertical. Contrarily to most of African events over the IP, none of these events occurred during the summertime, when African dust outbreaks are more common but the jet streams are by far less frequent over the study area. The interaction of upperlevel troughs and cut-off lows with the Atlas Mountains as well as the interaction of tropical plumes and/or the subtropical jet with the Hoggar Massif were the largescale relevant processes, but the precise location of the meteorological features is found to be crucial as indicated by the differences among the three episodes. In the case of October 2008, a cut-off low moving westwards over the Atlas was the cause of both dust mobilization (cold pool and density current formation, leading to a haboob that ran parallel to the SE slope of the Atlas) and transport to the IP. On September 2007, a low to the SW of St. Vincent Cape in combination with the North African high advected dust to the IP at mid-low tropospheric altitude after mobilization due to moist convection produced to the SW and N of the Hoggar. On February 2016, three main synoptic features at midupper levels were involved: a cut-off low to the SW of St. Vincent Cape, which was mostly responsible for the dust advection to the IP during the first part of the event; a tropical moist plume accompanied by an intense southwesterly subtropical jet streak that interacted with the Hoggar and triggered the instability at low levels; and the North African high (extended to the north when the trough was cut-off and the polar front jet retreated northwards) which advected the dust laden air masses to the east of the IP in the second part of the event. The analysis of back-trajectories calculated at multiple heightsfor the event of February 2016 in connection with aerodrome present-weather reports and satellite observations shows the orographic uplifting of the dust laden flows in the southern slope of the Saharan Atlas and their entrance into the IP at mid-low levels, in good agreement with the backscatter coefficient profiles of the CEAMA (Granada) ceilometer (Cazorla et al., submitted). This work was supported by the Spanish MINECO under grant CGL2015-70741-R (FRESA). Cazorla et al. (submitted to ACP), Near real time processing of ceilometer network data: characterizing and extraordinary dust outbreak over the Iberian Peninsula