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High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude alone is not a sufficient parameter to characterize the ABL influence. In this study, a topography analysis is performed allowing calculation of a newly defined index called ABL-TopoIndex. The ABL-TopoIndex is constructed in order to correlate with the ABL influence at the high altitude stations and long-term aerosol time series are used to assess its validity. Topography data from the global digital elevation model GTopo30 were used to calculate five parameters for 43 high and 3 middle altitude stations situated on five continents. The geometric mean of these five parameters determines a topography based index called ABL-TopoIndex, which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or high plains, if the slopes around the station are steep, and finally if the inverse drainage basin potentially reflecting the source area for thermally lifted pollutants to reach the site is small. All stations on volcanic islands exhibit a low ABL-TopoIndex, whereas stations in the Himalayas and the Tibetan Plateau have high ABL-TopoIndex values. Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABL-TopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (SS) correlations are found between the 5th and 50th percentiles of all aerosol parameters and the ABL-TopoIndex, whereas no SS correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a SS correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex. © 2018 Author(s). ; The authors gratefully acknowledge the following persons and organizations. – Wolfgang Schwanghart, the programmer of the TopoToolBox for putting his codes as a freeware and for all the kind and always rapid support he offers. – CHC: the Chacaltaya consortium and Laboratory for Atmospheric Physics at UMSA for taking care of the station and collecting the data. Swedish participation was supported by the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) and the Swedish research council FORMAS. – CMN: the European Commission funded ACTRIS, ACTRIS2 EU project and NEXTDATA National project funded by MIUR. – IZO: Global Atmospheric Watch program funded by AEMET and by the project AEROATLAN (CGL2015-66299-P), funded by the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund (ERDF). – JFJ: International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat (HFSJG) and the Federal Office of Meteorology and Climatology, MeteoSwiss, within the Swiss program of the Global Atmosphere Watch (GAW) of the World Meteorological Organization, funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 654109 (ACTRIS2), and the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract no. 15.0159-1. The opinions expressed and arguments employed herein do not necessarily reflect the official views of the Swiss Government. – LLN: the Taiwan Environmental Protection Administration. – MSA and MSY: MINECO (Spanish Ministry of Economy and Competitiveness), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment), the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA) and FEDER funds under the PRISMA project (CGL2012-39623-C02/00). This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654109. Marco Pandolfi is funded by a Ramón y Cajal Fellowship (RYC-2013-14036) awarded by the Spanish Ministry of Economy and Competitiveness. ; Peer reviewed

Highlights • Aerosol scattering simulated under variable microphysical properties (visible range). • Sensitivity of scattering to microphysical properties depends on particle mean size. • Scattering spectral dependence is slightly affected by microphysical assumptions. • Size distribution and real refractive index have strongest impact on scattering. • Comparison with source scattering observations constrained microphysical assumptions. ; This work has been funded by the Spanish Ministry of Economy and Competitiveness [grant: CGL2013-46736-R] and by the ACTRIS Research Infrastructure Project of the European Union's Horizon 2020 research and innovation programme [grant agreement: No. 654109]. Further support has been provided by the Severo Ochoa Program, awarded by the Spanish Government [grant: SEV-2011-00067]. This work was also supported by the MINECO (Spanish Ministry of Economy and Competitiveness), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment) and FEDER funds under the PRISMA project [grant: CGL2012-39623-C02/00]. Further fundings have been provided by the European Union's Horizon 2020 research and innovation programme [grant agreement: No. 654109]. Vincenzo Obiso is funded by the Spanish Ministry of Economy and Competitiveness ['FPI-SO' grant: SVP-2013-067953]. Marco Pandolfi is funded by the Spanish Ministry of Economy and Competitiveness ['Ramón y Cajal' grant: RYC-2013-14036]. We thank Noemi Pérez for providing the PM chemical speciated data for the Montseny station and the developers of the T-Matrix code for making it freely available. ; Peer Reviewed ; Postprint (author's final draft)

Highlights • Aerosol scattering simulated under variable microphysical properties (visible range). • Sensitivity of scattering to microphysical properties depends on particle mean size. • Scattering spectral dependence is slightly affected by microphysical assumptions. • Size distribution and real refractive index have strongest impact on scattering. • Comparison with source scattering observations constrained microphysical assumptions. ; This work has been funded by the Spanish Ministry of Economy and Competitiveness [grant: CGL2013-46736-R] and by the ACTRIS Research Infrastructure Project of the European Union's Horizon 2020 research and innovation programme [grant agreement: No. 654109]. Further support has been provided by the Severo Ochoa Program, awarded by the Spanish Government [grant: SEV-2011-00067]. This work was also supported by the MINECO (Spanish Ministry of Economy and Competitiveness), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment) and FEDER funds under the PRISMA project [grant: CGL2012-39623-C02/00]. Further fundings have been provided by the European Union's Horizon 2020 research and innovation programme [grant agreement: No. 654109]. Vincenzo Obiso is funded by the Spanish Ministry of Economy and Competitiveness ['FPI-SO' grant: SVP-2013-067953]. Marco Pandolfi is funded by the Spanish Ministry of Economy and Competitiveness ['Ramón y Cajal' grant: RYC-2013-14036]. We thank Noemi Pérez for providing the PM chemical speciated data for the Montseny station and the developers of the T-Matrix code for making it freely available. ; Peer Reviewed ; Postprint (author's final draft)

Aerosol light scattering (σsp), backscattering (σbsp) and absorption (σap) were measured at Montsec (MSC; 42°3' N, 0°44' E, 1570 m a.s.l.), a remote high-altitude site in the western Mediterranean Basin. Mean (±SD) σsp, σbsp and σap were 18.9 ± 20.8, 2.6 ± 2.8 and 1.5 ± 1.4 Mmĝ̂'1, respectively at 635 nm during the period under study (June 2011-June 2013). Mean values of single-scattering albedo (SSA, 635 nm), the scattering Ångström exponent (SAE, 450-635 nm), backscatter-to- scatter ratio (B/S, 635 nm), asymmetry parameter (g, 635 nm), black carbon mass absorption cross section (MAC, 637 nm) and PM2.5 mass scattering cross section (MSCS, 635 nm) were 0.92 ± 0.03, 1.56 ± 0.88, 0.16 ± 0.09, 0.53 ± 0.16, 10.9 ± 3.5 m2 gĝ̂'1 and 2.5 ± 1.3 m2 gĝ̂'1, respectively. The scattering measurements performed at MSC were in the medium/upper range of values reported by Andrews et al. (2011) for other mountaintop sites in Europe due to the frequent regional recirculation scenarios (SREG) and Saharan dust episodes (NAF) occurring mostly in spring/summer and causing the presence of polluted layers at the MSC altitude. However, the development of upslope winds and the possible presence of planetary boundary layer air at MSC altitude in summer may also have contributed to the high scattering observed. Under these summer conditions no clear diurnal cycles were observed for the measured extensive aerosol optical properties (σsp, σbsp and σap). Conversely, low σsp and σap at MSC were measured during Atlantic advections (AA) and winter regional anticyclonic episodes (WREG) typically observed during the cold season in the western Mediterranean. Therefore, a season-dependent decrease in the magnitude of aerosol extensive properties was observed when MSC was in the free troposphere, with the highest free-troposphere vs. all-data difference observed in winter and the lowest in spring/summer. The location of MSC station allowed for a reliable characterization of aerosols as a function of the main synoptic meteorological patterns. The SAE was the lowest during NAF and showed an inverse correlation with the outbreak intensity, indicating a progressive shift toward larger particles. Moreover, the strength of NAF episodes in the region led to a slope of the scattering vs. absorption relationship among the lowest reported for other mountaintop sites worldwide, indicating that MSC was dominated by dust aerosols at high aerosol loading. As a consequence, SSA showed a nearly monotonic increase with increasing particle concentration and scattering. The SAE was the highest during SREG, indicating the presence of polluted layers dominated by smaller particles. Correspondingly, the asymmetry parameter was lower under SREG compared with NAF. The MAC and MSCS were significantly higher during NAF and SREG compared to AA and WREG, indicating an increase of absorption and scattering efficiencies associated with the summer polluted scenarios. The optical measurements performed at the MSC remote site were compared with those simultaneously performed at a regional background station in the western Mediterranean Basin located at around 700 m a.s.l. upstream of the MSC station. ; This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment) and FEDER funds: the projects PRISMA (CGL2012-39623- C02/00) and VAMOS (CLG2010-19464). The research received funding from the European Union Seventh Framework Programme (FP7/2007–2013) ACTRIS under grant agreement no. 262254. M. Pandolfi was funded by the JAE-Doc CSIC programme, co-funded by the European Social Fund (ESF). The authors wish to thank D. C. Carslaw and K. Ropkins for providing the Openair software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012). The authors wish to thank A. Tobias for the statistical analysis related with Fig. 5. The authors are grateful for the use of the HYSPLIT model, SKIRON, BSC-DREAM and NRL-NAAPS aerosol maps, NCEP/NCAR meteorological database NOAA NAO index data, and image products from MODIS and SeaWiFs NASA satellites used in this publication. The authors would like to extend their gratitude to the personnel from the OAdM.We would also like to express our gratitude to METEOCAT for the meteorological data. ; Peer reviewed

A reliable assessment of the optical properties of atmospheric black carbon is of crucial importance for an accurate estimation of radiative forcing. In this study we investigated the spatio-temporal variability of the mass absorption cross-section (MAC) of atmospheric black carbon, defined as light absorption coefficient (σap) divided by elemental carbon mass concentration (mEC). σap and mEC have been monitored at supersites of the ACTRIS network for a minimum period of one year. The 9 rural background sites considered in this study cover southern Scandinavia, central Europe and the Mediterranean. σap was determined using filter based absorption photometers and mEC using a thermal-optical technique. Homogeneity of the data-set was ensured by harmonization of all involved methods and instruments during extensive intercomparison exercises at the European Center for Aerosol Calibration (ECAC). Annual mean values of σap at a wavelength of 637 nm vary between 0.66 and 1.3 Mm−1 in southern Scandinavia, 3.7–11 Mm−1 in Central Europe and the British Isles, and 2.3–2.8 Mm−1 in the Mediterranean. Annual mean values of mEC vary between 0.084 and 0.23 μg m−3 in southern Scandinavia, 0.28–1.1 in Central Europe and the British Isles, and 0.22–0.26 in the Mediterranean. Both σap and mEC in southern Scandinavia and Central Europe have a distinct seasonality with maxima during the cold season and minima during summer, whereas at the Mediterranean sites an opposite trend was observed. Annual mean MAC values were quite similar across all sites and the seasonal variability was small at most sites. Consequently, a MAC value of 10.0 m2 g−1 (geometric standard deviation = 1.33) at a wavelength of 637 nm can be considered to be representative of the mixed boundary layer at European background sites, where BC is expected to be internally mixed to a large extent. The observed spatial variability is rather small compared to the variability of values in previous literature, indicating that the harmonization efforts resulted in substantially increased precision of the reported MAC. However, absolute uncertainties of the reported MAC values remain as high as ± 30–70% due to the lack of appropriate reference methods and calibration materials. The mass ratio between elemental carbon and non-light-absorbing matter was used as a proxy for the thickness of coatings around the BC cores, in order to assess the influence of the mixing state on the MAC of BC. Indeed, the MAC was found to increase with increasing values of the coating thickness proxy. This provides evidence that coatings do increase the MAC of atmospheric BC to some extent, which is commonly referred to as lensing effect. © 2016 The Authors ; The research leading to these results has received funding from the European Union Seventh Framework Programme (ACTRIS, FP7/2007-2013, grant agreement no. 262254 ). ACTRIS-2 is a European Project supported by the European Commission Horizon 2020 Research and Innovation Framework Programme (ACTRIS-2, H2020-INFRAIA-2014-2015, grant agreement no. 654109 ). This work was also supported by grants from Labex OSUG@2020 (PhD investissements d'avenir – ANR10 LABX56) and from the European Research Council (ERC-CoG 615922-BLACARAT). Appendix A. ; Peer reviewed

PM10 and PM1 samples were collected at an urban site in southeastern Spain during 2006-2010. The chemical composition of all samples has been determined and analyzed by Positive Matrix Factorization (PMF) technique for fine and coarse source identification. The PMF results have been analyzed for working and non-working days in order to evaluate the change in PM sources contribution and possible future abatement strategies. A decreasing trend in PM10 levels and in its constituents has been observed, being partly associated to a reduction in anthropogenic activities due to the economic crisis. The use of fine and coarse PM in the PMF analysis allowed us for the identification of additional sources that could not be identified using only one size fraction. The mineral dust source was identified in both fractions and comprised 36 and 22% of the total mass in the coarse and fine fractions, respectively. This high contribution of the mineral source to the fine fraction may be ascribed to contamination of the source profile. The regional re-circulation source was traced by secondary sulfate, V and Ni. It was the most important source concerning PM1 mass concentration (41% of the total mass in this fraction). Although V and Ni are commonly associated to fuel oil combustion the seasonality of this source with higher concentrations in summer compared with winter suggest that the most important part of this source can be ascribed to regional pollution episodes. A traffic exhaust source was identified but only in the fine fraction, comprising 29% of the fine mass. The celestite mines source associated with nearby open-pit mines was typified by strontium, sulfate and mineral matter. PM10-1 levels were higher in working days, whereas PM1 levels remained fairly constant throughout the whole week. As a conclusion, traffic seems to be the main source to target in Granada. ; This work was supported by the Andalusia Regional Government through projects P12-RNM-2409 and P10-RNM-6299, by the Spanish Ministry of Science and Technology through projects CGL2010-18782, CSD2007-00067, CGL2011-13580-E/CLI and CGL2011-15008-E; and by EU through ACTRIS project (EU INFRA-2010-1.1.16-262254). Information on African dust available on CALIMA is obtained in the framework of a contract between CSIC and the Spanish Ministry of Agriculture, Food and Environment (MAGRAMA). We would like to thanks also Openair project. G. Titos was funded by Spanish Ministry of Economy and Competitiveness – Secretariat of Science Innovation and Development. ; Peer reviewed

Further research is needed to reduce the existing uncertainties on the effect that specific aerosol particle sources have on light extinction and consequently on climate. This study presents a new approach that aims to quantify the mass scattering and absorption efficiencies (MSEs and MAEs) of different aerosol sources at urban (Barcelona - BCN), regional (Montseny - MSY) and remote (Montsec - MSA) background sites in the north-western (NW) Mediterranean. An analysis of source apportionment to the measured multi-wavelength light scattering (σsp) and absorption (σap) coefficients was performed by means of a multilinear regression (MLR) model for the periods 2009-2014, 2010-2014 and 2011-2014 at BCN, MSY and MSA respectively. The source contributions to PM10 mass concentration, identified by means of the positive matrix factorization (PMF) model, were used as dependent variables in the MLR model. With this approach we addressed both the effect that aerosol sources have on air quality and their potential effect on light extinction through the determination of their MSEs and MAEs. An advantage of the presented approach is that the calculated MSEs and MAEs take into account the internal mixing of atmospheric particles. Seven aerosol sources were identified at MSA and MSY, and eight sources at BCN. Mineral, aged marine, secondary sulfate, secondary nitrate and V-Ni bearing sources were common at the three sites. Traffic, industrial/metallurgy and road dust resuspension sources were isolated at BCN, whereas mixed industrial/traffic and aged organics sources were identified at MSY and MSA. The highest MSEs were observed for secondary sulfate (4.5 and 10.7 m2 g-1, at MSY and MSA), secondary nitrate (8.8 and 7.8 m2 g-1) and V-Ni bearing source (8 and 3.5 m2 g-1). These sources dominated the scattering throughout the year with marked seasonal trends. The V-Ni bearing source, originating mainly from shipping in the area under study, simultaneously contributed to both σsp and σap, being the second most efficient light-absorbing source in BCN (MAE = 0.9 m2 g-1). The traffic source at BCN and the industrial/traffic at MSY exhibited the highest MAEs (1.7 and 0.9 m2 g-1). These sources were major contributors to σap at BCN and MSY; however at MSA, secondary nitrate exerted the highest influence on σap (MAE = 0.4 m2 g-1). The sources which were predominantly composed of fine and relatively dark particles, such as industrial/traffic, aged organics and V-Ni, were simultaneously characterized by low single scattering albedo (SSA) and a high scattering Ångström exponent (SAE). Conversely, mineral and aged marine showed the lowest SAE and the highest SSA, being scattering the dominant process in the light extinction. The good agreement found between modelled and measured particle optical properties allowed the reconstruction of σsp and σap long-term series over the period 2004-2014 at MSY. Significant decreasing trends were found for the modelled σsp and σap (-4.6 and -4.1 % yr-1). © Author(s) 2018. ; Acknowledgements. This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness) and FEDER funds under the PRISMA project (CGL2012-39623-C02/00) by the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment) and by the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA). This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 654109. Marco Pandolfi is funded by a Ramón y Cajal Fellowship (RYC-2013-14036) awarded by the MINECO. The authors would like to express their gratitude to Karl Ropkins and David C. Carslaw for providing the OpenAir software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012). ; Peer reviewed

Further research is needed to reduce the existing uncertainties on the effect that specific aerosol sources have on radiative forcing, thus supporting the assessment of future mitigation strategies which should be focused on both air quality and climate, and not acting separately. This study presents a new approach aimed at quantifying the mass scattering and absorption efficiencies (MSE and MAE) of different aerosol sources at urban (Barcelona-BCN), regional (Montseny-MSY) and remote (Montsec-MSA) background sites in the northwestern (NW) Mediterranean. An analysis of source apportionment to the measured scattering and absorption coefficients was performed by means of a multilinear regression (MLR) model during 2010–2014 at BCN and MSY and during 2011–2014 at MSA. The source contributions to PM10 mass, identified by means of the Positive Matrix Factorization (PMF) model, were used as dependent variables in the MLR model in order to take into account the internal mixing state of atmospheric aerosols. Seven aerosol sources were obtained at MSA and MSY and 8 sources at BCN. Mineral, Aged marine, Ammonium sulfate, Ammonium nitrate and V-Ni bearing sources were common at the three sites. Traffic, Industrial/metallurgy and Road-resuspension were isolated at BCN, whereas Industrial/Traffic and Aged organics were solely identified at MSY and MSA. The highest MSE were found for Ammonium sulfate (4.5 and 10.7 m2 g−1), Ammonium nitrate (8.8 and 7.8 m2 g−1) and V-Ni (8 and 3.5 m2 g−1) at MSY and MSA respectively, dominating the scattering throughout the year with marked seasonal trends. V-Ni bearing, originated mainly from shipping emissions, simultaneously contributed to both scattering and absorption being the second most efficient absorptive source in BCN (0.9 m2 g−1). The Traffic source at BCN and the equivalent Industrial/Traffic at MSY and MSA mainly governed the light absorption and exhibited the highest MAE (1.7, 0.9 and 0.2 m2 g−1, respectively). Sources predominantly composed by fine and relatively dark particles such as Industrial/Traffic, Aged organics and V-Ni were simultaneously characterized with low single scattering albedo (SSA) and high scattering Angstrom exponent (SAE). Conversely, Mineral and Aged marine showed the highest SAE and the lowest SSA, being scattering the dominant process in the light extinction. The good agreement between modeled and measured optical properties allowed for the reconstruction of scattering, absorption and SSA time series by means of the PMF-MLR technique for the period 2004–2014 at MSY. Significant decreasing trends were found for the modeled scattering and absorption (−4.6 and −4.1 % y−1) coefficients. Interestingly, the observed reduction in the SSA (−0.11 % y−1) might suggests a less effectiveness of the air quality strategies focused on reducing pollutants containing black carbon (BC) particles, which highly contribute to light absorption and thus climate warming. ; This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness) and FEDER funds under the PRISMA project (CGL2012-39623-C02/00), by the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment) and by the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA). This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654109. Marco Pandolfi is funded by a Ramón y Cajal Fellowship (RYC-2013-14036) awarded by the MINECO. The authors would like to express their gratitude to D. C. Carslaw and K. Ropkins for providing the OpenAir software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012). ; Peer reviewed

In this work for the first time data from two twin stations (Barcelona, urban background, and Montseny, regional background), located in NE of Spain, were used to study the trends of the concentrations of different chemical species in PM10 and PM2.5 along with the trends of the PM10 source contributions from Positive Matrix Factorization (PMF) model. Eleven years of chemical data (2004–2014) were used for this study. Trends of both specie concentrations and source contributions were studied using the Mann-Kendall test for linear trends and a new approach based on multi-exponential fit of the data. Despite the fact that different PM fractions (PM1, PM2.5, PM10) showed linear decreasing trends at both stations, the contributions of specific sources of pollutants and the related chemical tracers showed exponential (single or double) decreasing trends. The different types of trends observed reflected the different effectiveness and/or time of implementation of the measures taken to reduce the concentrations of atmospheric pollutants (i.e. those implemented in the Industrial Emission Directives and in the Large Combustion Plants Directive). Moreover, the trends of the contributions from specific sources such as those related with industrial activities and with primary energy consumption mirrored the effect of the financial crisis in Spain from 2008. The sources that showed statistically significant downward trends at both Barcelona (BCN) and Montseny (MSY) during 2004–2014 were Ammonium sulfate, Ammonium nitrate, and V-Ni bearing source. The contributions from these sources decreased exponentially during the considered period indicating that the observed decrease was not gradual and consistent over time. Moreover, statistically significant decreasing trends were observed for the contributions to PM from the Industrial/Traffic source at MSY (mixed metallurgy and road traffic) and from the Industrial (metallurgy mainly) source at BCN. These sources were clearly linked with anthropogenic activities and the observed decreasing trends confirmed the effectiveness of pollution control measures implemented at EU or regional/local levels. The general trends observed for the calculated PMF source contributions well reflected the trends observed for the chemical tracers of these pollutant sources. ; This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment), the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA) and FEDER funds under the PRISMA project (CGL2012‐39623‐ C02/00). The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654109 and previously from the European Union Seventh Framework Programme (FP7/2007‐2013) under grant agreement n° 262254. NO2 map analyses and visualizations used in this paper were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. The authors would like to express their gratitude to D. C. Carslaw and K. Ropkins for providing the Openair software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012). ; Peer reviewed

The study of Saharan dust events (SDE) and biomass burning (BB) emissions are both topic of great scientific interest since they are frequent and important polluting scenarios affecting air quality and climate. The main aim of this work is evaluating the feasibility of using near real-time in situ aerosol optical measurements for the detection of these atmospheric events in the Western Mediterranean Basin (WMB). With this aim, intensive aerosol optical properties (SAE: scattering Ångström exponent, AAE: absorption Ångström exponent, SSAAE: single scattering albedo Ångström exponent, and g: asymmetry parameter) were derived from multi-wavelength aerosol light scattering, hemispheric backscattering and absorption measurements performed at regional (Montseny; MSY, 720 m a.s.l.) and continental (Montsec; MSA, 1570 m a.s.l.) background sites in the WMB. A sensitivity study aiming at calibrating the measured intensive optical properties for SDE and BB detection is presented and discussed. The detection of Saharan dust events (SDE) by means of the SSAAE parameter and Ångström matrix depended on the altitude of the measurement station, and on SDE intensity. At MSA (mountain-top site) SSAAE detected around 85% of SDE compared with 50% at MSY station, where pollution episodes dominated by fine anthropogenic particles frequently masked the effect of mineral dust on optical properties during less intense SDE. Furthermore, an interesting feature of SSAAE was its capability to detect the presence of mineral dust after the end of SDE. Thus, resuspension processes driven by summer regional atmospheric circulations and dry conditions after SDE favored the accumulation of mineral dust at regional level having important consequences for air quality. On average, SAE, AAE and g ranged between -0.7 and 1, 1.3 and 2.5, and 0.5 and 0.75, respectively, during SDE. Based on the Aethalometer model, biomass burning (BB) contribution to equivalent black carbon (BC) accounted for 36% and 40% at MSY and MSA respectively. Linear relationships were found between AAE and bb, with AAE values reaching around 1.5 when bb was higher than 50%. BB contribution to organic matter (OM) at MSY was around 30%. Thus FF combustion sources showed important contributions to both BC and OM in the region under study. Results for OM source apportionment showed good agreement with simultaneous biomass burning organic aerosol (BBOA) and hydrocarbon-like organic aerosol (HOA) calculated from Positive Matrix Factorization (PMF) applied to simultaneous Aerosol Mass Spectrometer (ACSM) measurements. A wildfire episode was identified at MSY, showing AAE values up to 2 when daily BB contributions to BC and OM were 73% and 78% respectively. ; This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment), the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA) and FEDER funds under the PRISMA project (CGL2012-39623- C02/00). This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654109. The authors would like to express their gratitude to D. C. Carslaw and K. Ropkins for providing the Openair software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012). ; Peer reviewed

The study of Saharan dust events (SDEs) and biomass burning (BB) emissions are both topics of great scientific interest since they are frequent and important polluting scenarios affecting air quality and climate. The main aim of this work is evaluating the feasibility of using near-real-time in situ aerosol optical measurements for the detection of these atmospheric events in the western Mediterranean Basin (WMB). With this aim, intensive aerosol optical properties (SAE: scattering Ångström exponent, AAE: absorption Ångström exponent, SSAAE: single scattering albedo Ångström exponent and AAE: asymmetry parameter) were derived from multi-wavelength aerosol light scattering, hemispheric backscattering and absorption measurements performed at regional (Montseny; MSY, 720 m a.s.l.) and continental (Montsec; MSA, 1570 m a.s.l.) background sites in the WMB. A sensitivity study aiming at calibrating the measured intensive optical properties for SDEs and BB detection is presented and discussed. The detection of SDEs by means of the SSAAE parameter and Ångström matrix (made up by SAE and AAE) depended on the altitude of the measurement station and on SDE intensity. At MSA (mountain-top site) SSAAE detected around 85% of SDEs compared with 50% at the MSY station, where pollution episodes dominated by fine anthropogenic particles frequently masked the effect of mineral dust on optical properties during less intense SDEs. Furthermore, an interesting feature of SSAAE was its capability to detect the presence of mineral dust after the end of SDEs. Thus, resuspension processes driven by summer regional atmospheric circulations and dry conditions after SDEs favoured the accumulation of mineral dust at regional level having important consequences for air quality. On average, SAE, AAE and g ranged between-0.7 and 1, 1.3 and 2.5 and 0.5 and 0.75 respectively during SDEs. Based on the aethalometer model, BB contribution to equivalent black carbon (BC) accounted for 36 and 40 % at MSY and MSA respectively. Linear relationships were found between AAE and bb, with AAE values reaching around 1.5 when bb was higher than 50%. BB contribution to organic matter (OM) at MSY was around 30%. Thus fossil fuel (FF) combustion sources showed important contributions to both BC and OM in the region under study. Results for OM source apportionment showed good agreement with simultaneous biomass burning organic aerosol (BBOA) and hydrocarbon-like organic aerosol (HOA) obtained by applying a positive matrix factorization model (PMF) to simultaneous Aerosol Chemical Speciation Monitor (ACSM) measurements. A wildfire episode was identified at MSY, showing AAE values up to 2 when daily BB contributions to BC and OM were 73 and 78% respectively. © 2016 The Author(s). ; This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment), the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA) and FEDER funds under the PRISMA project (CGL2012- 39623-C02/00). This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654109. Marco Pandolfi is funded by a Ramón y Cajal Fellowship (RYC-2013-14036) awarded by the Spanish Ministry of Economy and Competitiveness. The authors would like to express their gratitude to D. C. Carslaw and K. Ropkins for providing the OpenAir software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012). ; Peer reviewed

Time variation of mass particulate matter (PM1 and PM1&minus10), black carbon (BC) and number of particles (N3: number of particles with an aerodynamic diameter higher than 3 nm, andN10: higher than 10 nm) concentrations at the high-altitude site of Montsec (MSC) in the southern Pyrenees was interpreted for the period 2010-2012. At MSC, PM10 (12 Î1/4g mĝ̂'3) andN7 (2140 # cmĝ̂'3) three-year arithmetic average concentrations were higher than those measured at other high-altitude sites in central Europe during the same period (PM10: 3-9 Î1/4g mĝ̂'3 andN: 634-2070 # cmĝ̂'3). By contrast, BC concentrations at MSC (0.2 Î1/4g mĝ̂'3) were equal to or even lower than those measured at these European sites (0.2-0.4 Î1/4g mĝ̂'3). These differences were attributed to the higher relevance of Saharan dust transport and to the higher importance of the biogenic precursor emissions and new particle formation (NPF) processes, and to the lower influence of anthropogenic emissions at MSC. The different time variation of PM and BC concentrations compared with that ofNsuggests that these aerosol parameters were governed by diverse factors at MSC. Both PM and BC concentrations showed marked differences for different meteorological scenarios, with enhanced concentrations under North African air outbreaks (PM1&minus10: 13 Î1/4g mĝ̂'3, PM1: 8 Î1/4g mĝ̂'3 and BC: 0.3 Î1/4g mĝ̂'3) and low concentrations when Atlantic advections occurred (PM1−10: 5 Î1/4g mĝ̂'3, PM1: 4 Î1/4g mĝ̂'3 and BC: 0.1 Î1/4g mĝ̂'3). PM and BC concentrations increased in summer, with a secondary maximum in early spring, and were at their lowest in winter, due to the contrasting origin of the air masses in the warmer seasons (spring and summer) and in the colder seasons (autumn and winter). The maximum in the warmer seasons was attributed to long-range transport processes that mask the breezes and regional transport breaking the daily cycles of these pollutants. By contrast, PM and BC concentrations showed clear diurnal cycles, with maxima at midday in the colder seasons. A statistically significant weekly variation was also obtained for the BC concentrations, displaying a progressive increase from Tuesday to Saturday, followed by a significant decrease on Sunday and Monday. Nconcentrations depended more on local meteorological variables such as temperature and solar radiation intensity than on the origin of the air mass. Therefore, arithmetic averages as a function of meteorological episodes showed the highest concentrations ofNduring summer regional episodes (N3: 4461 # cmĝ̂'3 andN7: 3021 # cmĝ̂'3) and the lowest concentrations during winter regional scenarios (N3: 2496 # cmĝ̂'3 and $N7: 1073 # cmĝ̂'3). This dependence on temperature and solar radiation also accounted for the marked diurnal cycle ofNconcentrations throughout the year, with a peak at midday and for the absence of a weekly pattern. Measurements carried out at MSC enabled us to characterize the tropospheric background aerosols in the western Mediterranean basin (WMB). Our results highlight the importance of the NPF processes in southern Europe, underline the high contribution of long-range dust transport with respect to central Europe and its prevalence in elevated layers, and reveal that MSC is much less affected by anthropogenic emissions than other high-altitude sites in central Europe. ; This study was supported by the Ministry of Economy and Competitiveness and FEDER funds under the PRISMA (CGL2012-39623-C02-0), and CARIATI (CGL2008- 06294/CLI) projects, and by the Generalitat de Catalunya (AGAUR 2009 SGR8 and DGQA). The research received funding from European Union Seventh Framework Programme (FP7/ 2007-2013) ACTRIS under grant agreement no. 262254. The authors would like to extend their gratitude to the personnel form the OAdM. We would also like to express our gratitude to METEOCAT for the meteorological data, to ACTRIS Data Center for the data from the high-altitude central Europe sites, to the Laboratory of Atmospheric Chemistry at the Paul Scherrer Institute for providing the Jungfraujoch BC and N data, the International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat (HFSJG), and the support by MeteoSwiss within the Swiss program of the WMO Global Atmosphere Watch (GAW), to the NABEL (FOEN/Empa) for providing the PM data from the Swiss sites, to the background monitoring network of Umweltbundesamt, Vienna, for providing the Vorhegg PM10 data, to the OPGC (Observatoire de Physique du Globe de Clermont-Ferrand) for providing the Puy de Dôme BC and N data, and to the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model, and the boundary layer height calculation used in this publication. M. C. Minguillón and M. Pandolfi were funded by the JAE-Doc CSIC program, co-funded by the European Social Fund (ESF). ; Peer reviewed

An outstanding Saharan dust event affected the Iberian Peninsula during winter 2016 (20 to 25 February). The impact of this event on ambient PM10 surface levels and its spatial and temporal evolution was investigated by means of data from 250 air quality stations across mainland Spain and the Balearic Islands. The event had a significant impact on surface PM10 levels, and on 22 February, 90% of the air quality monitoring sites registered PM10 concentrations above the EU daily limit value of 50 µg/m3. The study of the attenuated backscattering vertical profiles associated with African dust evidenced a complex structure, with a thick aerosol layer that was at higher altitudes over the pre-Pyrenees compared to the coastal area of Barcelona but closer to the surface than typically observed at both sites. Optical and physical properties of dust particles were investigated at the continental background Global Atmosphere Watch mountain observatory of Montsec (MSA) in the pre-Pyrenees. Good agreement was found between in situ and passive remote sensing methodologies once the aloft dust layer reached the MSA station. Scattering Ångström exponent values decreased to values close to zero (even below zero for surface in situ measurements) indicating the predominance of coarse particles. On the contrary, absorption Ångström exponent values increased during the Saharan dust outbreak denoting an absorption enhancement at shorter wavelengths, characteristic of mineral dust particles. Furthermore, the performance of NMMB/BSC-Dust and BSC-DREAM8b models has been qualitatively evaluated for the dust spatial distribution across Spain and the vertical structure over MSA and Barcelona showing good agreement. © 2017. The Authors. ; This work was supported by the MINECO (Spanish Ministry of Economy and Competitiveness, and FEDER funds, under the PRISMA project (CGL2012- 39623-C02/00) and project TEC2015- 63832-P), the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment), the Generalitat de Catalunya (AGAUR 2014 SGR33, and the Departament de Territori I Sostenibilitat). This work has received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement 654109 (ACTRIS-2). M. Pandolfi is funded by a Ramón y Cajal Fellowship (RYC-2013-14036) awarded by the Spanish Ministry of Economy and Competitiveness. G. Titos is funded by the Spanish Ministry of Economy and Competitiveness under postdoctoral programme Juan de la Cierva-formación (FJCI-2014-20819). We thank Alberto Cazorla for in-depth discussion about the ceilometer data processing. Ana Calvo is thankfully acknowledged for helpful discussion on the optical spec trometer and providing the diameters correction. Th.View More ; Peer reviewed