Comparison between simulated and observed chemical composition of fine aerosols in Paris (France) during springtime: contribution of regional versus continental emissions

Hourly concentrations of inorganic salts (ions) and carbonaceous material in fine aerosols (aerodynamic diameter, A.D. <2.5 μm) have been determined experimentally from fast measurements performed for a 3-week period in spring 2007 in Paris (France). The sum of these two chemical components (ions and carbonaceous aerosols) has shown to account for most of the fine aerosol mass (PM<sub>2.5</sub>). This time-resolved dataset allowed investigating the factors controlling the levels of PM<sub>2.5</sub> in Paris and showed that polluted periods with PM<sub>2.5</sub> > 15 μg m<sup>−3</sup> were characterized by air masses of continental (North-Western Europe) origin and chemical composition made by 75% of ions. By contrast, periods with clean marine air masses have shown the lowest PM<sub>2.5</sub> concentrations (typically of about 10 μg m<sup>−3</sup>); carbonaceous aerosols contributing for most of this mass (typically 75%). <br><br> In order to better discriminate between local and continental contributions to the observed chemical composition and concentrations of PM<sub>2.5</sub> over Paris, a comparative study was performed between this time-resolved dataset and the outputs of a chemistry transport model (CHIMERE), showing a relatively good capability of the model to reproduce the time-limited intense maxima observed in the field for PM<sub>2.5</sub> and ion species. Different model scenarios were then investigated switching off local and European (North-Western and Central) emissions. Results of these scenarios have clearly shown that most of the ions observed over Paris during polluted periods, were either transported or formed in-situ from gas precursors transported from Northern Europe. On the opposite, long-range transport from Europe appeared to weakly contribute to the levels of carbonaceous aerosols observed over Paris. <br><br> The model failed to properly account for the concentration levels and variability of secondary organic aerosols (SOA) determined experimentally by the EC-tracer method. The abundance of SOA (relatively to organic aerosol, OA) was as much as 75%, showing a weak dependence on air masses origin. Elevated SOA/OA ratios were also observed for air masses having residence time above ground of less than 10 h, suggesting intense emissions and/or photochemical processes leading to rapid formation of secondary organic aerosols

Overview of the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Forcing on the Mediterranean Climate (ChArMEx/ADRIMED) summer 2013 campaign

The Chemistry-Aerosol Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr) is a collaborative research program federating international activities to investigate Mediterranean regional chemistry-climate interactions. A special observing period (SOP-1a) including intensive airborne measurements was performed in the framework of the Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region (ADRIMED) project during the Mediterranean dry season over the western and central Mediterranean basins, with a focus on aerosol-radiation measurements and their modeling. The SOP-1a took place from 11 June to 5 July 2013. Airborne measurements were made by both the ATR-42 and F-20 French research aircraft operated from Sardinia (Italy) and instrumented for in situ and remote-sensing measurements, respectively, and by sounding and drifting balloons, launched in Minorca. The experimental setup also involved several ground-based measurement sites on islands including two ground-based reference stations in Corsica and Lampedusa and secondary monitoring sites in Minorca and Sicily. Additional measurements including lidar profiling were also performed on alert during aircraft operations at EARLINET/ACTRIS stations at Granada and Barcelona in Spain, and in southern Italy. Remote-sensing aerosol products from satellites (MSG/SEVIRI, MODIS) and from the AERONET/PHOTONS network were also used. Dedicated meso-scale and regional modeling experiments were performed in relation to this observational effort. We provide here an overview of the different surface and aircraft observations deployed during the ChArMEx/ADRIMED period and of associated modeling studies together with an analysis of the synoptic conditions that determined the aerosol emission and transport. Meteorological conditions observed during this campaign (moderate temperatures and southern flows) were not favorable to producing high levels of atmospheric pollutants or intense biomass burning events in the region. However, numerous mineral dust plumes were observed during the campaign, with the main sources located in Morocco, Algeria and Tunisia, leading to aerosol optical depth (AOD) values ranging between 0.2 and 0.6 (at 440 nm) over the western and central Mediterranean basins. One important point of this experiment concerns the direct observations of aerosol extinction onboard the ATR-42, using the CAPS system, showing local maxima reaching up to 150Mm(-1) within the dust plume. Non-negligible aerosol extinction (about 50Mm(-1)) has also been observed within the marine boundary layer (MBL). By combining the ATR- 42 extinction coefficient observations with absorption and scattering measurements, we performed a complete optical closure revealing excellent agreement with estimated optical properties. This additional information on extinction properties has allowed calculation of the dust single scattering albedo (SSA) with a high level of confidence over the western Mediterranean. Our results show a moderate variability from 0.90 to 1.00 (at 530 nm) for all flights studied compared to that reported in the literature on this optical parameter. Our results underline also a relatively low difference in SSA with values derived near dust sources. In parallel, active remote-sensing observations from the surface and onboard the F-20 aircraft suggest a complex vertical structure of particles and distinct aerosol layers with sea spray and pollution located within the MBL, and mineral dust and/or aged North American smoke particles located above (up to 6–7 km in altitude). Aircraft and balloon-borne observations allow one to investigate the vertical structure of the aerosol size distribution showing particles characterized by a large size (> 10 μm in diameter) within dust plumes. In most of cases, a coarse mode characterized by an effective diameter ranging between 5 and 10 μm, has been detected above the MBL. In terms of shortwave (SW) direct forcing, in situ surface and aircraft observations have been merged and used as inputs in 1-D radiative transfer codes for calculating the aerosol direct radiative forcing (DRF). Results show significant surface SW instantaneous forcing (up to (-90)Wm(-2) at noon). Aircraft observations provide also original estimates of the vertical structure of SW and LW radiative heating revealing significant instantaneous values of about 5 K per day in the solar spectrum (for a solar angle of 30 ) within the dust layer. Associated 3-D modeling studies from regional climate (RCM) and chemistry transport (CTM) models indicate a relatively good agreement for simulated AOD compared with observations from the AERONET/PHOTONS network and satellite data, especially for long-range dust transport. Calculations of the 3-D SW (clear-sky) surface DRF indicate an average of about -10 to -20Wm(-2) (for the whole period) over the Mediterranean Sea together with maxima (-50Wm(-2)) over northern Africa. The top of the atmosphere (TOA) DRF is shown to be highly variable within the domain, due to moderate absorbing properties of dust and changes in the surface albedo. Indeed, 3-D simulations indicate negative forcing over the Mediterranean Sea and Europe and positive forcing over northern Africa. Finally, a multiyear simulation, performed for the 2003 to 2009 period and including an ocean–atmosphere (O–A) coupling, underlines the impact of the aerosol direct radiative forcing on the sea surface temperature, O–A fluxes and the hydrological cycle over the Mediterranean.French National Research Agency (ANR) ANR-11-BS56-0006ADEMEFrench Atomic Energy CommissionCNRS-INSU and Meteo-France through the multidisciplinary programme MISTRALS (Mediterranean Integrated Studies aT Regional And Local Scales)CORSiCA project - Collectivite Territoriale de Corse through Fonds Europeen de Developpement Regional of the European Operational ProgramContrat de Plan Etat-RegionEuropean Union's Horizon 2020 research and innovation program 654169Spanish Ministry of Economy and Competitivity TEC2012-34575Science and Innovation UNPC10-4E-442European Union (EU)Department of Economy and Knowledge of the Catalan Autonomous Government SGR 583Andalusian Regional Government P12-RNM-2409Spanish Government CGL2013-45410-R 26225

Composition of gaseous organic carbon during ECOCEM in Beirut, Lebanon: new observational constraints for VOC anthropogenic emission evaluation in the Middle East

The relative importance of eastern Mediterranean emissions is suspected to be largely underestimated compared to other regions worldwide. Here we use detailed speciated measurements of volatile organic compounds (VOCs) to evaluate the spatial heterogeneity of VOC urban emission composition and the consistency of regional and global emission inventories downscaled to Lebanon (European Monitoring and Evaluation Programme, EMEP; Atmospheric Chemistry and Climate Model Intercomparison Project, ACCMIP; and MACCity, Monitoring Atmospheric Composition and Climate and megaCITY Zoom for the Environment). The assessment was conducted through the comparison of the emission ratios (ERs) extracted from the emission inventories to the ones obtained from the hourly observations collected at a suburban site in Beirut, Lebanon, during summer and winter ECOCEM (Emissions and Chemistry of Organic Carbon in the Eastern Mediterranean) campaigns. The observed ERs were calculated using two independent methods. ER values from both methods agree very well and are comparable to the ones of the road transport sector from near-field measurements for more than 80 % of the species. There is no significant seasonality in ER for more than 90 % of the species, unlike the seasonality usually observed in other cities worldwide. Regardless of the season, ERs agree within a factor of 2 between Beirut and other representative cities worldwide, except for the unburned fuel fraction and ethane. ERs of aromatics (except benzene) are higher in Beirut compared to northern post-industrialized countries and even the Middle Eastern city Mecca. The comparison of the observed ER to the ones extracted from the ACCMIP and MACCity global emission inventories suggests that the overall speciation of anthropogenic sources for major hydrocarbons that act as ozone and secondary organic aerosol (SOA) precursors in ACCMIP is better represented than other species. <br><br> The comparison of the specific road transport ERs, relative to acetylene derived from near-field measurements, to ERs from ACCMIP and EMEP emission inventories for the road transport sector showed that ERs of more reactive species are usually consistent within a factor of 2 with EMEP, while xylenes and toluene are underestimated by over a factor of 2 by ACCMIP. <br><br> The observed heterogeneity of anthropogenic VOC emission composition between Middle Eastern cities can be significant for reactive VOCs but is not depicted by global emission inventories. This suggests that systematic and detailed measurements are needed in the eastern Mediterranean Basin in order to better constrain emission inventory

Wet Deposition Fluxes of Mineral Dust and Their Relation With Cold Pools in the Central Sahel

International audienceBased on a large number of in-situ measurements performed over a 9-years period in twoSahelian stations, we investigate the drivers of the dust wet deposition in relation to the meteorologicalsituations and the PM10 (Particulate Matter with diameter lower than 10 μm) surface concentrations.Precipitation associated with cold pools (CP) contribute to more than 90% of the precipitation amountassociated with the collected wet deposition samples. The wet deposition events associated with these CPcontrol by far the wet deposition, that is, 66% and 81%, depending on the station. The dust washout ratios (WR)corresponding to the most convective events under high level of dust concentrations were found to be in therange of 319–766 while WR of other kind of events are depending on the dilution effect. This range of value arein the lower range of WR previously estimated and used in dust modeling studies (200–2000)

Evidence a major source of water insoluble secondary organic aerosols in the region of Paris (France) during wintertime

International audienceNear real-time measurements of carbonaceous aerosols were performed in fine aerosols for a 10-day period during winter at a suburban site of Paris (France). These measurements were performed using an OCEC Sunset Field instrument for elemental carbon (EC) and organic carbon (OC); a Particle-Into-Liquid-Sampler coupled with a Total Organic Carbon (PILS-TOC) instrument for water-soluble OC (WSOC); and a 7-lambda aethalometer for absorption. A successful comparison was performed with filter sampling performed in parallel for EC, OC, and WSOC, providing further confidence on the results obtained by the online analyzers. A modified version of the aethalometer model was used to derive hourly concentrations of 3 organic aerosol (OA) sources: fossil fuel, wood burning, and secondary. This source apportionment was validated for primary OA (fossil fuel, wood burning) using time-resolved measurements of specific tracers (including levoglucosan, water-soluble potassium and methanol for wood burning) and showed that secondary organic aerosols (SOA) were the most abundant OA species during our study. Water-soluble properties of these different OA sources were investigated from the reconstruction of experimentally determined water-soluble/insoluble OC. About 23% of WSOC was found to be of a secondary (photochemical) origin. A large fraction of SOA was assigned as water-insoluble and could originate from semi-volatile primary OA from wood burning and/or anthropogenic emissions. These results have been obtained at a typical suburban site in France and may be then representative of a larger European area. They bring new light on the commonly accepted idea that SOA is mainly water-soluble

Rain, Wind, and Dust Connections in the Sahel

International audienceThe Sahel is a dust source region where dust emission could be drastically modified in the future due to climatic and land use changes. Based on observations of meteorological parameters and dust concentration for about 1,000 rain events, we investigated the processes leading to dust emission during the rainy season when Mesoscale Convective Systems (MCSs) regularly cross the Sahel. We show that the highest wind speed is strongly linked to the MCS cold pool intensity, which is characterized by a drop in surface temperature. This is observed during the premonsoon period (∼May to June) when the midtroposphere is still sufficiently dry to allow intense evaporation of raindrops. Because this coincides with the time of the year that the surface is the least protected by the vegetative residue, the premonsoon wind speed leads to the highest observed dust concentration in our record. Most of the highest wind speed occur before or just at the beginning of a rainy event allowing a large part of the dust raised to be transported ahead the rain limiting dust removal by wet scavenging. Finally, we show that the number of 5-min dust concentration higher than 5,000 μg m−3 is almost only occurring during the rainy season. These results suggest that until the dust models fail to correctly resolve MCS, it will be difficulty to obtain reliable estimates of dust emission from the Sahel for the present or future scenarios

The Respective Roles of Wind Speed and Green Vegetation in Controlling Sahelian Dust Emission during the Wet Season

International audienceBased on 10 years of continuous measurements of wind speed, rainfall, and PM10 concentrations (i.e., concentrations of the particulate matter having a diameter lower than 10 μm) performed in two Sahelian stations, we examine how wind speed and vegetation interact during the wet season to control the dust concentration when it is due to local dust emissions. The results clearly show that the frequency of the high wind speeds is higher at the beginning of the wet season and is the main driver of the seasonal dust emission. During the second part of the wet season, the frequency of high wind speeds is much lower and, in addition, their efficiency for wind erosion and dust emission is strongly affected by the vegetation whose growth reduces progressively PM10 concentrations up to 80%

The Respective Roles of Wind Speed and Green Vegetation in Controlling Sahelian Dust Emission during the Wet Season

International audienceBased on 10 years of continuous measurements of wind speed, rainfall and PM10 concentrations 29 (i.e., concentrations of the particulate matter having a diameter lower than 10 µm) performed in 30 two Sahelian stations, we examine how wind speed and vegetation interact during the wet season 31 to control the dust concentration when it is due to local dust emissions. The results clearly show 32 that the frequency of the high wind speeds is higher at the beginning of the wet season and is the 33 main driver of the seasonal dust emission. During the second part of the wet season, the 34 frequency of high wind speeds is much lower and, in addition, their efficiency for wind erosion 35 and dust emission is strongly affected by the vegetation whose growth reduces progressively 36 PM10 concentrations up to 80%

Les vents érosifs au Sahel Central : une analyse fondée sur 10 années de suivi météorologique à haute résolution temporelle

International audienceDepuis 2006 trois stations de suivi des poussières désertiques, installées dans le cadre du programme AMMA et pérennisées au sein du SNO INDAAF (cf Poster) fournissent la vitesse, la direction du vent et la pluviométrie au pas de temps de 5 mn. Une analyse détaillée de ces données collectées sur 10 années au Mali et au Niger a permis de décrire le cycle journalier et saisonnier des vents supérieurs aux vitesses seuil d'érosion éolienne sur un sol nu et du potentiel de soulèvement de poussière (Dust Uplift Potentiel-DUP) (Bergametti et al. 2017). Tout au long de l'année des vents érosifs associés à la dissipation du jet nocturne par la turbulence thermique soufflent en milieu et fin de matinée. Mais ces vents ne contribuent que très peu au DUP. L'essentiel du potentiel d'érosion est produit par des vents très intenses, principalement nocturnes, associés aux systèmes convectifs de méso-échelle qui balaient le Sahel en saison des pluies. Ceci produit une très forte saisonnalité du DUP dont plus de 70% se concentrent dans les 90 jours de début de saison des pluies de mi-avril à mi-juillet. De façon encore plus remarquable, la durée des vents forts est très courte : plus de 80% de ces vents durent moins de 3 heures. Ceci suggère que la fréquence de mesures aux stations synoptiques n'est pas assez élevée pour quantifier la contribution réelle de ces vents forts au DUP. Enfin, l'effet inhibiteur des pluies sur l'érosion éolienne a été pris en compte grâce aux données pluviométriques (Bergametti et al. 2016). L'inhibition par les pluies affecterait ainsi environ un quart du DUP total. La durée totale des vents érosifs dans le Sahel central ne représente qu'environ 1% de l'année. Seul un suivi continu à une fréquence élevée, et avec des capteurs précis et stables peut permettre une description réaliste des vents érosifs au Sahel. Le défi est de maintenir ces mesures sur le long terme pour pouvoir appréhender l'évolution climatique de l'érosion éolienne. Bergametti, G., et al., 2016. How long does precipitation inhibit wind erosion in the Sahel? Geophysical Research Letters, 43, 6643-6649. Bergametti G. et al., 2017. Dust uplift potential in the Central Sahel: an analysis based on 10 years of meteorological measurements at high temporal resolution