ALBERT

Description: The study of aerosols in the troposphere and in the stratosphere is of major importance both for climate and air quality studies. Among the numerous instruments available, aerosol particles counters provide the size distribution in diameter range from few hundreds of nm to few tens of μm. Most of them are very sensitive to the nature of aerosols, and this can result in significant biases in the retrieved size distribution. We describe here a new versatile optical particle/sizer counter (OPC) named LOAC (Light Optical Aerosols Counter), which is light and compact enough to perform measurements not only at the surface but under all kinds of balloons in the troposphere and in the stratosphere. LOAC is an original OPC performing observations at two scattering angles. The first one is around 12°, and is almost insensitive to the nature of the particles; the second one is around 60° and is strongly sensitive to the refractive index of the particles. By combining measurement at the two angles, it is possible to retrieve accurately the size distribution and to estimate the nature of the dominant particles (droplets, carbonaceous, salts and mineral particles) in several size classes. This speciation is based on calibration charts obtained in the laboratory. Several campaigns of cross-comparison of LOAC with other particle counting instruments and remote sensing photometers have been conducted to validate both the size distribution derived by LOAC and the retrieved particle number density. The speciation of the aerosols has been validated in well-defined conditions including urban pollution, desert dust episodes, fog, and cloud. Comparison with reference aerosol mass monitoring instruments also shows that the LOAC measurements can be successfully converted to mass concentrations. All these tests indicate that no bias is present in the LOAC measurements and in the corresponding data processing.

Description: In a companion (Part 1) paper (Renard et al., 2015), we have described and evaluated a new versatile optical particle counter/sizer named LOAC (Light Optical Aerosols Counter) based on scattering measurements at angles of 12 and 60°. that allows some speciation of particles (droplets, carbonaceous, salts, and mineral dust) in addition to size segregated counting in a large diameter range from 0.2 up to possibly more than 100 μm depending on sampling conditions. Its capabilities overwhelm those of preceding optical particle counters (OPCs) allowing the characterization of all kind of aerosols from submicronic-sized absorbing carbonaceous particles in polluted air to very coarse particles (〉 10–20 μm in diameter) in desert dust plumes or fog and clouds. LOAC light and compact design allows measurements under all kinds of balloons, on-board unmanned aerial vehicles (UAV) and at ground level. We illustrate here the first LOAC airborne results obtained from an unmanned aerial vehicle (UAV) and a variety of scientific balloons. The UAV was deployed in a peri-urban environment near Bordeaux in France. Balloon operations include (i) tethered balloons deployed in urban environments in Wien (Austria) and Paris (France), (ii) pressurized balloons drifting in the lower troposphere over the western Mediterranean (during the Chemistry-Aerosol Mediterranean Experiment – ChArMEx campaigns), (iii) meteorological sounding balloons launched in the western Mediterranean region (ChArMEx) and from Aire-sur-l'Adour in south-western France (VOLTAIRE-LOAC campaign). More focus is put on measurements performed in the Mediterranean during (ChArMEx) and especially during African dust transport events to illustrate the original capability of balloon-borne LOAC to monitor in situ coarse mineral dust particles. In particular, LOAC has detected unexpected large particles in desert sand plumes.

Description: Long range transport of biomass burning (BB) aerosols between North America and the Mediterranean region took place in June 2013. A large number of ground based and airborne lidar measurements were deployed in the Western Mediterranean during the Chemistry-AeRosol Mediterranean EXperiment (ChArMEx) intensive observation period. A detailed analysis of the potential North American aerosol sources is conducted including the assessment of their transport to Europe using forward simulations of the FLEXPART Lagrangian particle dispersion model initialized using satellite observations by MODIS and CALIOP. The three dimensional structure of the aerosol distribution in the ChArMEx domain observed by the ground-based lidars (Menorca, Barcelona and Lampedusa), a Falcon-20 aircraft flight and three CALIOP tracks, agree very well with the model simulation of the three major sources considered in this work: Canadian and Colorado fires, a dust storm from Western US and the contribution of Saharan dust streamers advected from the North Atlantic trade wind region into the Westerlies region. Four aerosol types were identified using the optical properties of the observed aerosol layers (aerosol depolarization ratio, lidar ratio) and the transport model analysis of the contribution of each aerosol source: (I) pure BB layer, (II) weakly dusty BB, (III) significant mixture of BB and dust transported from the trade wind region (IV) the outflow of Saharan dust by the subtropical jet and not mixed with BB aerosol. The contribution of the Canadian fires is the major aerosol source during this episode while mixing of dust and BB is only significant at altitude above 5 km. The mixing corresponds to a 20–30 % dust contribution in the total aerosol backscatter. The comparison with the MODIS AOD horizontal distribution during this episode over the Western Mediterranean sea shows that the Canadian fires contribution were as large as the direct northward dust outflow from Sahara.

Description: 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 Forcing on the Mediterranean Climate (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 set-up 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 modelling 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 produce high level of atmospheric pollutants nor intense biomass burning events in the region. However, numerous mineral dust plumes were observed during the campaign with main sources located in Morocco, Algeria and Tunisia, leading to aerosol optical depth (AOD) values ranging between 0.2 to 0.6 (at 440 nm) over the western and central Mediterranean basins. Associated aerosol extinction values measured on-board the ATR-42 within the dust plume show local maxima reaching up to 150 Mm−1. Non negligible aerosol extinction (about 50 Mm−1) was also been observed within the Marine Boundary Layer (MBL). By combining ATR-42 extinction, absorption and scattering measurements, a complete optical closure has been made revealing excellent agreement with estimated optical properties. Associated calculations of the dust single scattering albedo (SSA) have been conducted, which show a moderate variability (from 0.90 to 1.00 at 530 nm). 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-salt 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 show particle size distributions characterized by large aerosols (〉 10 μm in diameter) within dust plumes. 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 direct radiative forcing (DRF). Results show significant surface SW instantaneous forcing (up to −90 W m−2 at noon). Associated 3-D modeling studies from regional climate (RCM) and chemistry transport (CTM) models indicate a relatively good agreement for simulated AOD compared with measurements/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 −20 W m−2 (for the whole period) over the Mediterranean Sea together with maxima (−50 W m−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.

Description: The Mediterranean basin is characterized by large concentrations of aerosols from both natural and anthropogenic sources. These aerosols change the optical properties of the atmosphere, therefore affecting tropospheric photochemistry through the photolytic rates. Two simulations of the atmospheric composition at basin-scale have been performed with the CHIMERE chemistry-transport model for the period from 1 June to 15 July 2013 covered by the ADRIMED campaign, a campaign of intense measurements in the western Mediterranean basin. One simulation takes into account the radiative effect of the aerosols on photochemistry, the other one does not. These simulations are compared to satellite and ground-based measurements, with a particular focus on the area of Lampedusa. Values of the Aerosol Optical Depth (AOD) are obtained from the MODIS instrument on the AQUA and TERRA satellites as well as from stations in the AERONET network and from the MFRSR sun photometer deployed at Lampedusa. Additional measurements from instruments deployed at Lampedusa either permanently or exceptionnally are used for other variables: MFRSR sun photometer for AOD, diode array spectrometer for actinic fluxes, LIDAR for the aerosol backscatter, sequential sampler for speciation of aerosol and Brewer spectrophotometer for the total ozone column. It is shown that CHIMERE has a significant ability to reproduce observed peaks in the AOD, which in Lampedusa are mainly due to dust outbreaks during the ADRIMED period, and that taking into account the radiative effect of the aerosols in CHIMERE improves considerably the ability of the model to reproduce the observed day-to-day variations of J(O1D) and J(NO2). While in the case of J(O1D) other variation factors such as the stratospheric ozone column are very important in representing correctly the day-to-day variations, the day-to-day variations of J(NO2) are captured almost completely by the model when the optical effects of the aerosols are taken into account. Finally, it is shown that the inclusion of the direct radiative effect of the aerosols in the CHIMERE model leads to reduced J(O1D) and J(NO2) values over all the simulation domain, which ranges from a few percents over continental Europe and the northeast Atlantic Ocean to about 20% close to and downwind from saharan dust sources. The effect on the modelled ozone concentration is twofold, with the effect of aerosols leading to reduced ozone concentrations over the Mediterranean Sea and continental Europe, close to the sources of NOx, and on the contrary to increased ozone concentrations over remote areas such the Sahara and the tropical Atlantic Ocean.

Description: In this work, impact of aerosol solar extinction on the photochemistry over eastern Europe during the 2010 wildfires episode is discussed for the period from 5 to 12 August 2010, which coincides to the peak of fire activity. The methodology is based on an on-line coupling between the chemistry-transport model CHIMERE (extended by an aerosol optical module) and the radiative transfer code TUV. Results of simulations indicate an important influence of the aerosol solar extinction, in terms of intensity and spatial extent, with a reduction of the photolysis rates of NO2 and O3 up to 50% (in diurnal-averaged) along the aerosol plume transport. At a regional scale, these changes in photolysis rates lead to a 3–15% increase in the NO2 daytime concentration and to an ozone reduction near the surface of 1–12%. The ozone reduction is shown to occur over the entire boundary layer, where aerosols are located. Also, comparisons of simulations with air quality measurements over Moscow show that the inclusion of the aerosol feedback tends to slightly improve performance of the model in simulating NO2 and O3 ground concentrations. In term of air quality prediction, the O3 peak reduction when including aerosol feedback results in a non-negligible difference in the predicted exceedance of alert threshold compared to the simulation without aerosol feedback, in coherence with measurements. Finally, the total aerosol mass concentration (PM10) is shown to be decreased by 1–2 %, on average during the studied period, caused by a reduced formation of secondary aerosols such as sulphates and secondary organics (4–10%) when aerosol impact on photolysis rates is included.

Description: Over the past decade, Aerosol Optical Depth (AOD) observations based on satellite and ground measurements have shown a significant increase over Arabia and the Arabian Sea, attributed to an intensification of regional dust activity. Recent studies have also suggested that west Asian dust forcing could induce a positive response of Indian monsoon precipitations on a weekly time scale. Using observations and a regional climate model including interactive slab ocean and dust aerosol schemes, the present study investigates possible climatic links between the increasing June-July-August-September (JJAS) Arabian dust activity and precipitation trends over southern India during the 2000–2009 decade. Meteorological reanalysis and AOD observations suggest that the observed decadal increase of dust activity and a simultaneous intensification of summer precipitation trend over southern India are both linked to a deepening of JJAS surface pressure conditions over the Arabian Sea. We show that the model skills in reproducing this trends and patterns are significantly improved only when an increasing dust emission trend is imposed on the basis of observations. We conclude that although climate variability might primarily determine the observed regional pattern of increasing dust activity and precipitation during the 2000–2009 decade, the associated dust radiative forcing might however induce a critical dynamical feedback contributing to enhanced regional moisture convergence and JJAS precipitation over Southern India.

Description: In the framework of the ChArMEx (the Chemistry-Aerosol Mediterranean Experiment, http://charmex.lsce.ipsl.fr/) program, the seasonal variability of the aerosol optical, microphysical and radiative properties is examined in two regional background insular sites in the western Mediterranean Basin (WMB): Ersa (Corsica Island, France) and Palma de Mallorca (Mallorca Island, Spain). A third site in Alborán (Alborán Island, Spain) with only a few months of data is considered for exploring the possible Northeast–Southwest (NE–SW) gradient of the aforementioned aerosol properties. The dataset is exclusively composed of AERONET (Aerosol Robotic Network; http://aeronet.gsfc.nasa.gov/) products during a four-year period (2011–2014). AERONET fluxes are validated with ground- and satellite-based flux measurements. To the best of our knowledge this is the first time that AERONET fluxes are validated at the top of the atmosphere. Products such as the aerosol optical depth (AOD), the fraction fine mode to total AOD, the particle size distribution, the sphericity, the radiative forcing and the radiative forcing efficiency show a clear annual cycle. The main drivers of the observed annual cycles are mineral dust outbreaks in summer and the transport of European continental aerosols in spring. A NE–SW gradient is observed on 6 parameters (3 extensive and 3 intensive) out of the 18 discussed in the paper. The NE–SW gradient of the AOD, the Ångström exponent, the coarse mode volume concentration, the sphericity and the radiative forcing at the surface are related to mineral dust outbreaks, while the NE–SW gradient of the coarse mode volume median radius is related to the decreasing influence of European continental aerosols along the NE–SW axis. The fact that two thirds of the parameters discussed in the paper do not present a NE–SW gradient is partly explained by two relevant findings: (1) a homogeneous spatial distribution of the fine particle loads over the three sites in spite of the distances between the sites and the differences in local sources, and (2) low values and the absence of spectral dependency of the absorption found in the southwesternmost site.

Description: This study presents in situ aircraft measurements of Saharan mineral dust transported over the western Mediterranean basin in June–July 2013 during the ChArMEx/ADRIMED (the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) airborne campaign. Dust events differing in terms of source region (Algeria, Tunisia and Morocco), time of tranport (1–5 days) and height of transport were sampled. Mineral dust were transported above the marine boundary layer, which conversely was dominated by pollution and marine aerosols. The dust vertical structure was extremely variable and characterized by either a single layer or a more complex and stratified structure with layers originating from different source regions. Mixing of mineral dust with pollution particles was observed depending on the height of transport of the dust layers. Dust layers carried higher concentration of pollution particles at intermediate altitude (1–3 km) than at elevated altitude (〉 3 km), resulting in scattering Angstrom exponent up to 2.2 within the intermediate altitude. However, the optical properties of the dust plumes remained practically unchanged with respect to values previously measured over source regions, regardless of the altitude. Moderate light absorption of the dust plumes was observed with values of aerosol single scattering albedo at 530 nm ranging from 0.90 to 1.00 ± 0.04. Concurrent calculations from the aerosol chemical composition revealed a negligible contribution of pollution particles to the absorption properties of the dust plumes that was due to a low contribution of refractory black carbon in regards to the fraction of dust and sulfate particles. This suggests that, even in the presence of moderate pollution, likely a persistent feature in the Mediterranean, the optical properties of the dust plumes could be assimilated to those of native dust in radiative transfer simulations, modeling studies and satellite retrievals over the Mediterranean. Measurements also showed that the coarse mode of mineral dust was conserved even after 5 days of transport in the Mediterranean, which contrasts with the gravitational depletion of large particles observed during the transport of dust plumes over the Atlantic. Simulations with the WRF mesoscale meteorological model highlighted a strong vertical turbulence within the dust layers that could prevent deposition of large particles during their atmospheric transport. This has important implications for the dust radiative effects due to surface dimming, atmospheric heating and cloud formation. The results presented here add to the observational dataset necessary for evaluating the role of mineral dust on the regional climate and rainfall patterns in the western Mediterranean basin.

Description: Marine nitrogen fixation is co-limited by the supply of iron and phosphorus in large areas of the global ocean. Up to 75 % of marine nitrogen fixation may be limited by iron supply due to the relatively high iron requirements of planktonic diazotrophs (Berman-Frank et al., 2001). The deposition of soluble aerosol iron can initiate nitrogen fixation and trigger toxic algal blooms in nitrate-poor tropical waters. There is a large variability in estimates of soluble iron, related to the mixing of aerosol iron sources. Most studies assume that mineral dust represents the primary source of soluble iron in the atmosphere. However, seasonal biomass burning in tropical regions is a potential source of aerosol iron that could explain the large variability of soluble iron in those regions. To investigate aerosol iron sources to the adjacent tropical waters of Australia, the fractional solubility of aerosol iron was determined during the Savannah Fires in the Early Dry Season (SAFIRED) campaign at Gunn Point, Northern Territory, Australia during the dry season in 2014. The source of particulate matter less than 10 µm (PM10) aerosol iron was a mixture of mineral dust, fresh biomass burning aerosol, sea spray and anthropogenic pollution. The mean soluble and total aerosol iron concentrations were 40 and 500 ng m−3 respectively. Fractional Fe solubility was relativity high for the majority of the campaign and averaged 8 % but dropped to 3 % during the largest and most proximal fire event. Fractional Fe solubility and proxies for biomass burning (elemental carbon, levoglucosan, oxalate and carbon monoxide) were unrelated throughout the campaign. An explanation of the lack of correlation between fractional Fe solubility and elemental carbon at the biomass burning source is due to the physical properties of elemental carbon, i.e., fresh elemental carbon aerosols are initially hydrophobic, however they can disperse in water after aging and coating with water soluble species in the atmosphere. Combustion aerosols are thought to have a high factional Fe solubility, which can increase during atmospheric transport from the source. Although, biomass burning derived particles may not be a direct source of soluble iron, they can act indirectly as a surface for aerosols iron to bind during atmospheric transport and subsequently be released to the ocean upon deposition. In addition, biomass burning derived aerosols can indirectly impact the fractional solubility of mineral dust. Fractional Fe solubility was highest during dust events at Gunn Point, and could have been enhanced by mixing with biomass burning derived aerosols. Iron in dust may be more soluble in the tropics compared to higher latitudes due to the presence higher concentrations of biomass burring derived reactive organic species in the atmosphere, such as oxalate, and their potential to enhance the fractional Fe solubility of mineral dust. As the aerosol loading is dominated by biomass burning emissions over the tropical waters in the dry season, additions of biomass burning derived soluble iron could have harmful consequences for initiating nitrogen fixing toxic algal blooms. Future research is required to quantify biomass burning derived particle sources of soluble iron over tropical waters.